# Terrarium Humidity & Ventilation Calculator Turn enclosure dimensions and vent areas into airflow, air changes, and the water you have to replace each day. --- - **Canonical URL:** https://dothecalculation.com/calculators/terrarium-humidity-ventilation-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 --- ## Work Out How Much Water Your Enclosure Loses Every Day Turn enclosure dimensions, vent areas and target conditions into airflow, air changes, and the millilitres of water you have to replace to hold your humidity target. - Airflow from the standard stack-effect formula, not a rule of thumb - Water loss in ml per day, compared against your actual misting routine - Vapour pressure deficit for the planted side of the hobby ## Quick Answer — How Much Ventilation Does a Terrarium Need? There is no published standard for terrarium ventilation, and anyone quoting one is guessing. What can be calculated exactly is the consequence: how fast air moves through the enclosure, and how much water that air carries out. Once you know the water loss in millilitres per day, the ventilation question answers itself — you need enough air exchange to keep the air fresh and little enough that you can replace what evaporates. Airflow comes from the stack effect: warm air inside rises and leaves through the upper vent, drawing room air in through the lower one. Three things control it — the **smaller** of the two vent areas, the vertical distance between them, and the temperature difference. **Three enclosures at their default settings:** • **45 × 45 × 60 cm, 20 cm² vents, 40 cm apart, 24 °C at 85% over a 21 °C room at 45%** — 12.5 air changes per hour, one complete change every 4.8 minutes, **324 ml of water lost per day** • **30 × 30 × 45 cm, 6 cm² vents, 30 cm apart, 23 °C at 90% over a 20 °C room at 50%** — one change every 6.1 minutes, **81 ml per day** • **90 × 45 × 45 cm, 120 cm² vents, 30 cm apart, 28 °C at 60% over a 21 °C room at 40%** — one change every 0.94 minutes, **2,238 ml per day** That last row is the lesson. A large, warm, heavily vented enclosure loses well over two litres of water a day, which is why screen-top setups need a fogger or an automated misting system rather than a spray bottle. ## How to Use This Calculator: A 45 × 45 × 60 cm Planted Vivarium Enter the internal dimensions, then the substrate depth — here 8 cm. The enclosure is **121.5 litres** gross, but the substrate takes up 16.2 litres, so only **105 litres** of it is air. That air volume is what turns over, and it is what the air-change figure is measured against. Enter the vent free areas next. Free area is the open area, not the panel size: a 40 cm² opening covered in mesh with a 50% open weave gives 20 cm² of free area. With 20 cm² top and bottom, 40 cm apart, at 24 °C inside and 21 °C in the room, airflow is **21.96 litres per minute** — **12.5 air changes per hour**, or one complete change every **4.8 minutes**. Now the humidity side. At 24 °C and 85% relative humidity the air inside holds **18.46 grams of water per cubic metre**. The room, at 21 °C and 45%, holds **8.23 g/m³**. Every cubic metre that leaves takes the **10.23 g/m³** difference with it, which works out to **324 ml of water a day**. Finally, enter your misting routine. Two sessions of 250 ml is 500 ml a day, comfortably above the 324 ml the enclosure loses, so the calculator reports the target holds with 176 ml to spare. If it did not, you would know before the animals or the plants told you. ## The Formulas This Calculator Uses **Saturation vapour pressure**, in hectopascals, from the Alduchov-Eskridge refinement of the Magnus formula: \(e_s = 6.1094\,e^{\frac{17.625T}{T + 243.04}}\). It is accurate to well under half a percent across every temperature an enclosure will ever see. **Absolute humidity**, in grams of water per cubic metre: \(AH = \frac{216.7 \times \frac{RH}{100} \times e_s}{T + 273.15}\). At 20 °C and 100% humidity this returns 17.25 g/m³, matching the standard reference figure. **Stack-effect airflow**, in cubic metres per second: \(Q = C_d \times A \times \sqrt{\frac{2gh\,\Delta T}{T_{inside}}}\), with \(C_d = 0.65\) for sharp-edged openings, \(A\) the **smaller** of the two free areas, \(h\) the vertical separation in metres, and temperatures in kelvin. **Air changes per hour** is \(\frac{Q \times 3600}{\text{air volume}}\), and **water loss** is \(Q \times 3600 \times (AH_{inside} - AH_{room})\) grams per hour. **Vapour pressure deficit**, in kilopascals, is \(\frac{e_s \times (1 - RH/100)}{10}\) — the number growers work to rather than relative humidity, because it describes how hard the air is pulling water out of a leaf. ## A Second Example: Why Vent Height Beats Vent Area Take the same 45 × 45 × 60 cm enclosure and change one thing: move the upper vent from 40 cm above the lower one down to 20 cm, leaving both vent areas at 20 cm². Airflow falls by a factor of **1.414** — exactly the square root of two, because flow scales with the square root of the separation. Halving the height between the vents cuts turnover by about 29%, and doubling it raises turnover by about 41%, with no change in vent area whatsoever. This is the most useful thing the model tells you, and it is counter-intuitive. Builders reach for bigger vents when an enclosure feels stuffy, but on a tall enclosure the cheaper fix is often to move the existing vents further apart. Conversely, an enclosure whose vents sit at nearly the same height barely ventilates at all no matter how large they are, because there is no buoyancy pressure to drive flow. The other asymmetry worth knowing: only the **smaller** vent counts. Doubling the top vent while leaving a small bottom vent unchanged does nothing, because the restriction governs the flow. Vents should be sized in matched pairs. ## Reading the Numbers Against Real Enclosures Air changes per hour is a building-services unit, and a 100-litre box turns its air over far faster than any room ever does. Twelve air changes an hour sounds enormous next to a house, and in an enclosure it is unremarkable. That is why this calculator reports **minutes per complete air change** alongside it — a number you can picture. The bands attached to that figure are this tool's own labels, not a published standard. Under a minute per change behaves like a screen top and will not hold humidity without continuous input. One to five minutes needs frequent misting or a fogger. Five to twenty minutes is where most planted tropical vivariums sit. Twenty to sixty minutes holds humidity easily but is worth watching for stagnation and mould. Beyond an hour, the enclosure is effectively sealed. Vapour pressure deficit is the input to watch on the planted side. Most tropical foliage is comfortable somewhere between roughly 0.4 and 0.8 kPa. The default enclosure sits at 0.45 kPa; the warm 28 °C example at 60% humidity sits at 1.51 kPa, which is a demanding environment for soft-leaved plants even though 60% humidity does not sound low. Substrate matters more than most builders expect, and in two directions. It displaces air, which raises the air-change rate for a given airflow, and a deep damp substrate is also a large evaporative reservoir that buffers humidity between mistings. This model counts the first effect and not the second, so a well-planted enclosure with deep damp substrate will hold humidity somewhat better than the bare numbers suggest. ## Limitations This is a steady-state model of buoyancy-driven flow, and a real enclosure is neither steady nor purely buoyancy-driven. A misting cycle spikes humidity and then decays; a heat lamp switching on changes the temperature difference and therefore the airflow; a fan, a room draught, or an open door all add flow the stack-effect term knows nothing about. The discharge coefficient of 0.65 is the standard figure for a sharp-edged opening. Fine mesh, foam, and filter media all restrict flow further, sometimes substantially, so an enclosure with heavily filtered vents will exchange less air than the model predicts. Treat the free area you enter as an upper bound. Water loss counts only what leaves through the vents. It does not model absorption into substrate and wood, uptake and transpiration by plants, drinking, or a water feature, all of which move water around inside the enclosure. The misting balance is therefore a guide to whether your routine is in the right region, not a precise water budget. Nothing here is a husbandry recommendation. Target temperature and humidity for a particular species come from that species, from a keeper who has kept it, and from a veterinarian — not from a calculator. This tool takes your target as given and tells you what it costs in air exchange and water. ## Related Calculators Enclosed-habitat keeping runs into the same closed-system arithmetic whether the medium is air or water. The [aquarium volume calculator](/calculators/aquarium-volume-calculator) turns tank dimensions into litres and gallons the same way this one turns enclosure dimensions into air volume, and the [aquarium bioload and stocking calculator](/calculators/aquarium-bioload-stocking-calculator) answers the other half of the question — how much life a fixed volume can support. For dissolved gas rather than airborne water, the [aquarium CO2 calculator](/calculators/aquarium-co2-calculator) works through concentration and injection rate in a planted tank. ## Frequently asked questions ### How much ventilation does a terrarium need? There is no published standard, so the useful question is what the ventilation costs you in water. A typical planted tropical vivarium turns its air over every five to twenty minutes; a screen-topped enclosure turns over in under a minute and will not hold humidity without a fogger or automated misting. Calculate the daily water loss and check it against what you can actually replace. ### Why does moving the vents further apart help more than making them bigger? Airflow scales with the square root of the vertical separation but directly with the smaller vent area. Doubling the separation raises turnover by about 41 percent; doubling only one vent raises it by nothing at all, because the smaller opening governs the flow. Vents should be sized in matched pairs and placed as far apart vertically as the build allows. ### How much water does a terrarium lose per day? It depends on size, ventilation and the humidity gap. The default 45 by 45 by 60 cm enclosure at 24 degrees and 85 percent over a 21 degree room at 45 percent loses about 324 ml a day. A 90 by 45 by 45 cm enclosure with large vents at 28 degrees loses about 2,238 ml a day — nearly seven times as much. ### What is absolute humidity and why does the calculator use it? Absolute humidity is the actual mass of water in a cubic metre of air, in grams. Relative humidity cannot be compared between two different temperatures, but absolute humidity can, and the difference between inside and room air is exactly what drives evaporation out of the vents. It is the only way to turn a humidity target into millilitres. ### What is a good vapour pressure deficit for terrarium plants? Most tropical foliage is comfortable roughly between 0.4 and 0.8 kPa. The default enclosure here sits at 0.45 kPa. A reading well above 0.8 means the air is pulling water out of leaves faster than the plants like, which can happen at a relative humidity that looks perfectly reasonable if the enclosure is running warm. ### Does the substrate affect the calculation? Yes, in one direction. It displaces air, so an enclosure with 8 cm of substrate has less air to turn over and a higher air-change rate for the same flow. The calculator counts that. It does not count the substrate acting as a damp reservoir that buffers humidity between mistings, so a well-planted setup with deep substrate holds humidity a little better than the numbers suggest. ### Can I use this for a sealed bioactive terrarium? Partly. With no vents and no temperature difference the model floors airflow at a small diffusion term rather than reporting zero, because a genuinely sealed box does not exist. For a closed jar terrarium the water loss figure stops being meaningful — the water cycles internally rather than leaving — though the vapour pressure deficit reading is still useful for the plants. ### Why does my enclosure feel stuffy even though the numbers look fine? Air changes measure how much air enters and leaves, not how well it mixes inside. A large enclosure with vents at one end can turn over on paper while a corner stays stagnant, which is where mould appears. A small circulation fan fixes mixing without adding exchange, and it does not appear in this model because it moves air within the enclosure rather than through it. ## Related concepts - **Stack Effect** — Buoyancy-driven airflow: warm air rises out of the upper vent and pulls room air in through the lower one. It is what ventilates an enclosure with no fan. - **Absolute Humidity** — The mass of water vapour per cubic metre of air, in grams. Unlike relative humidity it can be compared across temperatures, which is what makes water loss calculable. - **Vapour Pressure Deficit** — How hard the air pulls moisture out of a leaf, in kilopascals. Most tropical foliage is comfortable between roughly 0.4 and 0.8 kPa. ## Related guides - [Aquarium Volume Guide: Gallons, Liters, and Usable Water Capacity](https://dothecalculation.com/blog/hobby/aquarium-volume-guide) — Calculate rectangular aquarium capacity from dimensions, convert between gallons and liters, and adjust gross volume by fill percentage. - [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. ## Related calculators - [Home Brewing Water Chemistry Calculator](https://dothecalculation.com/calculators/home-brewing-water-chemistry-calculator) — Turn source water, RO dilution and salt additions into a full ion profile, sulfate-chloride ratio and residual alkalinity. - [Wood Shrinkage & Seasonal Movement Calculator](https://dothecalculation.com/calculators/wood-shrinkage-radial-calculator) — Model exact wood shrinkage and seasonal dimensional changes based on relative humidity, temperature, and wood species type. - [Aquarium Volume Calculator](https://dothecalculation.com/calculators/aquarium-volume-calculator) — Calculate your exact fish tank capacity in gallons and liters based on length, width, and height for accurate water volume. - [Camping Pack Weight Calculator](https://dothecalculation.com/calculators/camping-pack-weight-calculator) — Calculate your total pack weight from base gear, food, water, and fuel, and compare it to body-weight guidelines. - [Coffee-to-Water Brew Ratio Calculator](https://dothecalculation.com/calculators/coffee-brew-ratio-calculator) — Calculate the exact coffee-to-water ratio for pour-over, French press, drip, or cold brew, with gram and tablespoon results. - [Epoxy Resin Mixing Calculator](https://dothecalculation.com/calculators/epoxy-resin-calculator) — Calculate exact resin and hardener volume and weight for tabletop pours, coasters, and river tables from your mold dimensions and mix ratio. --- _The physics here is standard and named: the Alduchov-Eskridge form of the Magnus equation for vapour pressure, and the conventional stack-effect expression for buoyancy-driven flow through two openings. What is not standard is any target for terrarium ventilation, because none is published — the bands attached to the air-change figure are this tool's own descriptive labels. The model is steady-state and counts only air leaving through the vents, so it ignores misting cycles, fans, room draughts, and water absorbed by substrate and plants. Target temperature and humidity for a species should come from that species, from experienced keepers, and from a veterinarian, not from a calculator._ --- _Source: [Do The Calculation](https://dothecalculation.com/calculators/terrarium-humidity-ventilation-calculator). Quote freely with attribution and a link to this page._