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Dehumidifier Size Calculator

This dehumidifier size calculator works out the pints or litres per day to remove in a grow room, a drying room or a damp basement. Pick the space, then enter your watering, harvest or floor area.

Room
Units
I know
L

days

Watering every few days? The plants keep releasing that water in between, so the daily load is the amount divided by the days.

More options
%

Water that drains out of the pots.

L/kWh

Litres removed per kWh (“IEF”), on the US yellow EnergyGuide label or the spec sheet. Empty: we use ENERGY STAR’s minimum for that size.

%

Headroom for test-versus-room differences and uneven days.

L/day

From the box or the manual. Leave empty to skip the check.

Dehumidifier capacity needed

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Water released into the air
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Suggested unit
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Electricity it uses
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Notes

    Next: Work out what the dehumidifier costs to run

    How the size is worked out

    1. Water applied per day, minus the runoff that drains away.
    2. About 99% of what the plants take up is transpired; the rest becomes plant tissue.
    3. A safety margin on top, because real-world removal at grow-room temperatures is lower than the label rating.
    4. Rounded up to the next common size: 22, 35, 50, 70, 95, 120, 155, 205 or 250 pints per day.

    Most of that load arrives while the lights are on, but the hardest hour is just after lights-off, when the room cools and humidity jumps.

    Why dehumidifier ratings dropped in 2019

    US dehumidifiers used to be tested at 80 °F (27 °C) and 60% humidity. Since 2019 they are tested at 65 °F (18 °C), closer to a real basement. Cooler air gives up less water, so the same machine now carries a smaller pint rating. Nothing about the machine changed. ENERGY STAR’s guide to the change:

    Portable dehumidifier ratings before and after the 2019 test change (pints per day)
    Rated before 2019About the same machine now
    30 pints/day20 pints/day
    40 pints/day25 pints/day
    50 pints/day30 pints/day
    55 pints/day30–35 pints/day
    60 pints/day35 pints/day
    70 pints/day40–45 pints/day
    90 pints/day55 pints/day

    Actual changes vary by model. The sizes this calculator suggests are in today’s (2019 and later) pints.

    Drying rooms

    A drying harvest only loses water. Its dry matter is the wet weight × (1 − water content going in). When it reaches the water content you dry to, it weighs that dry matter ÷ (1 − target). The difference is the water your dehumidifier removes, here spread evenly over the drying days.

    Using the dehumidifier size calculator: start with the water

    A grow room and a basement get damp for different reasons, so the calculator sizes them differently. In a grow room the water comes from you. About 99% of what the plants take up goes back into the air through the leaves. So the daily load is close to the water you give, less the runoff that drains away. A drying room is the same idea: the harvest loses only water, and the calculator spreads it over the drying days.

    A home or basement has no watering to count. There the calculator uses ENERGY STAR's chart, by floor area and how damp the space is.

    What size dehumidifier for a 4×4 grow tent

    Tent size does not set the load. Watering does. Say you grow 4 plants in a 4×4 tent and give each 2 litres (about half a US gallon) a day:

    • Water in: 8 litres a day, with 10% runoff
    • Released into the air: 7.1 litres (15 pints) a day
    • With the 30% safety margin: 9.3 litres, or 20 pints, a day
    • Suggested unit: 22 pints a day (10 litres), the smallest common size
    • Electricity: about 4.2 kWh a day or less, at ENERGY STAR's minimum of 1.70 litres per kWh for that size

    Double the watering to 16 litres and the need rises to 39 pints a day, which points to a 50-pint unit.

    Dehumidifier size chart for basements and homes

    ENERGY STAR's sizing chart, in pints per day:

    • Moderately damp (50–75% humidity, musty in humid weather): 20–30 up to 2,000 sq ft (186 m²); 30 or more above that
    • Very damp (75–90%, musty smell, damp spots): 25–40; 40 or more above 2,000 sq ft
    • Wet (90–100%, wet floor or walls, seepage): 30–50; 50 or more above 2,000 sq ft

    Two things are easy to miss in that chart. First, the range does not grow with floor area below 2,000 sq ft, so a 500 sq ft basement and a 1,500 sq ft one get the same answer; dampness matters more. Second, the chart does not say whether its pints are the old rating or the one used since 2019. Today's ratings are lower for the same machine, so pick from the top of your range.

    Buying a unit after the 2019 rating change

    Compare like with like. The sizes this calculator suggests are in today's pints, tested at 65 °F (18 °C). A box or a second-hand listing may still quote the old 80 °F (27 °C) figure, which is larger: an old 50-pint unit is about a 30-pint unit now. If a rating looks generous for the price, check which test it uses.

    Then look at the energy factor on the label, in litres per kWh. A higher figure removes the same water for less electricity. Type it under More options to see the daily kWh, and take that to the electricity cost calculator. Enter your unit's rating in the last field to see whether it covers the need.

    Common sizing mistakes

    • Using the home chart for a grow room. ENERGY STAR's chart is built for homes and knows nothing about watering. Size a grow room or tent from the water you give.
    • Forgetting the heat. Every watt a dehumidifier draws becomes heat. Add it in the air conditioner calculator.
    • Ignoring lights-off. Humidity jumps as the room cools. The dew point calculator shows how close your surfaces get to condensing.
    • Drying the air too far. Check the targets on the VPD chart. If clones or seedlings need more moisture, use the humidifier size calculator.

    FAQ

    Questions growers ask

    What size dehumidifier do I need?

    For a grow room, roughly the water you give each day, less runoff, plus a 30% margin: 8 litres a day with 10% runoff needs about 20 pints a day, so a 22-pint unit. For a home, ENERGY STAR's chart goes by dampness, from 20 to 50 pints a day for spaces up to 2,000 sq ft.

    What size dehumidifier for a 4×4 grow tent?

    About 20 pints a day if 4 plants get 2 litres (about half a US gallon) each a day, so the smallest common size, 22 pints, covers it. The tent's size does not decide this; the watering does. Give twice the water and the need rises to about 39 pints a day, which calls for a 50-pint unit.

    What size dehumidifier for a basement?

    20–30 pints a day for a moderately damp basement up to 2,000 sq ft, 25–40 if very damp, and 30–50 if wet, with seepage or wet walls, according to ENERGY STAR. Above 2,000 sq ft it gives at least 30, 40 or 50. Choose from the top of the range, since today's ratings run lower.

    What size dehumidifier for 1,000 sq ft?

    Between 20 and 50 pints a day, depending on dampness: 20–30 if moderately damp, 25–40 if very damp and 30–50 if wet. ENERGY STAR's chart uses the same ranges for any space up to 2,000 sq ft, so dampness matters more than area. Pick the top of your range, as the chart does not name its rating test.

    What size dehumidifier for a crawl space?

    Use the same chart by floor area and dampness: up to 2,000 sq ft, 20–30 pints a day if moderately damp, 25–40 if very damp and 30–50 if wet. A crawl space with a wet floor, wet walls or seepage belongs in the wet row. Enter its floor area in the Home mode to see yours.

    Why do dehumidifier pint ratings look smaller since 2019?

    Because the US test got colder, not because the machines got weaker. Portable units were rated at 80 °F and are now rated at 65 °F, both at 60% humidity, and cooler air gives up less water. A unit sold as 50 pints before 2019 is about 30 pints today, and a 70 is about 40–45.

    Should the dehumidifier go inside the grow tent or outside?

    Inside, if the tent has little or no exhaust, so it removes the water where the plants release it. With strong exhaust, a unit drying the room the tent draws its air from also works. Either way, all the power it draws becomes heat where it stands, so count it in your cooling.

    Does a dehumidifier add heat?

    Yes. Every watt it draws ends up as heat in the room, so a 500 W unit adds about 1,706 BTU/h. Add its watts to your other equipment when you size cooling. Removing 7.1 litres a day at ENERGY STAR's minimum of 1.70 litres per kWh uses about 4.2 kWh, all of which becomes heat.

    Sources

    The formulas and target ranges on this page come from these references.

    1. Kochhar & Gujral (2020). Plant Physiology: Theory and Applications, Cambridge University Press, chapter 3 (about 99% of water taken up is lost as vapour). doi.org/10.1017/9781108486392
    2. ENERGY STAR: Dehumidifier testing and capacity (portable units are now tested at 65 °F rather than 80 °F, which reports a smaller capacity). www.energystar.gov/products/dehumidifier_testing_and_capacity
    3. ENERGY STAR: Dehumidifiers key product criteria, Version 6.0 (minimum Integrated Energy Factor, litres per kWh, by capacity). www.energystar.gov/products/dehumidifiers/key_efficiency_criteria
    4. ENERGY STAR: Dehumidifiers, sizing chart (pints per day by dampness, under or over 2,000 sq ft). www.energystar.gov/products/dehumidifiers
    5. NIST Handbook 44 (2026), Appendix C: units and conversion factors. www.nist.gov/document/2026-nist-handbook-44-appendix-c

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