Short answer, in plain words. Almost all the water you give your plants comes back out of the leaves as water vapour. So the dehumidifier has to pull out about as much water as you pour in each day, minus what drains away. Measure it, turn it into pints, add a quarter for the cool, damp lights-off hours, and buy a unit rated at least that much. A 4 × 4 ft tent in flower often needs about 12–24 pints a day. US labels changed in 2019, so new labels show smaller numbers for the same machine. Supported (plant physiology and the DOE test rule; water use is an assumption you should measure).
Use the calculator: dehumidifier calculator.
Why does watering equal humidity?
Plants transpire most of the water their roots take up; only a small share is kept in new tissue [1]. In a grow room that water ends up in the air. So, in steady state:
Water to remove per day ≈ water given − runoff − water left in the medium + evaporation from the reservoir, trays and the medium surface.
For a quick estimate, use water given − runoff. Supported [1].
Plain words: the water goes in the pot, up the plant and out into the air. The dehumidifier catches it again.
What are the formulas and conversions?
- 1 L = 2.113 US pints = 0.264 US gal; 1 US gal = 3.785 L = 8 pints.
- 1 mm of water over 1 m² = 1 L [4]. Canopy water use in L/m²/day is the same as mm/day.
- Daily water (L/day) = water use (L/m²/day) × canopy area (m²). Area in m² = ft² × 0.0929.
- Lights-on rate (L/h) = daily water × share used in lights-on ÷ lights-on hours.
- Rated capacity to buy (pints/day) ≈ daily water in pints × 1.25 (headroom for lights-off and peaks). Grower practice.
- Heat the dehumidifier puts back in the room: its electrical watts × 3.412 BTU/h, plus about 2,320 BTU per litre of water condensed (1,050 BTU/lb) [7]. See AC sizing.
How much water does a grow room transpire?
Measure yours: log water in and runoff out for a week. For planning, indoor cannabis in flower often uses about 3–6 L per m² of canopy per day (0.3–0.55 L per ft²), higher under strong light and dry air, lower in early veg. This range is an assumption from grower logs, not a cannabis study; use your own numbers as soon as you have them.
Night transpiration is real but small for most plants: stomata don't close completely in the dark [5]. For sizing we assume about 90% of the day's water is transpired in a 12-hour lights-on period (an assumption; check with your own RH logs).
How big a dehumidifier for each size?
Flower, 12/12, 3–6 L/m²/day.
| Space | Canopy | Water per day | Pints per day | Lights-on rate | Rated capacity to buy (2019 DOE pints/day) |
|---|---|---|---|---|---|
| 2 × 4 ft tent | 8 ft² (0.74 m²) | 2.2–4.5 L (0.6–1.2 gal) | 5–9 | 0.17–0.34 L/h | ~6–12 (the smallest compressor units are bigger than this) |
| 4 × 4 ft tent | 16 ft² (1.49 m²) | 4.5–8.9 L (1.2–2.4 gal) | 9–19 | 0.34–0.68 L/h | ~12–24 |
| 5 × 5 ft tent | 25 ft² (2.32 m²) | 7.0–13.9 L (1.8–3.7 gal) | 15–29 | 0.53–1.06 L/h | ~19–37 |
| 4 × 8 ft tent | 32 ft² (2.97 m²) | 8.9–17.8 L (2.4–4.7 gal) | 19–38 | 0.68–1.35 L/h | ~24–47 |
| 10 × 10 ft room | 100 ft² (9.29 m²) | 28–56 L (7.4–14.7 gal) | 59–118 | 2.1–4.2 L/h | ~74–147 |
3–6 L/m²/day is a planning assumption; measure your own water in minus runoff.
Source: Size table above; 1 mm = 1 L/m² [4].
Show the numbers
| Space | 3–6 L/m²/day (L/day) |
|---|---|
| 2 × 4 tent | 2.2–4.5 |
| 4 × 4 tent | 4.5–8.9 |
| 5 × 5 tent | 7.0–13.9 |
| 4 × 8 tent | 8.9–17.8 |
| 10 × 10 room | 27.9–55.7 |
- 5.9 L/dayWater to remove
- 12.6 pintsPer day
- ~16 pintsRated capacity to buy (+25%)
- 0.44 L/hLights-on rate
Worked example, 4 × 4 at 4 L/m²/day: 4 × 1.49 = 5.9 L/day = 12.6 pints. Lights-on rate: 5.9 × 0.9 ÷ 12 = 0.44 L/h. Buy: 12.6 × 1.25 ≈ 16 pints/day rated, so a 20-pint (2019 rating) unit.
In a tent, the dehumidifier usually sits in the room, not the tent, and the tent's exhaust carries moist air to it. If the room air is drier than the tent target, extra exhaust alone may do the job (see ventilation).
What changed with the 2019 pint ratings?
Since 7 June 2019, US dehumidifiers must be rated with a DOE test at 65 °F (18.3 °C) and 60% RH, instead of 80 °F (26.7 °C) and 60% RH [2][3]. Cooler air holds less water, so the same machine removes less in the test and the label shows fewer pints. A unit once sold as "70 pints" is often labelled about 50 pints now [2]. The machine didn't change; the test did.
What that means in a grow room. Capacity depends mostly on the air's moisture content (its dew point) [6]:
| Conditions | Dew point (approx.) |
|---|---|
| Old test: 80 °F, 60% RH | ~65 °F (18 °C) |
| Mid flower, lights on: 77 °F (25 °C), 55% RH | ~60 °F (15.5 °C) |
| New test: 65 °F, 60% RH | ~51 °F (10.5 °C) |
| Late flower, lights off: 70 °F (21 °C), 45% RH | ~48 °F (9 °C) |
So in mid-flower lights-on, a unit removes more than its new label; in late flower at night it can remove less than its new label. Size from the new (2019) rating with the 25% headroom, and don't assume the old, bigger numbers. Supported (test rule and psychrometrics); the exact capacity curve is model-specific, so check the maker's performance chart.
Why does humidity spike at lights-off?
At lights-off the air cools, the AC often stops, and cooler air holds less water, so the same moisture shows as a higher RH [6]. Plants keep transpiring a little [5]. In late flower, that spike is when bud rot gets its chance (see botrytis).
What to do:
- Keep the dehumidifier running through lights-off; don't put it on the lights timer.
- Keep the night drop small in late flower: a few degrees, not 10 °C.
- Keep air moving through the canopy all night.
- Size for night conditions as well as daytime.
- Water early in the lights-on period, not right before lights-off. Grower practice.
Common mistakes
- Buying by room size instead of water use. The plants set the load, not the floor area alone.
- Comparing an old 70-pint label with a new 50-pint label as if they're different machines [2].
- Putting the dehumidifier on the light timer, so it's off during the lights-off spike.
- Forgetting the dehumidifier's heat when sizing the AC [7].
- Ignoring open reservoirs, wet floors and trays, which add evaporation on top of transpiration.
- Draining into a bucket and letting it fill; run a continuous drain or a condensate pump.
