What a dehumidifier is actually sized against
A dehumidifier is sized against a moisture balance, not against floor area. Water enters a space from three places — humid outdoor air leaking in, people and activities inside, and evaporation from wet ground or fresh concrete — and the machine has to remove all of it, every day, to hold the space at your target humidity.
Square-footage charts on retail packaging skip that balance entirely. They cannot know whether your basement leaks 20 CFM or 200, whether the crawl space floor is covered with 10-mil poly or bare clay, or whether your summers are 85 °F at 70% RH or 95 °F at 30%. Two 1,200 ft² basements in the same city can differ by a factor of four in real moisture load.
The quantity that makes the arithmetic work is humidity ratio — pounds of water vapour per pound of dry air, usually quoted in grains (7,000 grains to the pound). Relative humidity on its own tells you nothing about moisture transfer, because 70% RH at 85 °F carries roughly twice the water of 70% RH at 60 °F. If you want to see that conversion on its own, the humidity ratio calculator and the dew point calculator both work from the same psychrometric relations used here.
The formula and why it has that shape
Start with the air. Infiltration airflow is the air change rate multiplied by the volume of the space, divided by 60 to get cubic feet per minute:
CFM = ACH × area × height ÷ 60
Every cubic foot of that air carries about 0.075 lb of dry air, so 60 × 0.075 = 4.5 lb of dry air pass through per hour for each CFM. Multiply by the humidity ratio difference and you have pounds of water per hour:
lb/h = 4.5 × CFM × (W_out − W_target)
That is the same 4.5 constant that appears in the latent heat equation Q = 4,840 × CFM × ΔW, where 4,840 is 4.5 multiplied by the latent heat of vaporisation. Multiply the hourly figure by 24 for a daily load, add the internal sources, and divide by 1.0432 lb — the weight of a US liquid pint of water — to land in the units the appliance industry uses.
Occupants are handled the same way. ASHRAE lists roughly 200 BTU/h of latent gain for an adult at seated, very light activity. Divide by the 1,060 BTU/lb latent heat used here and each person contributes about 0.19 lb of water per hour, or 4.5 lb a day.
The one term you cannot calculate is evaporation from a bare earth crawl space floor, because it depends on soil moisture, ground temperature and how much of the floor is covered. That belongs in the other moisture sources field, measured rather than guessed: run a unit with a known pint counter for a week and read the number off it.
Worked example: a 1,200 ft² basement in a humid summer
Take a 1,200 ft² basement with 8 ft ceilings, leaking at 0.35 air changes per hour. Outdoors is 85 °F at 70% RH; you want 75 °F and 50% RH inside; two people are usually down there.
- Volume. 1,200 × 8 = 9,600 ft³.
- Infiltration airflow. 0.35 × 9,600 ÷ 60 = 56 CFM.
- Outdoor humidity ratio. At 85 °F the saturation pressure is 0.5964 psia, so pv = 0.70 × 0.5964 = 0.4175 psia and W = 0.621945 × 0.4175 ÷ (14.696 − 0.4175) = 0.01818 lb/lb, or 127.3 gr/lb.
- Target humidity ratio. At 75 °F the saturation pressure is 0.4302 psia, pv = 0.2151 psia, W = 0.00924 lb/lb, or 64.7 gr/lb.
- Difference. 127.3 − 64.7 = 62.6 gr/lb, which is 0.008946 lb/lb.
- Infiltration moisture. 4.5 × 56 × 0.008946 = 2.254 lb/h, and ×24 = 54.1 lb/day.
- Occupants. 2 × 200 ÷ 1,060 × 24 = 9.06 lb/day.
- Total. 54.1 + 9.06 = 63.2 lb/day ÷ 1.0432 = 60.5 pints per day.
- Latent load. 63.2 ÷ 24 × 1,060 = 2,790 BTU/h — about a quarter of a ton of cooling that your air conditioner is not being asked to do.
- Nameplate size. 60.5 × 1.25 = 75.6, so the next common size is a 80-pint unit.
Every number above is reproducible on paper from the two saturation pressures, which you can read off any psychrometric chart.
How to read the result and pick a unit
The headline pints-per-day figure is the real removal duty at the conditions you entered. The nameplate number on a dehumidifier is not the same thing. Since 2019 the US Department of Energy has required residential dehumidifiers to be rated at 65 °F and 60% relative humidity under the ANSI/AHAM DH-1 test method. Older units carry a rating taken at 80 °F and 60% RH, which is why a "70-pint" machine from 2015 and a "50-pint" machine from 2020 can be the same physical box.
Capacity falls as the air gets cooler and drier, because both the moisture available at the coil and the temperature difference driving condensation shrink. That is what the sizing allowance covers. A unit working in a 75 °F, 55% RH basement is near its rating point and 1.15–1.25 is generous; one working in a 60 °F crawl space at 55% RH is well below it, and the honest move is to read the manufacturer's performance table at your actual conditions rather than to keep inflating a multiplier.
Also check the duty cycle. A unit sized exactly to the load runs continuously, which gives the best moisture removal per kilowatt-hour but leaves no margin for a wet week. A unit sized at twice the load short-cycles, removes less water per unit of energy, and swings the humidity. Landing between 1.2 and 1.5 times the calculated load is the usual compromise.
If the space is served by air conditioning, part of this load is already being removed by the cooling coil. The condensate rate calculator shows how much, and the sensible heat ratio calculator tells you whether the coil has any latent capacity left to give.
Moisture in air at common conditions
| Dry bulb | 30% RH | 50% RH | 60% RH | 70% RH | 80% RH |
|---|---|---|---|---|---|
| 55 °F | 19.1 | 31.9 | 38.4 | 44.8 | 51.3 |
| 60 °F | 22.9 | 38.3 | 46.1 | 53.8 | 61.6 |
| 65 °F | 27.3 | 45.8 | 55.0 | 64.4 | 73.7 |
| 70 °F | 32.5 | 54.5 | 65.6 | 76.7 | 87.9 |
| 75 °F | 38.6 | 64.7 | 77.8 | 91.1 | 104.4 |
| 80 °F | 45.6 | 76.5 | 92.1 | 107.8 | 123.7 |
| 85 °F | 53.7 | 90.2 | 108.7 | 127.3 | 146.1 |
| 90 °F | 63.0 | 106.1 | 127.9 | 150.0 | 172.2 |
Read across for the outdoor condition, read your indoor target, subtract, and multiply by 4.5 × CFM to get pounds of water per hour.
Mistakes that make a dehumidifier the wrong size
- Sizing from square feet. Area appears in this calculation only through volume, and volume only sets the infiltration airflow. A tight 2,000 ft² basement can be an easier job than a leaky 800 ft² one.
- Comparing a pre-2019 rating to a post-2019 rating. The DOE test point moved from 80 °F to 65 °F, which cut nameplate numbers by roughly a third for the same hardware. Check which rating point a listing quotes before comparing prices.
- Ignoring the ground. A bare crawl space floor can evaporate more water than every other source combined. A sealed vapour barrier is a cheaper fix than any machine, and it changes this calculation more than any input on the page.
- Setting an unreachable target. A refrigerant dehumidifier struggles below about 35% RH and frosts in air below roughly 60 °F unless it has hot-gas defrost. Desiccant units cover that range and are rated differently.
- Forgetting the sensible heat. A dehumidifier rejects both the latent heat it removes and its own compressor power into the room. Expect the space to run several degrees warmer than it did — welcome in a cool basement, unwelcome in a finished room.
- Draining into a bucket. An 80-pint duty is a 10-gallon bucket a day. Any unit sized from this calculation needs a gravity or condensate-pump drain.
Which standards this follows
The psychrometric relations — saturation pressure over liquid water and the humidity ratio from partial pressures — are those given in the ASHRAE Handbook—Fundamentals, Chapter 1. The occupant latent gain of about 200 BTU/h for seated, very light activity is from Chapter 18. Dehumidifier capacity ratings in the United States are established under ANSI/AHAM DH-1 and the DOE test procedure at 10 CFR Part 430, which since 2019 rate residential units at 65 °F and 60% relative humidity.
When a dehumidifier is the wrong tool
Dehumidification is the last step, not the first. Three fixes beat it on cost and permanence.
Stop the water. Grading, gutters, downspout extensions and a crawl space vapour barrier remove liquid water before it ever evaporates. No appliance competes with a 10-mil poly ground cover on a dirt floor.
Stop the air. Every input on this page scales linearly with the air change rate. Halving infiltration halves the infiltration term. Air-sealing the rim joist and the crawl space vents is usually a one-day job; the air changes per hour calculator converts a blower door result into the ACH figure this page needs.
Use the cooling you already have. An air conditioner is a dehumidifier with a bigger coil. If it runs long enough, it removes the latent load for free. Short-cycling on an oversized system is the most common reason a house is cool and clammy at the same time — the latent heat load calculator shows how much moisture the coil should be taking out.
Where dehumidification genuinely earns its place is in spaces with no cooling load to piggyback on: crawl spaces, unconditioned basements, wine rooms, indoor pools, and drying-out work after a flood. In those cases size from the balance above and specify a unit with a real drain, a humidistat you trust, and a performance table published at your operating temperature.
Key terms
- Humidity ratio
- Pounds of water vapour per pound of dry air, often quoted in grains (7,000 gr = 1 lb). It does not change when air is heated or cooled without condensation, which is why moisture calculations use it instead of relative humidity.
- Latent load
- The heat associated with changing water between vapour and liquid, about 1,060 BTU per pound at room conditions. Removing 60 pints a day is a latent load of roughly 2,800 BTU/h.
- AHAM rating point
- The standard test condition at which a dehumidifier's pints-per-day capacity is measured — 65 °F and 60% RH under the current DOE procedure, 80 °F and 60% RH before 2019.
- Air change per hour (ACH)
- The number of times the full volume of a space is replaced by outdoor air in an hour. Natural infiltration in a house typically runs a small fraction of one air change per hour.