Grow Room Calculators

Depth is the number that moves the answer. A 4 × 8 bed filled 6 inches deep takes 16 cubic feet; filled to the top of a 12-inch wall it takes 32, and a bed built 24 inches deep takes 64. Measure the depth you will actually fill — inside the walls, stopping an inch below the rim — not the height printed on the box. Deep beds are usually better with a cheaper base in the bottom third, compost or native soil or a coarse bulk mix, and the good stuff on top; run the calculator once for each layer.

Bagged mix settles. Peat and coco fluff up in the bag and drop 10–20% once they are wet and the air is out, which is why the allowance defaults to 10% — buy it now or top up later, but the bed will not stay level on its own. Compressed bales go the other way: the 3.8 cubic feet on a Pro-Mix or Sunshine bale is the compressed size and breaks out to about 7, so a 32-cubic-foot bed is five bales, not nine. And a “5 gallon” nursery pot is a trade name, not a measurement — moulded nursery pots run anywhere from 10 to 35% under their name, fabric pots run close to true gallons, and the pot mode here assumes true gallons with an inch of headroom left at the top.

VPD is the difference between how much water vapor the air could hold at leaf temperature and how much it actually holds. Low VPD and the plant stops drinking — transpiration stalls, calcium stops moving, and you are one cool night away from botrytis. High VPD and the stomata shut to protect the plant, which stops photosynthesis just as hard.

The leaf offset matters more than people expect. Leaves sit below air temperature because evaporation cools them, and under LED that gap is usually 2–4 °F. Under HID, radiant heat closes the gap to nearly nothing. Get the offset wrong and your VPD number is wrong by a tenth or more, which is the whole width of a target band.

EC is the real measurement. A meter passes a current through the solution and reads its conductivity in millisiemens per centimeter; parts-per-million is a conversion applied on top, and the three scales use different multipliers because they assume different reference salts. The same reservoir reads 1,200 ppm on a Hanna meter and 1,680 ppm on a Eutech one. Nothing changed but the math.

If your tap or well water already carries 0.3 mS/cm of dissolved minerals, that is 0.3 you did not add and cannot feed with. Put it in the starting-water field and the last line tells you how much EC your nutrient is actually contributing — which is the number a feed chart is written against. Work in EC if you possibly can. It is the one number that means the same thing on every meter in the room.

The conversion is simple — DLI = PPFD × hours × 3600 ÷ 1,000,000 — but the consequence is not. A veg room at 18 hours and 400 PPFD delivers more daily light than a flower room at 12 hours and 550. Photoperiod does half the work, which is why turning a light down in veg costs less than people fear and why pushing PPFD in flower is the only lever you have left once the clock is fixed at 12.

Measure the average across the canopy, not the brightest point. A quantum meter held in nine spots on a grid and averaged will tell you something a single reading under the center of the fixture never will — usually that your corners are 40% down. Above roughly 45 DLI you need supplemental CO2 for the plant to use the light, and without it you are paying for photons it cannot spend.

Two things cost people more grows than any nutrient ever has. The first is mixing into a tank that is not actually full — a 50-gallon reservoir filled to 40 and dosed for 50 is a 25% overfeed, and the slider above exists so that never happens. The second is adding concentrates together: silica and calcium will lock up in front of you if they meet undiluted. Add one product, circulate, then add the next, in the order the chart gives, and never pour two into the same measuring cup.

Mix, circulate, then read pH and EC — in that order, and not before the tank has turned over a few times. A reading taken thirty seconds after the last bottle went in is a reading of the last bottle.

The base figure is just room volume divided by how fast you want the air replaced. Everything after that is loss. A carbon filter is a wall of packed charcoal and costs you roughly a quarter of the fan's rating; every bend and every foot of flex duct costs more, and flex duct costs far more than rigid pipe for the same run. The rating printed on the box is what the fan does with nothing attached to it, which is not a state any grow room is ever in.

Buy the next size up and run it slower. An 8-inch fan at 40% is quieter, lasts longer and moves more air than a 6-inch at 100%, and it leaves you headroom for the July week when the room will not come down. If the fan is on a temperature or VPD controller, that headroom costs nothing on the power bill the rest of the year.

A plant transpires something like 95% of the water it takes up; only a few percent ends up as tissue. So if 30 gallons go in and 6 of those run to waste, 24 gallons are going into the air — about 182 pints a day. A "70 pint" home unit in that room is not slightly small, it is a third of what is needed, and the room will sit at 70% RH every night while you wonder why the tops are going soft.

Two things worth knowing. Manufacturer pint ratings are measured at 80 °F and 60% RH; at lights-off temperatures the same machine pulls noticeably less, so size against the room you have at night, not the spec sheet. And a dehumidifier is a heater — every pint it condenses releases latent heat back into the room, which is why big flower rooms end up needing the dehumidification and the cooling sized together rather than one at a time.