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VPD Calculator

Free tool

Humidifier Size Calculator

This humidifier size calculator shows how much water a grow tent, grow room or whole home needs each day. In a tent, the fresh air your fan pulls in sets most of the load.

Space
Units
Common tent sizes

Fills in a typical height too; change it if your tent is taller or shorter.

m

m

m

°C

%

Where the fresh air comes from: the room around the tent, or outside. Not the reading inside.

Target
%

Fresh air
m³/h

What the fan actually moves after its filter: the rating times the share left (the exhaust fan calculator works it out).

More options
°C

Optional. Cold winter air holds less water even at the same humidity, so it needs more added. Empty: same as the room.

%

Ratings are measured in ideal conditions; real output is lower. 25% is our starting point.

Water to add

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Target humidity
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Humidity gap
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Room volume
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To raise the room once
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Buy a humidifier rated at least
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Air changes per hour
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Notes

    Next: Check the VPD you get at the new humidity

    How it works

    Each fresh-air change swaps humid air for air at today’s humidity. The humidifier has to put back the difference in absolute humidity (grams of water per cubic metre) for every cubic metre that is replaced, plus a one-off amount to bring the room up to the target in the first place.

    For the opposite problem, too much humidity, use the dehumidifier calculator.

    Formula
    AH(T, RH) = e / (461.5 × (T + 273.15))   g/m³
Water per hour = (AH_target − AH_now) × volume × air changes
Water per day  = water per hour × 24

    What size humidifier do I need for a grow tent?

    Filling a tent with humid air once takes almost nothing. Keeping it humid is the hard part. Every air change swaps your moist air for whatever comes in, and the humidifier has to put the difference back. So the answer depends far more on your exhaust fan and the air around the tent than on the tent's size.

    The calculator sizes for the worst case, with nothing else adding moisture: an empty room, or clones and seedlings that give off little water. Once the plants are big they add plenty of their own.

    Humidifier for a 4×4 grow tent: a worked example

    Take a 4×4 tent, 6.5 ft tall (104 ft³). The air inside is 24 °C (75.2 °F), the room air coming in is at 40%, and you want 65%. The exhaust fan moves 52 CFM, enough to change the air every 2 minutes.

    • Humidity gap: 5.4 g of water per cubic metre
    • Water to add: 11.5 litres (3.0 US gallons) a day
    • Buy a humidifier rated for at least 14.4 litres (3.8 gallons) a day, with the default 25% headroom
    • To raise the tent once: 0.016 litres, next to nothing

    Slow the fan so the air changes every 10 minutes (about 10 CFM) and the need falls to 2.3 litres (0.6 gallons) a day. If your lights do not need that much fresh air for cooling, slowing the fan costs less than a bigger humidifier. A 5×5 tent on the 2-minute setting needs 18.0 litres (4.8 gallons) a day.

    Boxes quote output in different units, so convert before you compare. 14.4 litres a day is 600 ml an hour, or 3.8 US gallons a day. Check the tank too: a small one may need refilling more than once a day at that rate, unless the unit can take a water line.

    Humidifier size chart for a whole home

    Choose Whole home for a house. It uses the AHRI Guideline F chart, made to hold 35% humidity in winter with 8 ft (2.4 m) ceilings. US gallons per day for a house of average build:

    • 500 sq ft (46 m²): 1.3
    • 1,000 sq ft (93 m²): 4.5
    • 1,500 sq ft (139 m²): 7.8
    • 2,000 sq ft (186 m²): 11.1
    • 2,500 sq ft (232 m²): 14.3
    • 3,000 sq ft (279 m²): 17.6

    How well the house is sealed matters as much as its size. At 1,000 sq ft a tight house needs 2.2 gallons and a loose, draughty one 7.2. Between chart points the calculator draws a straight line. Holding more than 35% in cold weather can wet windows and walls; check your coldest surface with the dew point calculator.

    Common mistakes with a humidifier size calculator

    • Entering the humidity inside the tent. The field wants the air coming in: the room around the tent, or outside. That is what the humidifier has to make up for.
    • Forgetting cold intake air. In the example, air drawn in at 10 °C instead of 24 °C, still at 40%, carries much less water. The need rises to 22.0 litres a day, nearly double.
    • Using the fan's box rating. Enter what the fan moves after its carbon filter and ducting. The exhaust fan calculator works that out.
    • Guessing the target. Choose a growth stage instead of a humidity and the calculator aims for the middle of its leaf VPD band. For clones at 24 °C that is about 63%. The VPD chart shows every stage.
    • Keeping it running into flower. Big plants release most of the water you give them. Then the job flips, and the dehumidifier size calculator takes over.

    FAQ

    Questions growers ask

    What size humidifier do I need?

    One that adds the water fresh air carries out, plus some headroom. The calculator multiplies the humidity gap in grams per cubic metre by the room volume and the air changes per hour, then by 24 for a day. For a house, a 1,000 sq ft home of average build needs about 4.5 US gallons a day in winter.

    What size humidifier for a 4×4 tent?

    About 11.5 litres (3 US gallons) a day for a 4×4 tent at 24 °C, raising 40% air to 65% with the air changed every 2 minutes. With 25% headroom, buy one rated for at least 14.4 litres (3.8 gallons) a day. Change the air every 10 minutes instead and it needs only 2.3 litres.

    Do I need a humidifier in a grow tent?

    Only if the tent stays drier than your target. Clones and seedlings aim for a leaf VPD of 0.5–1.0 kPa, about 63% humidity at 24 °C with leaves 2 °C cooler. If the air coming in is drier than that, the tent will sit below it unless you add water. Larger plants add their own.

    Warm or cool mist for a grow room?

    Cool mist, in most grow rooms. A warm-mist unit heats water into steam, and that heat stays in the room along with the rest of its electricity, which your cooling then has to remove. In a cold room in winter the warmth can help. Size either type by the litres or gallons per day shown here.

    Why is there white dust around my humidifier?

    It is minerals from the water. An ultrasonic humidifier breaks water into a fine mist, and anything dissolved in it, mostly from hard tap water, dries into white dust on leaves, lights and walls. Distilled or reverse-osmosis water stops it. A demineralising cartridge, if your model takes one, reduces it.

    Where should a humidifier go in a grow tent?

    Near where the fresh air comes in, so incoming dry air picks up moisture before it reaches the plants. Keep it well away from the exhaust fan, which would pull the moisture straight out. Point the mist away from leaves, lights and electrics, so nothing gets wet and the water ends up in the air.

    How much water does a whole-house humidifier need?

    About 4.5 US gallons (17 litres) a day for a 1,000 sq ft house of average build, to hold 35% humidity in winter with 8 ft ceilings. A tight, well-sealed house of that size needs about 2.2 gallons and a loose, draughty one about 7.2. At 2,000 sq ft the average figure is 11.1 gallons.

    How big a humidifier do I need?

    Enough to add the water that fresh air carries away. Each air change swaps your humid air for drier air, so the load is the humidity gap (in grams per cubic metre) × the room volume × the air changes per hour.

    Sources

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

    1. American Meteorological Society, Glossary of Meteorology: gas constant (water vapour, about 461 J kg⁻¹ K⁻¹). glossary.ametsoc.org/wiki/Gas_constant
    2. Allen, Pereira, Raes & Smith (1998). Crop evapotranspiration, FAO Irrigation and Drainage Paper 56, chapter 3, equations 11 and 19. www.fao.org/4/x0490e/x0490e07.htm
    3. Corredor-Perilla, Kwon & Park (2025). Elevated relative humidity significantly decreases cannabinoid concentrations while delaying flowering development in Cannabis. Frontiers in Plant Science (leaf VPD equation 3, and the cannabis VPD ranges it reports from earlier work). doi.org/10.3389/fpls.2025.1678142
    4. Pulse Grow (2020): VPD guide for cannabis growers. pulsegrow.com/blogs/learn/vpd
    5. Honeywell TrueSTEAM humidifier reference guide, chart 2, from AHRI Guideline F (US gallons per day to hold 35% RH, 8 ft ceilings, by floor area and how tightly the house is built). resource.ecmdi.com/is/content/Watscocom/Gemaire/honeywell_hm609a1000u_article_1422491579767_en_ss.pdf
    6. NIST Handbook 44 (2026), Appendix C: units and conversion factors. www.nist.gov/document/2026-nist-handbook-44-appendix-c

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