Grow Room Environment Calculator by Urban Root
Enter the size of your room or tent, your lights and your plants. The calculator sizes the cooling, the dehumidifier, the exhaust fan and the CO2 supply for that space, and it checks VPD and light.
It runs in the browser at growroomcalc.com. You don't need an account. The VPD and cooling tabs are free. Everything else, including a downloadable Environment Plan, unlocks with one $34.99 payment.
Imperial or metric units. Power cost in USD, EUR, GBP, ZAR, AUD, CAD or NZD.
Each tab reads the same room. Change the lights on one tab and the cooling, dehumidifier, power and DLI figures follow.
Air and leaf vapour pressure deficit in kPa from air temperature, humidity and a leaf offset, on a heatmap, with a verdict against the band for your growth stage.
Heat load in BTU per hour and tons, with a 20 % margin and a class of air conditioner. Free counts lights and walls. Paid adds dehumidifiers, people, other equipment, a CO2 burner and insulation grade.
Water to remove in pints (or litres) per day from plant count, plant size, light watts and stage, with a 30 % margin.
Exhaust CFM (or m³/h) from room volume and 1, 2 or 3 air changes per minute, plus 25 % for a carbon filter, 5 % per duct bend and 0.5 % per foot of duct. It names a fan size and how many fans.
CO2 to reach your target ppm, the flow per hour to hold it, and how many days a tank lasts at 12 hours a day.
kWh per day and per month for lights, air conditioning, dehumidifiers, fans and other loads, and the monthly bill at your rate.
Place fixtures from a catalogue or enter your own. You get a PPFD heatmap of the floor, with the minimum, maximum, average, coverage and uniformity.
A timeline of stages with days, temperature, humidity, light hours and CO2, and the DLI for each stage. Presets cover photoperiod, autoflower and veg-only runs.
Type in what your meters read (temperature, humidity, CO2, PPFD, airflow, power cost). The tab shows how far each is from the calculated figure.
A downloadable Environment Plan as one HTML file, with every figure above. You can also save and load room designs.
DLI is average PPFD times light hours times 3,600, divided by 1,000,000. The average PPFD comes from the Light Planner grid when you have placed fixtures there. When you haven't, it comes from light watts times the efficacy for the light type, times 0.85, over the full floor area.
That second path changed on 7 September 2026. Before then it put more light on the floor than the fixture emits, so a room larger than about 3.6 x 3.6 ft read high and a smaller tent read low. The Cycle tab and the download print a dated note about the change, so a saved plan from before that date will not match a new one. A stage above 65 mol/m²/day gets an advisory, because that is about the outdoor midsummer peak in a desert. The figure itself is never capped.
Yes. The calculator works from length, width and height in feet or metres, so a 4 x 4 ft tent and a converted bedroom go through the same maths. Room volume drives the exhaust, CO2 and wall heat figures, and the floor area drives the light and DLI figures.
The VPD tab and the Cooling tab are free. The free cooling figure counts the lights and the heat through the walls. A licence adds dehumidifiers, people, other equipment, a CO2 burner and the insulation grade to that figure, and unlocks the Dehumidifier, Vent, CO2, Power, Light, Cycle, Actual and Report tabs.
Saturation vapour pressure comes from the Tetens equation. Leaf temperature is the air temperature plus a leaf offset you set (the default is 2 °F cooler than the air). The tab shows air VPD and leaf VPD in kPa and compares leaf VPD with the band for the stage you pick: 0.4 to 0.8 for propagation, 0.8 to 1.2 for vegetative and 1.2 to 1.6 for flowering.
DLI is average PPFD times the stage's light hours times 3,600, divided by 1,000,000. If you lay out fixtures in the Light Planner, the average PPFD comes from that grid. If you do not, it comes from light watts times the efficacy for the light type (LED 2.5, CMH 1.9, HPS 1.7, fluorescent 1.2 µmol/J) times 0.85, spread over the full floor area. A stage above 65 mol/m²/day gets an advisory note. The figure is not capped.
$34.99 USD, paid once through LemonSqueezy. There is no subscription and no account. The licence key arrives by email and unlocks every tab and the downloadable Environment Plan.
No. The figures are estimates from standard formulas and grow-room rules of thumb. Use them to shortlist equipment and to check a plan. For a commercial build, have an HVAC engineer size the system.
$34.99 once
VPD and cooling stay free. The licence unlocks the other eight tabs and the Environment Plan download. See the refund policy before you buy.