Start with the water you already know about
A plant is a wick. Almost all the water it takes up leaves through the leaves, so the moisture load in a sealed room is simply the water you poured in. There are 8 pints to the gallon, and that's the whole calculation:
Feed 2 gallons a day and the room gains about 16 pints of water a day. Add the usual 25% for peak transpiration and lights-off and you get 20 pints/day — which is where the common advice that a 4×4 tent needs "20 to 25 pints" comes from. It's a good number and it's genuinely useful.
It's also, for most people, the smaller half of the problem. It assumes the only water moving is the water you poured, and that is only true if nothing is breathing in or out of the room.
Your exhaust fan is the biggest dehumidifier you own
Every cubic foot of tent air your fan pushes out is replaced by a cubic foot of outside air, carrying whatever moisture it happens to hold. To compare the two you have to stop thinking in relative humidity and start thinking in absolute humidity — grams of water per cubic meter — because 60% RH at 80 °F and 60% RH outside at 50 °F are not remotely the same amount of water.
A tent at 80 °F and 60% RH carries 15.2 g/m³. At 200 CFM — an ordinary inline fan on a small tent — you are exchanging roughly 288 thousand cubic feet of air a day. The result is not a footnote:
| Outside air | Its moisture | The fan removes | Plants add | Left for the dehumidifier |
|---|---|---|---|---|
| Cool, dry (spring night, or a heated house in winter) 70 °F / 50% RH | 9.2 g/m³ | 102 pints/day | 16 pints/day | 0 pints/day — none needed |
| Mild (a temperate day) 72 °F / 60% RH | 11.8 g/m³ | 58 pints/day | 16 pints/day | 0 pints/day — none needed |
| Warm and humid (summer, or a basement in July) 85 °F / 70% RH | 20.6 g/m³ | adds 94 pints/day | 16 pints/day | 110 pints/day |
Computed at 200 CFM with a 80 °F / 60% RH tent feeding 2 gal/day, from absolute humidity at each condition. The dehumidifier column has the 25% margin left off so the raw balance is visible.
Vented into cool dry air, the fan removes about 102 pints a day against the 16 the plants are adding. It is not helping your dehumidifier; it has replaced it. Buying a unit for that room is buying a machine to do a job already done — which is why growers with good ventilation in a dry climate report that their dehumidifier "never fills up."
Run the identical tent in July, venting into 85 °F air at 70% RH, and the sign flips. That air holds 20.6 g/m³ — more than the tent does — so the fan now imports about 94 pints a day. The requirement goes from 16 pints/day to 110, for the same plants drinking the same water.
That is the honest headline of this page: in humid weather no portable dehumidifier will win against an open exhaust. You are removing water and your fan is bringing it back faster. The fix is to seal and recirculate, condition the intake, or accept the humidity and manage for mold — not to buy a bigger machine. Nearly every sizing chart online will happily sell you the bigger machine.
The pint number on the box moved in 2019
Now the part that makes the shopping confusing. A dehumidifier's pint rating is a measurement taken at one test condition, not a property of the machine — and the test condition changed. Until 2019 the industry tested at 80 °F and 60% RH. The current Department of Energy procedure tests at 65 °F and 60% RH, on the reasoning that dehumidifiers mostly live in basements.
Cooler air holds less water, so every machine's number fell. The equivalence the industry published: a unit sold as 70-pint before 2020 is labeled 50-pint today. Nothing about the hardware changed.
| Standard | Test condition | Same machine is called | What it is |
|---|---|---|---|
| Pre-2020 (AHAM / old DOE) | 80 °F / 60% RH | 70 pints/day | The scale almost every sizing chart still on the internet was written against. |
| Current DOE (2019 on) | 65 °F / 60% RH | 50 pints/day | The scale on the box you buy today. Same machine, smaller number. |
The two scales differ by 1.40×. Every sizing chart written before 2020 — and a great many written since, copied from older ones — is quoting the left-hand column at you while the shop is selling you the right-hand one.
So where does your room sit? The honest way to ask is to compare the moisture your air actually carries against each test point:
| Your room | Moisture in the air | vs the old 80 °F label | vs the current 65 °F label |
|---|---|---|---|
| Warm veg tent 80 °F / 60% RH | 15.2 g/m³ | 1.00× | 1.61× |
| Late-flower room 78 °F / 50% RH | 11.9 g/m³ | 0.78× | 1.26× |
| Lights-off, cooling down 68 °F / 65% RH | 11.2 g/m³ | 0.74× | 1.20× |
A figure above 1.00 means there is more water available than at that test condition, so a unit will tend to beat its label; below 1.00 it will fall short of it.
This is worth being careful about, because the most-quoted advice on the subject gets the direction wrong. You will read that the rating "overstates real-world capacity by 30 to 40% at flower-room temperatures". Against an old 80 °F label that is roughly the right idea but too pessimistic — a 78 °F room at 50% RH comes out at 0.78×, so the overstatement is about 22%, not 30 to 40. Against a current DOE label it is simply backwards: the same room reads 1.26×, because 78 °F is much warmer than the 65 °F the machine was rated at. The label understates.
Comparing absolute humidity is a first-order guide, not a manufacturer curve. Real capacity depends on how cold the coil runs and how far the air is above its dew point, not purely on how much water the air is carrying. You can see the gap in our own numbers: the moisture available at the two test conditions differs by 1.61×, while the industry's own relabeling was 1.40×. Same direction, not the same size.
Use the ratio to know which way you are wrong and roughly how much — not to shave a purchase to two decimal places.
Size it for lights-off, not for lights-on
The worst moment for humidity is lights-off. Transpiration carries on for a while after the lamps cut, the air cools, and cooler air holds less — so RH climbs steeply even though nobody added any water.
Your dehumidifier gets weaker at the same moment, for the same reason. Look again at the table above: the warm lit tent runs at 1.61× its DOE label, and the cooling dark one at 1.20× — you lose about 26% of your machine's output at precisely the hour the load peaks. It doesn't fall all the way to the label, because even a cool tent is far wetter than the 65 °F basement the test imagines. But the squeeze is real and it happens every night.
So size from the dark period and the lit period looks after itself. Size from the warm, well-lit room and you'll be short at 2 a.m., which is when mold decides things. It cuts the other way too: air that is too humid stops transpiration altogether, and a plant that isn't transpiring can't move calcium to its growing tips — which is why tipburn gets diagnosed as a nutrient problem when it is a humidity one. The deficiency guide has that argument in full.
For the sealed 80 °F tent above, needing 20 pints/day: at tent temperature a DOE-labeled unit does better than its number, so the arithmetic says a 12-pint label would do. Buy the 20-pint anyway. The margin costs a few dollars and buys you the dark period, the summer week, and the day you add two more plants.
What it costs to run, and why that settles the argument
A dehumidifier is not an occasional appliance. In a room that genuinely needs one it runs more or less continuously, and a portable unit in the 50-pint class draws somewhere around 500 W. That is 12 kWh a day — about $58 a month at $0.16/kWh, every month, running against your exhaust fan.
The fan it is arguing with draws about 30 W: roughly $3.46 a month. Turning the fan down a step costs nothing and removes load; the machine that compensates for leaving it up costs 17 times as much to run as the fan does. In a humid climate you can end up paying $58 a month to remove water your own fan is pulling in — which is the most expensive way there is to be wrong about this, and it doesn't show up anywhere on the sizing charts.
At 500 W running 24 h at $0.16/kWh; substitute your own rate and nameplate wattage, both of which are on the unit. Larger and ducted units draw more.
When the room is too dry instead
The same balance runs backwards. Seedlings and clones want humid air and have almost no roots to pull water with, so early in a grow the plants are adding nearly nothing and any ventilation at all is stripping the tent. That's why a humidifier is a seedling-stage purchase and a dehumidifier is a late-flower one, and why people who buy both in month one feel they've been sold a pair of machines that fight each other. They have.
Before buying either, turn the fan down. Ventilation is the largest term in the balance in both directions, it costs nothing to change, and a speed controller is cheaper than either appliance. Reach for a machine when the fan is already as low as your temperature and CO2 will tolerate — and if temperature is the thing stopping you from turning it down, that's a lighting problem wearing a humidity costume. The light calculator prints the heat load your fixture is putting into the tent, which is what the fan is really there to remove.
And read the result as a VPD, not as an RH number — the VPD calculator converts your temperature and humidity into what the plant actually experiences, which is the thing you're trying to control. Humidity is the lever; VPD is the reading.
What to actually do
- Work out your water: gallons fed × 8. For most tents this lands between 8 and 40 pints a day, and it is the floor, not the answer.
- Decide whether you're sealed or vented. If you vent, look up your outdoor dew point rather than the RH — that is the number that tells you whether the fan is helping or hurting. Cool, dry intake: you may need no dehumidifier at all. Warm, humid intake: seal the room, because you cannot out-buy an open door.
- Check which scale the chart you're reading is on. If it doesn't say, assume pre-2020 and divide by about 1.4 to get a modern label — or ignore charts and use your own water number.
- Size for the dark period. Take your requirement as the label, don't discount it for tent warmth, and buy the next size up if you're between two.
- Put it where the air moves and give it a drain hose. A unit filling a tank in a tent you check twice a day is a unit that spends half its life switched off and full.
Sources: the transpiration-equals-water-fed relationship and the 8-pints-per-gallon rule are standard grow-room practice and are widely published by suppliers; we compute rather than quote the rest. Absolute humidity is the standard psychrometric form, from the same saturation-vapor-pressure curve as the VPD calculator (Tetens as used in FAO-56). Test conditions and the 70-to-50 pint relabeling are the Department of Energy dehumidifier test procedure that took effect in 2019, as summarized by ENERGY STAR and by manufacturers explaining the change to customers. Ventilation figures are ours, computed from the absolute-humidity difference across the exchange rate; treat the capacity ratios as first-order guidance, for the reason given in the box above.