# No-Decompression Limits, Nitrox and MOD: The Numbers Explained

What an NDL is, where the published table comes from, how nitrox buys bottom time through equivalent air depth, and why the same gas imposes a hard depth ceiling.

---

- **Canonical URL:** https://dothecalculation.com/blog/hobby/no-decompression-limits-explained
- **Category:** Hobbies & Leisure Utilities
- **Author:** Do The Calculation Team
- **Published:** 2026-09-20
- **Reading time:** 17 min read
- **Publisher:** Do The Calculation (https://dothecalculation.com)
- **Methodology:** https://dothecalculation.com/methodology

---

> **Read this first** — This is an educational explanation of dive planning arithmetic. It is not dive training, it is not a dive plan, and it is not a substitute for a dive computer. Every figure here is for a single square-profile dive with no residual nitrogen from any previous dive and no altitude adjustment, which describes almost no real diving day. Dive within your certification, plan with your instructor or a current agency table, and follow your computer in the water.

With that said, the arithmetic behind a dive plan is worth understanding rather than trusting blindly to a wrist computer. There are three numbers, they interact, and two of them pull in opposite directions.

## What a no-decompression limit is

Breathing gas under pressure loads nitrogen into your tissues. Ascend slowly enough and that nitrogen comes back out through your lungs; ascend with too much of it dissolved and it comes out as bubbles. The no-decompression limit is the longest you can stay at a depth and still surface on a normal ascent without owing the water a decompression stop.

The table below is the one published by the United States federal government as Appendix A to Part 197 of Title 46 of the Code of Federal Regulations, which reproduces the US Navy air no-decompression limits. It is the source our calculator uses, and it is eleven rows — that is the entire table.

**Air no-decompression limits (46 CFR Part 197, Appendix A)**
| Depth (feet) | Limit (minutes) |
| --- | --- |
| 35 | 310 |
| 40 | 200 |
| 50 | 100 |
| 60 | 60 |
| 70 | 50 |
| 80 | 40 |
| 90 | 30 |
| 100 | 25 |
| 110 | 20 |
| 120 | 15 |
| 130 | 10 |

The regulation defines the limit as "the maximum bottom time in minutes that a diver can spend at that depth without requiring decompression beyond that provided by a normal ascent rate of 60 fsw per minute", and states that it applies to "single, no-decompression, air dives made within any 12-hour period".

> **Never interpolate a dive table** — If your depth falls between two rows, you use the deeper row. A dive to 85 feet is planned on the 90-foot line, which gives 30 minutes rather than something between 30 and 40. Rounding towards the conservative direction is how tables are taught and it is what our calculator does — the arithmetic mean of two rows is not a number anybody published.

Look at how fast the limits collapse. At 60 feet you have an hour. At 100 feet you have 25 minutes. At 130 feet — the recreational depth ceiling taught by every mainstream agency — you have ten. Depth is expensive, and the last thirty feet of a dive plan cost more bottom time than the first sixty.

Tool: [Look up a limit and check a gas](https://dothecalculation.com/calculators/scuba-diving-no-decompression-limit-calculator) — The published limit for your depth, with nitrox equivalent air depth, maximum operating depth and partial pressure of oxygen.

## What nitrox actually buys you

Nitrox is air with some of the nitrogen replaced by oxygen. Because the limits above are driven by nitrogen loading, breathing less nitrogen at the same depth loads you more slowly — so you get longer. The way to quantify it is equivalent air depth: the depth at which air would give you the same nitrogen partial pressure your nitrox gives you here.

**Equivalent air depth**

```
EAD = ((1 − FO₂) × (D + 33) ÷ 0.79) − 33
```
- D is the actual depth in feet of sea water, FO₂ is the oxygen fraction (0.32 for EAN32), and 0.79 is the nitrogen fraction of air.
- You then look the EAD up in the air table, rounding up to the next row as always.
- For air, FO₂ is 0.21 and the formula returns the actual depth, which is the arithmetic confirming itself.

**What EAN32 does, at a 1.4 ATA oxygen limit**
| Actual depth | Equivalent air depth | Table row used | Limit on air | Limit on EAN32 |
| --- | --- | --- | --- | --- |
| 80 ft | 64.3 ft | 70 ft | 40 min | 50 min |
| 100 ft | 81.5 ft | 90 ft | 25 min | 30 min |

Ten extra minutes at 80 feet and five at 100. Real, useful, and smaller than the marketing sometimes suggests — partly because the table rounds up to the next row, so some of the benefit gets rounded away.

## And what it costs you: the depth ceiling

The same oxygen that buys bottom time becomes the limiting factor as you go deeper. Oxygen at high partial pressure is toxic to the central nervous system, and the consequence underwater — a convulsion — is not survivable with a regulator in your mouth. So every nitrox mix has a hard maximum operating depth.

**Partial pressure and maximum operating depth**

```
ATA = depth in feet ÷ 33 + 1
ppO₂ = FO₂ × ATA
MOD (feet) = 33 × (ppO₂ limit ÷ FO₂ − 1)
```
- The conventional working limit is 1.4 ATA of oxygen, with 1.6 sometimes used for a decompression stop rather than for bottom time. Our calculator takes the limit as an input rather than assuming one.
- The richer the mix, the shallower the ceiling. This is the trade that makes gas selection a planning decision rather than a preference.

**Maximum operating depth at a 1.4 ATA oxygen limit**
| Mix | MOD | ppO₂ at 100 ft |
| --- | --- | --- |
| Air (21%) | 187 ft | 0.85 |
| EAN32 | 111 ft | 1.29 |
| EAN36 | 95 ft | 1.45 — over the limit |

> **The trap in that table** — EAN36 has a maximum operating depth of 95 feet. A diver who takes it to 110 feet is at a ppO₂ of 1.56, well past 1.4, on a gas chosen precisely to extend the dive. The richer mix that gives you the most bottom time at 60 feet is the one that must not go deep at all, and the depths where nitrox helps most are the depths where the wrong mix is most dangerous. Analyse every cylinder yourself and label it. Do not dive a mix you did not personally verify.

## What this arithmetic does not model

The gap between the numbers above and an actual dive is large, and it all runs in the same direction: the real limits are shorter than the table.

- Repetitive dives. The regulation is explicit that its limits apply to a single dive in a 12-hour period, and that nitrogen remaining from an earlier dive has to be accounted for. A second dive has a shorter limit than the table shows, always.
- Multi-level profiles. The table assumes a square profile — straight down, stay, straight up. Real dives spend time at varying depths, which is exactly what a dive computer tracks and a table cannot.
- Altitude. A dive in a mountain lake surfaces to lower atmospheric pressure, so sea-level tables are not valid without an altitude procedure.
- You. Tables are population models. Hydration, temperature, exertion, body composition, age and fatigue all affect nitrogen loading, and none of them is an input to any of the formulas above.
- Ascent rate and safety stops. The limits assume a 60 fsw per minute ascent. Every agency now also teaches a safety stop, which the table does not require but which is standard practice.

> **Which is why the computer wins** — A dive computer samples your actual depth continuously and recalculates tissue loading throughout the dive, so it credits you for the shallow parts of a multi-level profile and carries your residual nitrogen between dives. A table cannot do either. Use this arithmetic to understand what your computer is doing and to sanity-check a plan on the surface — then dive the computer.

## Pressure at depth: the number everything else comes from

Every calculation in this guide rests on one relationship. At the surface you are under one atmosphere of pressure. Every 33 feet of sea water adds another. The pressure at depth, in atmospheres absolute, is therefore depth in feet divided by 33, plus 1.

**Absolute pressure in sea water**
| Depth | Pressure (ATA) | What it means |
| --- | --- | --- |
| Surface | 1.00 | Reference |
| 33 ft | 2.00 | Pressure has doubled in the first 33 ft |
| 66 ft | 3.00 |  |
| 99 ft | 4.00 |  |
| 130 ft | 4.94 | The recreational depth ceiling |

The first 33 feet doubles the pressure; the next 33 only adds another half on top of that. This is why the largest changes in volume, and the greatest need to equalise, happen near the surface, and why the final few metres of an ascent deserve the most care. It is also the reason gas consumption, nitrogen loading and oxygen exposure all climb with depth, as the rest of this section shows.

## Why your gas runs out faster at depth

At twice the pressure, every breath you take contains twice as many molecules of gas, drawn from the same cylinder. Boyle's law makes the consequence exact: at a fixed breathing effort, gas consumption scales directly with absolute pressure.

**Gas use at depth**

```
Consumption at depth = surface consumption rate × pressure (ATA)
```
- Surface consumption rate, sometimes called SAC, is what you breathe per minute at the surface. It varies a great deal between people and with exertion, cold and stress; divers measure their own on training dives.
- The figures below use an illustrative rate of 0.7 cubic feet per minute and an 80 cubic foot cylinder, keeping a third of it in reserve for the illustration. They are not planning figures for you.

**Illustrative: how long two thirds of an 80 cu ft cylinder lasts at 0.7 cu ft/min surface rate**
| Depth | Consumption | Time to use two thirds |
| --- | --- | --- |
| Surface | 0.70 cu ft/min | about 77 min |
| 33 ft | 1.40 cu ft/min | about 38 min |
| 60 ft | 1.97 cu ft/min | about 27 min |
| 100 ft | 2.82 cu ft/min | about 19 min |
| 130 ft | 3.46 cu ft/min | about 16 min |

Put this beside the no-decompression table and a pattern appears. At 60 feet the table allows 60 minutes, but on these illustrative figures the gas plan runs out well before that. At 100 feet the table allows 25 minutes and gas allows about 19. For many recreational divers on a single cylinder, gas, not nitrogen, is the limit that arrives first. A dive plan has to respect whichever comes first, and that is usually the smaller of the two numbers.

## Using the table with metric depths

The regulatory table is in feet of sea water. If your gauge and dive plan are in metres, convert to feet at 3.28 feet per metre and then apply the same rule as always: use the next deeper row.

**Metric depths against the 46 CFR air table**
| Depth | In feet | Row used | No-decompression limit |
| --- | --- | --- | --- |
| 10 m | 32.8 ft | 35 ft | 310 min |
| 12 m | 39.4 ft | 40 ft | 200 min |
| 15 m | 49.2 ft | 50 ft | 100 min |
| 18 m | 59.1 ft | 60 ft | 60 min |
| 21 m | 68.9 ft | 70 ft | 50 min |
| 25 m | 82.0 ft | 90 ft | 30 min |
| 30 m | 98.4 ft | 100 ft | 25 min |
| 35 m | 114.8 ft | 120 ft | 15 min |
| 40 m | 131.2 ft | Beyond the table | No limit given |

Two rows are worth pausing on. At 25 metres, 82 feet is just past the 80-foot line, so the table sends you to the 90-foot row and a 30-minute limit rather than 40. Rounding up always costs time, and it is meant to. And 40 metres, a depth many people think of as a round recreational figure, is 131 feet: past the last row of this table. There is no no-decompression limit for it here at all.

## Nitrox across the whole range of depths

The body of this guide showed EAN32 at two depths. Here it is across the full range, using the equivalent air depth formula and rounding up to the next table row each time.

**EAN32 against air, 46 CFR air table, equivalent air depth rounded up**
| Actual depth | Equivalent air depth | Row used | Limit on EAN32 | Limit on air |
| --- | --- | --- | --- | --- |
| 50 ft | 38.4 ft | 40 ft | 200 min | 100 min |
| 60 ft | 47.1 ft | 50 ft | 100 min | 60 min |
| 70 ft | 55.7 ft | 60 ft | 60 min | 50 min |
| 80 ft | 64.3 ft | 70 ft | 50 min | 40 min |
| 90 ft | 72.9 ft | 80 ft | 40 min | 30 min |
| 100 ft | 81.5 ft | 90 ft | 30 min | 25 min |
| 110 ft | 90.1 ft | 100 ft | 25 min | 20 min |

The shallow end is where nitrox changes the numbers most, doubling the limit at 50 feet. At depth the gain shrinks to a few minutes, and the 110-foot row shows rounding at its harshest: an equivalent air depth of 90.1 feet is a tenth of a foot past the 90-foot line, so it takes the 100-foot row. It is also within a foot of EAN32's maximum operating depth at 1.4 ATA, which is 111 feet. The dive that looks like it benefits from nitrox is right at the edge of what the mix allows.

## Maximum operating depth for common mixes

**Maximum operating depth by oxygen fraction, from MOD = 33 × (ppO₂ limit ÷ FO₂ − 1)**
| Mix | At a 1.4 ATA limit | At a 1.6 ATA limit |
| --- | --- | --- |
| Air (21%) | 187 ft | 218 ft |
| EAN28 | 132 ft | 156 ft |
| EAN30 | 121 ft | 143 ft |
| EAN32 | 111 ft | 132 ft |
| EAN34 | 103 ft | 122 ft |
| EAN36 | 95 ft | 114 ft |
| EAN40 | 82 ft | 99 ft |

The 1.6 column is shown because the figure is often quoted, not as a planning limit for bottom time; the body of this guide explains that 1.4 is the conventional working figure and 1.6 is usually associated with decompression stops. Your training agency sets the limits you are qualified to use. The table is simply the arithmetic behind the numbers on a nitrox cylinder label.

## The best mix for a planned depth

**Best mix**

```
Best oxygen fraction = ppO₂ limit ÷ pressure at the planned maximum depth (ATA)
```
- This gives the richest mix whose maximum operating depth is exactly the planned depth. Divers normally round down to a standard fill and leave margin, never up.
- Worked at a 1.4 ATA limit: at 100 ft, pressure is 4.03 ATA, so the best mix is 1.4 ÷ 4.03 = 34.7%, rounded down to EAN34 or below.

**Best mix at a 1.4 ATA oxygen limit**
| Planned maximum depth | Best mix |
| --- | --- |
| 60 ft | 49.7% |
| 70 ft | 44.9% |
| 80 ft | 40.9% |
| 90 ft | 37.6% |
| 100 ft | 34.7% |
| 110 ft | 32.3% |

Rich mixes are only usable for shallow dives. A 50% mix would be ideal at 60 feet and dangerous at 80. That is why nitrox is always paired with a planned maximum depth, and why a dive with a variable profile needs a mix chosen for its deepest point, not its average depth.

## The ascent the table assumes

The regulation defines its limits against "a normal ascent rate of 60 fsw per minute". At that rate, surfacing from 60 feet takes one minute and from 130 feet a little over two. Recreational dive training commonly teaches a slower ascent than the regulation's figure, together with a safety stop near the surface. The table's limits do not include either, which is one more reason the numbers here are a ceiling, not a target. Follow the ascent rate and stops your own training taught you.

## Planning mistakes the arithmetic can catch

- **Planning on the no-decompression limit alone.** Gas supply is often the tighter limit, especially at depth. Plan to the smaller of the two.
- **Interpolating between rows.** Always use the next deeper row, even when the depth is only just past a line.
- **Converting metres loosely.** 25 metres is 82 feet, not 80, and it changes the row.
- **Choosing a mix for the average depth.** Nitrox is chosen for the deepest point of the dive.
- **Forgetting that the table is for a single dive.** Repetitive dives within 12 hours need a proper repetitive-dive procedure or a computer.
- **Treating the numbers as targets.** Every figure in this guide is an upper limit for a single square-profile dive under ideal assumptions. Real dives are planned well inside them.

## Putting it together: one dive, every number

Here is how the pieces combine for a single dive to 25 metres, first on air and then on EAN32, using the illustrative gas figures from earlier in this section. It shows the method, not a plan anyone should dive: real planning uses your own consumption rate, your agency's procedures and your computer.

**A single square-profile dive to 25 m (82.0 ft)**
| Step | On air | On EAN32 |
| --- | --- | --- |
| Pressure at depth | 3.49 ATA | 3.49 ATA |
| Oxygen partial pressure | 0.73 ATA | 1.12 ATA, inside 1.4 |
| Table row | 90 ft (82 ft rounds up) | 70 ft (equivalent air depth 66.0 ft rounds up) |
| No-decompression limit | 30 min | 50 min |
| Illustrative gas: two thirds of 80 cu ft at 0.7 cu ft/min surface rate | about 22 min | about 22 min |
| The limit that arrives first | Gas, at about 22 min | Gas, at about 22 min |

On these figures, nitrox raises the no-decompression limit from 30 to 50 minutes but changes nothing about the dive, because the gas supply ends it at about 22 minutes either way. That is a common real-world outcome and a useful correction to the idea that nitrox automatically means longer dives. It gives more margin on nitrogen, which is valuable, especially on repetitive dives; it does not give you more gas. A larger cylinder or a lower consumption rate is what lengthens this dive.

## The terms, in one place

- **ATA (atmospheres absolute):** total pressure, counting the atmosphere above the water. 1 ATA at the surface, 2 ATA at 33 feet of sea water.
- **fsw (feet of sea water):** depth expressed as the pressure of that much sea water, the unit the regulatory table uses. Metres of sea water (msw) are the metric equivalent.
- **NDL (no-decompression limit):** the longest time at a given depth from which you can ascend at the normal rate without a required decompression stop.
- **Square profile:** a dive modelled as going straight to one depth, staying there, and coming straight up. Tables assume it; real dives rarely follow it.
- **Repetitive dive:** any dive made while nitrogen from an earlier dive is still in your tissues. The 46 CFR table covers a single dive within 12 hours only.
- **EAN (enriched air nitrox):** air with extra oxygen. EAN32 is 32% oxygen.
- **ppO₂ (partial pressure of oxygen):** the oxygen fraction multiplied by the absolute pressure. It is what sets oxygen exposure limits.
- **EAD (equivalent air depth):** the depth on air that would give the same nitrogen partial pressure as a nitrox mix at the actual depth.
- **MOD (maximum operating depth):** the deepest a mix can be used before ppO₂ exceeds the chosen limit.
- **SAC (surface air consumption):** how much gas you breathe per minute at the surface, used to plan gas at depth by multiplying by ATA.

Every one of these connects back to the single pressure relationship at the top of this section. Once you are comfortable that pressure rises by one atmosphere every 33 feet, the rest of the arithmetic follows, and the numbers on your computer stop being a black box.

## Sources

- 46 CFR Part 197, Appendix A — Air No-Decompression Limits, the eleven-row table reproduced above, the definition of the limit and the 60 fsw per minute ascent rate, and the statement that the limits are for a single dive within any 12-hour period: https://www.law.cornell.edu/cfr/text/46/appendix-A_to_part_197
- The equivalent air depth, partial pressure and maximum operating depth relations are standard gas arithmetic and are computed from the formulas shown rather than taken from a chart.
- The 1.4 ATA working oxygen limit is the conventional recreational figure rather than a regulatory one, which is why our calculator takes it as an input.

## Common questions

**What is a no-decompression limit?**

The longest you can stay at a given depth and still ascend normally without owing a decompression stop. On the published air table it is 60 minutes at 60 feet, 40 at 80 feet, 25 at 100 feet and 10 at 130 feet. Those are for a single square-profile dive with no residual nitrogen.

**How much extra bottom time does nitrox give me?**

At 80 feet on EAN32, the equivalent air depth is 64.3 feet, which uses the 70-foot table row: 50 minutes instead of 40. At 100 feet it is 30 minutes instead of 25. Useful, but less than people expect, partly because rounding up to the next table row absorbs some of the gain.

**What is maximum operating depth?**

The deepest you can take a mix before the oxygen partial pressure exceeds your chosen limit. At a 1.4 ATA limit, EAN32 tops out at 111 feet and EAN36 at 95 feet. Air reaches 187 feet on oxygen grounds alone, which is far past the 130-foot recreational depth ceiling.

**What do I do if my depth is between two table rows?**

Use the deeper row. An 85-foot dive is planned on the 90-foot line, giving 30 minutes rather than 40. Dive tables are never interpolated — rounding always goes the conservative way.

**Can I use these limits for a second dive of the day?**

No. The regulation states these are for a single dive within any 12-hour period, and that nitrogen remaining from an earlier dive must be accounted for. A repetitive dive always has a shorter limit than the table shows. Use a current agency repetitive-dive procedure or your computer.

**Why does my dive computer give a different number?**

Because it is tracking your actual profile rather than assuming a square one, carrying residual nitrogen between dives, and applying its own conservatism setting and tissue model. It has far more information than a table does. Follow the computer.

**What is the no-decompression limit at 18 metres?**

18 metres is 59.1 feet, which uses the 60-foot row of the 46 CFR air table: 60 minutes. That is for a single square-profile air dive with no residual nitrogen, and your gas supply may run out first.

**Is 40 metres within the no-decompression table?**

No. 40 metres is about 131 feet, just past the table's last row at 130 feet, so the table gives no no-decompression limit for it at all.

**Why does gas run out faster at depth?**

Boyle's law. At depth each breath contains more gas, in proportion to the absolute pressure. At 33 feet, 2 ATA, you use gas twice as fast as at the surface; at 100 feet, about 4 ATA, about four times as fast.

**How do I work out the best nitrox mix for a dive?**

Divide the oxygen partial pressure limit by the absolute pressure at your planned maximum depth. At a 1.4 ATA limit and 100 feet, that is 1.4 ÷ 4.03 = 34.7%, so you would choose a standard fill at or below that, such as EAN32.

**Does nitrox always give more bottom time?**

It gives more no-decompression time at any depth within its maximum operating depth, and the gain is largest in the shallows: on the 46 CFR table, EAN32 doubles the limit at 50 feet but adds only five minutes at 100 feet. It does not change your gas supply, which may limit the dive first.

---

_Source: [Do The Calculation](https://dothecalculation.com/blog/hobby/no-decompression-limits-explained). Quote freely with attribution and a link to this page._