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 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: Premier Tech rates Pro-Mix at 2 : 1, so the 3.8 cubic feet on a bale breaks out to about 7.6, and a 32-cubic-foot bed is five bales, not nine. And a “5 gallon” nursery pot is a trade name, not a measurement — the nursery standard (ANSI Z60.1) lets a #5 hold anywhere from 3.4 to 5.4 gallons, while fabric pots are true gallons, which is why the pot mode asks which kind you have. Bags of potting mix are sold in dry quarts: 12 of them is just under half a cubic foot.

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 bands above are the ones Pulse, Dimlux, Black Dog and Metos publish; flower is the contested one — Pulse stops at 1.5, Fluence calls 1.16–1.35 “stress”, Dimlux goes to 1.6 — so we show 1.5–1.6 as an edge, not a target. Lights-off targets run about 0.2 kPa lower, and the thing that matters more than any single number is keeping the swing between lights-on and lights-off under 0.4 kPa.

The leaf offset matters more than people expect. Under LED, leaves sit 2–6 °F below the air because evaporation cools them and the fixture does not warm them (Pulse says 2–5, Dimlux 3–5, Quest 5, Black Dog 4–6; we default to 3). Under HPS or CMH the infrared heats the leaf directly and it can sit at or above air temperature, so the field accepts zero and negative numbers. Get the offset wrong and your VPD is off by a tenth or more, which is the whole width of a band. An infrared thermometer settles it in five seconds.

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 set to 0.5 and 1,680 ppm on a Bluelab reading ppm 700. Nothing changed but the math. Bluelab’s own advice is the right one: work in EC if you possibly can. It means the same thing on every meter in the room, and every meter reports it corrected to 25 °C.

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 the nutrient is actually contributing — which is the number a feed chart is written against (Front Row says it outright: every chart assumes RO water). The stage bands are judged on that nutrient-only number, and they are starting points: salt-based programs run veg at 2.6 where a general chart calls 1.8 full strength.

The conversion is simple — DLI = PPFD × hours × 3,600 ÷ 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. The bands are the ones Thrive, Hydrobuilder, Trimleaf and LED Grow Lights Depot publish, with the photoperiod they assume shown beside them.

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. The fixture mode divides the spec-sheet PPF by the footprint and knocks off what a tent or room wastes; nobody publishes that loss, so it is a rule of thumb (60–80% for open rooms, 85–100% for a reflective tent). On CO2: without enrichment, returns per photon start shrinking around 800–1,000 PPFD; with 1,000–1,500 ppm growers run 1,000–1,500 PPFD. A University of Guelph trial pushed yield up in a straight line to 1,800 PPFD at ambient CO2, so the ceiling is a cost question, not a wall.

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. Every manufacturer says the same thing in its own words — FoxFarm: “never mix pure concentrates together; always add water first”; Canna: “do not mix A & B concentrate directly”; Cultured Solutions: “do not combine any products in undiluted form”. The chemistry behind it is calcium meeting phosphate or sulfate: calcium nitrate, Part B, FloraMicro and cal-mag must never touch Part A, Bloom, Epsom salt or silica undiluted. Silica first, then cal-mag, then the base parts in the chart’s order, one product at a time into the full water volume, pH last. For Jack’s: 5-12-26 fully dissolved, then Epsom, then calcium nitrate.

Dry nutrients are dosed by weight, not by the spoon. Jack’s label allows “1 tsp ≈ 5 g” for its own products, but a teaspoon of calcium nitrate does not weigh what a teaspoon of 5-12-26 does, and the hobby doses (3.6, 2.4 and 1.1 g per gallon for the 3-2-1 parts) come to three quarters, half and a fifth of a teaspoon — buy a $15 scale. Liquid charts: FoxFarm rounds 1 tsp to 5 mL; this calculator uses the exact 4.93, which is 1.4% less and does not matter. Mix, circulate, then read pH and EC — in that order, and not before the tank has turned over a few times.

The base figure is just room volume divided by how fast you want the air replaced: once a minute for a tent (the convention every tent maker uses), every two minutes for a room, every three to five for a room with air conditioning, where the fan only has to handle odor and the odd hot afternoon. Everything after that is resistance. A carbon filter, every foot of flex duct and every elbow adds back-pressure, and a fan on back-pressure moves less than its rating — so the losses are divided out, not added on: a fan that loses 20% through the filter has to be rated 25% higher to deliver the same air, which is what the math above does. The percentages are Hydrobuilder’s engineering set (filter 20–25%, flex 4.5% per 10 ft, 5% a bend); Can-Filters, who make the filters, put the filter alone at 10–25%. Published fan curves say a good mixed-flow fan on a matched filter really loses 5–20%; the 60% figures in some fan-company blogs only happen with a cheap axial booster or a badly undersized filter.

Heat is the other side of the ledger. Nearly every watt of lamp power ends up as heat in the room, and holding a 10 °F rise over the intake air takes 3.4 BTU per watt divided by 1.08 per CFM — a 600 W light needs 190 CFM on heat alone, more than a 4 × 4 tent’s once-a-minute 112. Put the watts in and the calculator takes the larger of the two. Then buy the next size up and run it slower: an 8-inch fan at half speed moves about what a 6-inch does flat out, with less noise, longer bearing life, and headroom for the July week when the room will not come down. Leave the passive intake open at three to four times the duct area (AC Infinity’s rule) so the fan is not pulling against a vacuum.

A plant transpires about 95% of the water it takes up; only a few percent ends up as tissue (Desert Aire puts it at 99% of what reaches the roots). So if 4 gallons go in and 15% runs to a drain, 3.4 gallons are going into the air — about 26 pints a day — and a “50 pint” box from the hardware store is not the comfortable fit it looks like. That box was rated at 65 °F and 60% RH under the 2019 DOE test; grow-room brands rate at 80 °F and 60% (the old AHAM test, where the same machine reads 70); Ideal-Air’s Pro series is rated at 86 °F and 80%, where a machine reads 1.4 to 2 times what it does at 80/60 (Hydrofarm’s own re-ratings span that range; we use 2, the conservative end, which is also what the coil model gives). None of those is your room. The calculator derates the nameplate to your lights-on and lights-off conditions with a coil model fitted to Quest’s own published second ratings (a Quest 155 does 90 pints at 75 °F / 50%, and the model says 90), and shows the answer on all three labels.

Three more things the simple “gallons times eight” rule misses. The air conditioner is a dehumidifier while it runs — about 25–30% of its BTU rating goes into condensing water, which is 3–4 pints an hour from a 12,000 BTU unit — but it only runs when there is heat to remove, so it does nothing at lights-off, which is exactly when the room is coolest, the machine is weakest, and Quest says to size for. An exhaust fan carries water out when the room it draws from is drier than your target, and carries it in when a Massachusetts July basement is wetter; the vented mode works that out from the fan’s CFM and the intake air, and tells you where the tent would settle with no dehumidifier at all. And every pint condensed puts about 1,100 BTU of heat back into the room plus the compressor’s watts — roughly 1,500 BTU per pint for a Quest, 2,000 for a consumer unit — which is why big flower rooms get their dehumidification and cooling sized together.

Plants transpire at roughly a third of the daytime rate in the dark (the ratio that reproduces Desert Aire’s published load tables), so the lights-on hour carries most of the load; whichever period needs the bigger nameplate sets the size, with 20% headroom on top. The cross-check against Quest’s 0.5–2 pints per square foot of canopy per day catches a feed figure that is off. Runoff default is 15% (10–15% is what drain-to-waste coco and peat growers run); if the runoff sits in saucers it evaporates and counts again.