SettingsSettings for this calculationUS
The floor area of the space being dried.
For connected open spaces, use the total — air and moisture move freely between them.
Average ceiling height — area × height is the air volume that holds the water.
For sloped ceilings, use the average of the low and high sides. Stairwells open to the space count too — moisture does not stop at the landing.
Air temperature in the space. Warmer air holds more water at the same relative humidity.
This is why a cool basement at 70% RH holds LESS water than a warm room at 60% — relative humidity alone does not say how much water is there.
Tools needed: Thermometer or hygrometer
What the hygrometer reads now, in %.
Give the meter ten minutes to settle at room level, away from the dehumidifier's own outlet — reading the dry exhaust stream is the classic way to declare victory early.
Tools needed: Hygrometer
Where you want to hold the space — 50–55% is the usual comfort and mould-control band.
Below about 50% costs energy for little gain in an ordinary home; above 60% sustained is where mould risk climbs. Museums and instrument rooms have their own, tighter bands.
The litres-per-day figure on the unit's plate or box.
Note WHICH test standard it was measured under: the same hardware prints roughly half the number under the newer US DOE 65°F test as under the older 80°F one. European ratings usually quote 30°C/80% RH, the most flattering condition of all.
How much of the rated capacity your room's conditions actually deliver.
Extraction falls as the room gets cooler and drier than the test chamber. 70% is a placeholder for an ordinary heated room; check your unit's capacity table for its figure at your temperature and humidity — for a cool basement it can be under half. This site will not invent a curve for hardware it has never seen.
Water to remove from the air
0.0666 gal
This is the water in the AIR alone. Wet walls, wet contents and the continuous load from infiltration and occupants add far more over a day than the air itself holds — the pull-down being quick does not mean the job is small.
- Room air volume
- 127.41 yd³
- Water in the air now
- 0.31 gal
- Water in the air at the target
- 0.24 gal
- Pull-down time at derated capacity
- 0.43 hours
- Effective extraction rate
- 3.7 gal/day
They open the calculator with your figures already in it
Dehumidifier Water Removal Calculator: 0.0666 gal — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- Magnus saturation-vapour-pressure approximation, e_s = 6.112·exp(17.62·T/(243.12+T)) hPa — WMO-recommended coefficients
- Vapour mass from the ideal gas law on the vapour's own partial pressure, m = e·V/(R_v·T), with R_v = 461.5 J/(kg·K) — so no fixed air density enters and the figure stays exact across the whole temperature range
- Humidity ratio W = 0.622·e/(P−e) at standard sea-level pressure — psychrometric relations per ASHRAE Handbook — Fundamentals
- Rated dehumidifier capacity is defined at a standard test condition; the US DOE test procedure (10 CFR 430) moved from 80°F/60% RH to 65°F/60% RH, roughly halving printed ratings for the same hardware — the unit's own documentation governs its real extraction
Inputs used
- Floor area
- 430 sq ft
- Ceiling height
- 8 ft
- Room temperature
- 68 °F
- Current relative humidity (%)
- 70
- Target relative humidity (%)
- 55
- Dehumidifier rated capacity (L/day)
- 20
- Real-conditions extraction (% of rated)
- 70
Intermediate steps
- Room air volume
- 127.41 yd³
- Water in the air now
- 0.31 gal
- Water in the air at the target
- 0.24 gal
- Pull-down time at derated capacity
- 0.43 hours
- Effective extraction rate
- 3.7 gal/day
Confidence note: This is the water in the AIR alone. Wet walls, wet contents and the continuous load from infiltration and occupants add far more over a day than the air itself holds — the pull-down being quick does not mean the job is small.
What this calculation does not cover
- Only the water suspended in the air is counted. Moisture soaked into plaster, timber, screed, carpet and stored contents is excluded, and so is the continuous load — air leaking in from outside, ground moisture through a slab or crawlspace, showers, cooking and occupants. That continuous load, not the air's own water content, is what fills the tank day after day and what sizes the machine.
- This is not a drying-out calculation. Reaching the target room humidity does not mean a wet subfloor, wall cavity or plaster is dry; only a moisture meter in the material tells you that, and water still held in the fabric will push the room back up as soon as the unit stops.
- The pull-down time assumes a closed space with nothing adding moisture while the unit runs and a single extraction rate held constant throughout. It does not model infiltration during the run, doors opening, extraction falling as the room dries and cools, coil defrost cycles, or the unit's own humidistat cutting out short of the target.
- One temperature and one humidity are applied to the whole volume. Real spaces stratify — cold corners, outside walls, the space behind furniture and under a floor sit colder and wetter than the meter in the middle of the room, and reach the target long after it does.
- Not a condensation or mould-risk assessment. Mould grows at the surface humidity against a cold wall, window reveal or uninsulated slab, which can stay high while the room air sits comfortably at 55%. Judging that needs the surface temperature and a dew-point check, not a room average.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-09-02 · in the site-wide review of 2026-09-06 · v1.0.1
Regulatory standards & verification citations4
- Magnus saturation-vapour-pressure approximation, e_s = 6.112·exp(17.62·T/(243.12+T)) hPa — WMO-recommended coefficients
- Vapour mass from the ideal gas law on the vapour's own partial pressure, m = e·V/(R_v·T), with R_v = 461.5 J/(kg·K) — so no fixed air density enters and the figure stays exact across the whole temperature range
- Humidity ratio W = 0.622·e/(P−e) at standard sea-level pressure — psychrometric relations per ASHRAE Handbook — Fundamentals
- Rated dehumidifier capacity is defined at a standard test condition; the US DOE test procedure (10 CFR 430) moved from 80°F/60% RH to 65°F/60% RH, roughly halving printed ratings for the same hardware — the unit's own documentation governs its real extraction
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