# Scuba Diving No-Decompression Limit Calculator Look up no-stop bottom time for a planned depth on air or nitrox, from the published U.S. Navy air table. --- - **Canonical URL:** https://dothecalculation.com/calculators/scuba-diving-no-decompression-limit-calculator - **Category:** Hobbies & Leisure Utilities - **Publisher:** Do The Calculation (https://dothecalculation.com) - **Cost:** Free, no account or sign-up required - **Privacy:** Runs entirely in the browser; inputs are never sent to a server - **Methodology:** https://dothecalculation.com/methodology --- ## No-Decompression Limits from the Published Air Table Look up the maximum no-stop bottom time for a planned depth on air or nitrox, using the U.S. Navy air table reproduced in U.S. federal regulation. A pre-dive learning and planning tool, never a substitute for your dive computer. - Limits taken directly from 46 CFR Part 197, Appendix A — no invented numbers - Nitrox handled by equivalent air depth, with maximum operating depth alongside - Depths always round up to the deeper table row, the conservative direction ## Quick Answer — What Is the No-Decompression Limit at My Depth? A no-decompression limit, or NDL, is the maximum time you can spend at a given depth and still ascend directly to the surface at a normal rate without a required decompression stop. It shortens sharply with depth, because nitrogen loads into tissue faster as pressure rises. **Every row of the air table this calculator uses, from 46 CFR Part 197, Appendix A:** • **35 ft** — 310 minutes • **40 ft** — 200 minutes • **50 ft** — 100 minutes • **60 ft** — 60 minutes • **70 ft** — 50 minutes • **80 ft** — 40 minutes • **90 ft** — 30 minutes • **100 ft** — 25 minutes • **110 ft** — 20 minutes • **120 ft** — 15 minutes • **130 ft** — 10 minutes Read the shape of that list rather than any single row. Going from 60 ft to 80 ft cuts the limit from 60 minutes to 40; another 20 feet down to 100 ft cuts it to 25; by 130 ft, the recreational depth ceiling, you have ten minutes. Depth is not a linear cost. **This is a planning and learning reference, not a dive computer.** It plans one dive, in isolation, at a single depth. It does not track residual nitrogen from an earlier dive, does not adjust for altitude, and cannot see what actually happens in the water. Dive within your certification, follow your computer, and make a safety stop. ## How to Use This Calculator: An 80-Foot Dive on Air Set the units to feet, enter a planned maximum depth of **80 ft**, and a planned bottom time of **30 minutes**. Leave the gas on air at 21% oxygen and the oxygen partial pressure limit at 1.4 ata. On air, the equivalent air depth is simply the depth: **80 ft**. That lands exactly on the 80 ft row, so the no-decompression limit is **40 minutes** and the margin against your plan is **10 minutes**. The calculator also reports the pressure side of the dive. At 80 ft the ambient pressure is **3.42 ata**, so on air the oxygen partial pressure is **0.72 ata** — nowhere near a concern. The maximum operating depth for air at a 1.4 ata limit is **187 ft**, far deeper than any recreational dive, which is why oxygen is not the limiting factor on air. Now try 55 ft instead. The equivalent air depth is 55 ft, which falls between the 50 ft and 60 ft rows, so the calculator rounds **up** to the 60 ft row and returns **60 minutes** rather than something between 60 and 100. Rounding a dive table toward the shallower row would hand back a longer limit than the table supports, which is the one direction you never want an error to go. ## The Formulas This Calculator Uses Only one number is looked up — the limit itself. Everything around it is computed from standard dive physics. **Equivalent air depth** converts a nitrox dive into the air dive that carries the same nitrogen load: \(EAD = \frac{(1 - FO_2)(D + 33)}{0.79} - 33\), with depth in feet of sea water. For air, \(FO_2 = 0.21\), so the numerator and the 0.79 cancel and the EAD equals the depth. **Ambient pressure** in atmospheres absolute is \(\frac{D}{33} + 1\), because 33 feet of sea water adds one atmosphere. **Oxygen partial pressure** is \(ppO_2 = FO_2 \times ata\), and inverting it gives the **maximum operating depth**: \(MOD = 33 \times \left(\frac{ppO_2\ limit}{FO_2} - 1\right)\). **The limit itself** is a table lookup, never a formula: the calculator finds the shallowest listed depth at or beyond your equivalent air depth and reads off its minutes. Between-row depths always move to the deeper row. The distinction matters. Interpolating between 60 minutes at 60 ft and 50 minutes at 70 ft would produce a plausible-looking 55 minutes at 65 ft, and the table does not say that. Dive tables are lookups, and this one is treated as a lookup. ## A Second Example: The Same 80 Feet on EAN32 Keep the depth at 80 ft and the bottom time at 30 minutes, then switch the gas to EAN32 — nitrox with 32% oxygen. The equivalent air depth falls to **64.3 ft**, because the mix carries less nitrogen than air does. Rounded up to the 70 ft row, the no-decompression limit becomes **50 minutes** rather than 40, so the margin against your plan grows from 10 minutes to **20 minutes**. That is the entire benefit of nitrox: a shallower table depth for the same real depth. The trade shows up on the oxygen side. At 80 ft on EAN32 the oxygen partial pressure is **1.10 ata**, and the maximum operating depth at a 1.4 ata limit is **111.4 ft**. On EAN36 that ceiling drops to **95.3 ft**. The richer the mix, the more bottom time it buys and the shallower you have to stay — which is why a 130 ft dive on EAN32 is flagged here as exceeding the maximum operating depth even though its nitrogen limit looks generous. Nitrox does not make you safe from decompression sickness and it does not extend a dive indefinitely. It substitutes an oxygen constraint for part of the nitrogen one, and both constraints still bind. ## Why the Table Says What It Says Nitrogen dissolves into body tissue in proportion to the pressure you breathe it at. Descend and it loads in; ascend and it comes back out. Ascend faster than the tissue can offload it and the dissolved gas comes out of solution as bubbles, which is decompression sickness. A no-decompression limit is the point at which a direct ascent stops being safe without stopping on the way up. The limits in this table date from U.S. Navy work and were carried into U.S. federal regulation, where the same eleven rows have applied to commercial diving operations for decades. Recreational agencies publish their own tables from different models, and dive computers run continuous algorithms rather than tables at all — the Bühlmann family being the most common. Those all disagree with each other at the margins, which is a real feature of the field rather than a defect in any one of them. This tool uses one named source and shows you exactly which row it read. That is deliberate. A calculator that blended several tables into an average would produce numbers with no source behind them at all, and dive safety is the wrong place for a plausible-looking synthesis. None of it accounts for you specifically. Cold water, hard work, dehydration, age, body composition, fatigue, and a previous day of diving all change how your body handles nitrogen, and no table models any of them. That is why every agency teaches divers to plan well inside the limit rather than at it. ## Limitations **This calculator plans a single dive, and most diving is not a single dive.** It has no concept of a repetitive dive, a pressure group, a surface interval, or residual nitrogen. If you have already been in the water today, the limits shown here are too long — sometimes dramatically so — and you need a full repetitive table or a computer that has been tracking your day. It also models a square profile: straight down to your maximum depth, the whole bottom time there, straight up. Real dives are multi-level, and a dive computer credits you for the shallower parts. That difference makes this calculator conservative for a typical recreational profile, which is the right direction, but it means the number here is not the number your computer will show. There is no altitude adjustment. Diving at elevation reduces surface pressure and shortens every limit in the table, and a lake at 5,000 feet needs altitude tables or an altitude-aware computer, not this. The table stops at 130 ft, which is also the recreational depth ceiling. Beyond that the calculator returns nothing, because the honest answer is that the dive requires planned decompression and technical training rather than a longer table. Finally, and most importantly: **this is an educational and pre-dive planning reference.** It is not a substitute for a certified dive computer, for formal training, or for the judgement of a certified instructor or divemaster. In the water, follow your computer, stay well inside your limits, ascend slowly, and make a safety stop. ## Related Calculators Depth and time planning is one of several problems in this collection where a closed volume of water or air has to be modelled carefully. The [aquarium volume calculator](/calculators/aquarium-volume-calculator) works out tank capacity from dimensions, and the [aquarium CO2 calculator](/calculators/aquarium-co2-calculator) deals with dissolved gas concentration in water — the same physics of gas in solution, at a much smaller scale. For converting between feet and metres, bar and atmospheres, and the other units dive planning mixes freely, the [unit converter](/calculators/unit-converter) handles the arithmetic. And for planning effort and timing on the surface rather than under it, the [hiking time estimator](/calculators/hiking-time-estimator-calculator) applies the same kind of published-model approach to a route on land. ## Frequently asked questions ### What is a no-decompression limit? It is the maximum time you can stay at a given depth and still ascend directly to the surface at a normal rate without a required decompression stop. On the air table this calculator uses, it is 60 minutes at 60 feet, 40 minutes at 80 feet, 25 minutes at 100 feet, and 10 minutes at 130 feet. ### Where do these numbers come from? From the air no-decompression limits table published as Appendix A to Subpart C of 46 CFR Part 197, which reproduces the U.S. Navy Diving Manual limits. All eleven rows of that table are listed on this page. Nothing is interpolated or averaged, and depths between rows are rounded up to the deeper row. ### Can I use this instead of a dive computer? No. This is a pre-dive planning and learning tool. It models one square-profile dive with no residual nitrogen and no altitude adjustment, and it cannot see what happens once you are in the water. A dive computer tracks your actual depth continuously, accounts for previous dives, and is what you follow underwater. ### How much extra bottom time does nitrox give me? It depends on the depth. At 80 feet, EAN32 has an equivalent air depth of 64.3 feet, which moves the lookup from the 80-foot row to the 70-foot row and takes the limit from 40 minutes to 50. The gain comes entirely from carrying less nitrogen, and it is paid for with a shallower maximum operating depth. ### What is equivalent air depth? It is the depth at which breathing air would give the same nitrogen partial pressure as your nitrox mix does at your actual depth. The formula is EAD = ((1 - FO2) x (Depth + 33) / 0.79) - 33 in feet. Because a nitrox mix contains less nitrogen than air, its equivalent air depth is always shallower than the real depth. ### Why does my depth get rounded up to a deeper row? Because rounding toward the shallower row would return a longer limit than the table actually supports. A 55-foot dive is planned on the 60-foot row, giving 60 minutes rather than an interpolated figure. This is standard dive table practice and it is the only safe direction for the error to run. ### What is maximum operating depth and why does it matter on nitrox? It is the depth at which the oxygen partial pressure of your mix reaches your chosen limit, usually 1.4 ata for working depths. MOD = 33 x (limit / FO2 - 1) in feet. On EAN32 that is 111.4 feet and on EAN36 it is 95.3 feet — so a rich mix can hit its oxygen ceiling well before its nitrogen limit becomes the problem. ### Does this handle repetitive dives? No, and that is the single biggest limitation. It plans one dive with a clean nitrogen load. Any dive after the first one on the same day carries residual nitrogen that shortens every limit, and accounting for that needs a full repetitive table with pressure groups and surface intervals, or a dive computer that has been tracking you all day. ## Related concepts - **No-Decompression Limit (NDL)** — The maximum bottom time at a depth that still permits a direct ascent at a normal rate, without a required decompression stop on the way up. - **Equivalent Air Depth** — The air depth carrying the same nitrogen partial pressure as a nitrox mix at your real depth. It is what lets a single air table serve nitrox dives. - **Maximum Operating Depth** — The depth at which a mix reaches your oxygen partial pressure limit. The richer the nitrox, the shallower this ceiling sits — 111.4 feet on EAN32 at 1.4 ata. ## Related guides - [How to Use Do The Calculation Calculators: A Practical Step-by-Step Guide](https://dothecalculation.com/blog/site-guides/how-to-use-calculators) — Learn the fastest reliable workflow for using Do The Calculation calculators, reading results, checking formulas, and using save, print, share, and export actions correctly. - [Understanding Calculator Formulas: How DTC Turns Inputs into Results](https://dothecalculation.com/blog/site-guides/understanding-calculator-formulas) — Understand how Do The Calculation formulas are presented, what the explanation blocks mean, and how to verify calculator logic before using a result in a real decision. ## Related calculators - [Fish Tank Cycling Time Calculator](https://dothecalculation.com/calculators/fish-tank-cycling-time-calculator) — Estimate aquarium cycling time from temperature, pH, seeding, ammonia level and method, with a phase breakdown. - [Hiking Time Estimator (Naismith's Rule)](https://dothecalculation.com/calculators/hiking-time-estimator-calculator) — Estimate hiking time from distance and elevation gain using Naismith's Rule, with Aitken and Tranter corrections. - [Metal Detector Target Depth Estimator](https://dothecalculation.com/calculators/metal-detector-depth-calculator) — Estimate the maximum target detection depth for your metal detector based on coil size, frequency, and soil mineral type. - [Model Rocket Altitude & Motor Calculator](https://dothecalculation.com/calculators/model-rocket-altitude-calculator) — Estimate peak rocket altitude, maximum velocity, and exact motor burn time using your rocket specifications and motor data. - [Depth of Field & Hyperfocal Distance Calculator](https://dothecalculation.com/calculators/photography-dof-hyperfocal-calculator) — Calculate depth of field, near and far focus limits, and hyperfocal distance for any camera, lens, and aperture setting. - [RC Flight Time & Battery Power Calculator](https://dothecalculation.com/calculators/rc-flight-time-calculator) — Estimate RC multirotor or airplane flight duration, battery power draw, and thrust-to-weight ratio for your specific setup. --- _This calculator is an educational and pre-dive planning reference. It is not a substitute for a certified dive computer, formal dive training, or the guidance of a certified instructor. Every no-decompression limit shown comes from one named source, the air table published as Appendix A to Subpart C of 46 CFR Part 197, and depths between rows always round up to the deeper row. It models a single square-profile dive with no residual nitrogen from previous dives and no altitude adjustment, so the limits shown are too long for any repetitive dive. Dive conservatively, stay well inside your certification limits, follow your computer in the water, ascend slowly, and make a safety stop._ --- _Source: [Do The Calculation](https://dothecalculation.com/calculators/scuba-diving-no-decompression-limit-calculator). Quote freely with attribution and a link to this page._