Calculators · 10 tools · free, no account
Size your grow room properly.
Fans, intake, filters, cooling, dehumidifiers, CO₂, lights, circuits and nutrient mixes. Plain answers first, every formula one tap away, and a source for every number. No brands, nothing to buy, and nothing you type leaves your browser.
One job at a time
The house view, up front. Passive intake suits most growers. Oversize your lights and dim them. Watt for watt, every device heats the same. And size for the worst case: everything at 100%.
Every constant and rule of thumb
One shared module, calc-core.js, holds every number below, and every calculator and Calculate it properly guide uses the same ones. Machine-readable: calculators.json.
Show all 73 constants
| Constant | Value | What it is | Evidence |
|---|---|---|---|
BTU_PER_W | 3.412 BTU/h per W | Every watt drawn from the wall becomes 3.412 BTU/h of heat in the room, whatever the device | Established |
BTU_PER_TON | 12000 BTU/h per ton | One ton of cooling | Established |
FT3_PER_M3 | 35.3147 ft³ per m³ | Volume conversion | Established |
M3H_PER_CFM | 1.699 m³/h per CFM | Airflow conversion | Established |
L_PER_GAL | 3.78541 L per US gal | Water conversion | Established |
PINT_PER_L | 2.11338 US pints per L | Dehumidifier ratings are in US pints/day | Established |
KG_PER_LB | 0.453592 kg per lb | Mass conversion | Established |
SENSIBLE_AIR | 1.08 BTU/h per CFM per °F | Heat one CFM of standard air carries per °F of temperature rise (60 min × 0.075 lb/ft³ × 0.24 BTU/lb·°F) | Established |
EXCHANGE_MIN | 1 minutes | Default: replace the whole tent's air once a minute (the KB range is every 1–3 minutes) | Grower practice |
FILTER_STD | 1.25 × | Carbon filter airflow loss, typical | Grower practice |
FILTER_HEAVY | 1.5 × | Carbon filter loss, thick bed, pre-filter or an older loaded filter | Grower practice |
DUCT_PER_M | 0.03 + per metre | Duct run loss: about +3% per metre (+10% per 10 ft) of smooth duct | Grower practice (rule of thumb; real loss depends on duct type and speed) |
BEND_90 | 0.15 + per bend | Each 90° bend: about +15% (more for tight or crushed flex duct) | Grower practice (rule of thumb) |
HEADROOM | 0.25 + | Speed-controller headroom so the fan runs near 75–80% instead of flat out | Grower practice |
MIN_DT_F | 2 °F | Below about 2 °F (1 °C) between intake air and your maximum, exhaust alone cannot cool the space | Established (follows from q = 1.08 × CFM × ΔT) |
FAN_W_PER_CFM | 0.12 W per rated CFM | Rough power draw of an EC mixed-flow inline fan at full speed (for the planner's electrical total) | Estimate |
INTAKE_RATIO | 2.5 × exhaust duct area | Passive intake opening area (KB rule of thumb 2–3×; we default to the middle) | Grower practice |
MESH_FREE | 0.6 open fraction | Bug mesh or a pre-filter sock leaves roughly 60% of the hole open | Estimate |
INTAKE_FAN | 0.75 × exhaust airflow | If you do fit an intake fan, keep it at about 70–80% of the exhaust so the space stays under negative pressure | Grower practice |
ACTIVE_CFM | 800 CFM | Above about 800 CFM (an 8-inch fan running hard) passive ports get large: consider an active intake | Grower practice |
INTAKE_RUN_M | 3 m | Intake ducted from further than about 3 m (10 ft) away needs a fan to pull through it | Grower practice |
FILTER_MATCH_LO | 1 × fan CFM | Filter rated airflow should be at least the airflow you actually run | Grower practice |
FILTER_MATCH_HI | 1.5 × fan CFM | Up to ~1.5× is better: slower air through the bed means longer contact and longer carbon life | Grower practice |
PERSON_BTU | 450 BTU/h per person | Heat from a person doing light work (ASHRAE: about 250 sensible + 200 latent) | Established |
U_GOOD | 0.06 BTU/h·ft²·°F | Well-insulated room (about R-16 overall) | Established (typical assembly) |
U_AVG | 0.1 BTU/h·ft²·°F | Average insulated stud walls (about R-10 overall) | Established (typical assembly) |
U_POOR | 0.25 BTU/h·ft²·°F | Uninsulated framed walls (about R-4) | Established (typical assembly) |
U_SHED | 0.6 BTU/h·ft²·°F | Bare garage, shed or metal building | Estimate |
AC_MARGIN | 0.2 + | Cooling margin over the calculated load (homes keep within ~15%; grow rooms run near flat out, so 20% is common) | Grower practice |
AC_EER | 10 BTU/h per W | Rough efficiency of a room or portable AC, for the electrical estimate (mini-splits do better) | Estimate |
TRANSPIRED | 0.97 fraction | Share of the water a plant takes up that it transpires into the air | Established |
NIGHT_RATE | 0.1 × day rate | Night transpiration per hour as a share of the lights-on rate | Supported (other crops) |
DEHU_MARGIN | 1.25 × | Dehumidifier margin over the average daily water (the AC also pulls water while it runs) | Grower practice |
AHAM_TO_DOE | 0.7 × | Old AHAM pints/day (80 °F, 60% RH) to the 2019 DOE rating (65 °F, 60% RH): about 70 → 50 | Established |
DEHU_IEF_S | 1.3 L per kWh | Federal minimum efficiency, portable units up to 25 pints/day (2019 rating); used to estimate the heat when you don't know the watts | Established (regulation) |
DEHU_IEF_M | 1.6 L per kWh | Federal minimum, portable units 25–50 pints/day | Established (regulation) |
DEHU_IEF_L | 2.8 L per kWh | Federal minimum, portable units over 50 pints/day | Established (regulation) |
WATER_SEEDLING | 0.25 L per plant per day | Stage helper: seedling | Grower practice (varies with pot, light and VPD) |
WATER_VEG | 1 L per plant per day | Stage helper: veg | Grower practice |
WATER_FLOWER | 2 L per plant per day | Stage helper: flower (4 plants in a 4 × 4 ft tent ≈ 5.4 L/m²/day, inside the guides' 3–6 L/m²/day) | Grower practice |
CO2_AMBIENT | 420 ppm | Outdoor CO₂ | Established |
CO2_M | 44.01 g/mol | Molar mass of CO₂ (density from the ideal gas law at room temperature, ≈ 1.83 kg/m³ at 20 °C) | Established |
ACH_TIGHT | 0.1 air changes per hour | Sealed room, well taped | Estimate |
ACH_SEALED | 0.25 air changes per hour | Sealed room, average | Estimate |
ACH_LEAKY | 2 air changes per hour | Zipped tent or leaky room, exhaust off | Estimate |
CO2_UPTAKE | 4 g CO₂ per m² canopy per hour | Net canopy uptake at high light and ~1,000 ppm (≈ 25 µmol/m²/s) | Estimate from leaf data |
PROPANE_CO2 | 3 lb CO₂ per lb fuel | Propane burner: C₃H₈ + 5 O₂ → 3 CO₂ + 4 H₂O (132 ÷ 44.1) | Established (stoichiometry) |
PROPANE_H2O | 1.63 lb water per lb fuel | Propane burner: water vapour made | Established (stoichiometry) |
PROPANE_BTU | 21600 BTU per lb fuel | Propane burner: heat released (higher heating value) | Established |
NG_CO2 | 2.74 lb CO₂ per lb fuel | Natural gas burner (as methane): CH₄ + 2 O₂ → CO₂ + 2 H₂O (44 ÷ 16) | Established (stoichiometry) |
NG_H2O | 2.25 lb water per lb fuel | Natural gas burner: water vapour made | Established (stoichiometry) |
NG_BTU | 23900 BTU per lb fuel | Natural gas burner: heat released (higher heating value) | Established |
CO2_PEL | 5000 ppm | 8-hour workplace limit (OSHA PEL, NIOSH REL, ACGIH TLV) | Established |
CO2_STEL | 30000 ppm | 15-minute short-term limit (ACGIH STEL, NIOSH ST) | Established |
CO2_IDLH | 40000 ppm | Immediately dangerous to life or health | Established |
DLI_K | 0.0036 mol·s per µmol·h | DLI = PPFD × hours × 0.0036 | Established |
CANOPY_CAPTURE | 0.85 fraction | Share of the fixture's photons that land on the canopy in a reflective tent (assumption) | Estimate |
EFF_OLD | 1.5 µmol/J | Old or low-end LED at the wall (older-generation and blurple panels, low-bin no-name diodes driven hard): often worse than double-ended HPS; independent 2014 tests measured LED fixtures at 0.89–1.70 | Supported (independent tests; founder's field experience 1.3–1.7) |
EFF_HPS | 1.7 µmol/J | Double-ended HPS at the fixture (independent tests 1.66–1.72) | Established |
EFF_BUDGET | 2.3 µmol/J | Budget LED panel at the wall (low-bin chips driven hard, budget driver) | Supported (arithmetic from datasheets) |
EFF_MID | 2.6 µmol/J | Mid-tier LED at the wall | Supported |
EFF_TOP | 3 µmol/J | Top-tier LED at the wall (about 6% of listed fixtures in 2022) | Established |
GB_W_PER_FT2 | 50 W/ft² of dimmable capacity | GenetixBay recommendation: oversize and dim down. Too much is better than not enough | House position (founder) |
GAL_IMP | 4.54609 L per imperial gallon | Imperial (UK) gallon; most North American labels mean the US gallon (3.785 L) | Established |
P_FROM_P2O5 | 0.436 × | Elemental P from a label's P₂O₅ (2 × 30.974 ÷ 141.94) | Established |
K_FROM_K2O | 0.83 × | Elemental K from a label's K₂O (2 × 39.098 ÷ 94.20) | Established |
PPM500 | 500 ppm per EC 1.0 | Hanna/NaCl '500' scale (TDS meters) | Established |
PPM700 | 700 ppm per EC 1.0 | '700' (KCl) scale | Established |
WATER_USE_M2 | 4 L per m² canopy per day | Planning water use in flower (range 3–6 L/m²/day; 1 mm a day = 1 L/m²) | Grower practice |
RES_DAYS_LO | 3 days | Reservoir holds at least 3 days of water use, so EC and pH swing less | Grower practice |
RES_DAYS_HI | 5 days | …and up to 5 days | Grower practice |
CONTINUOUS | 0.8 × breaker rating | Continuous loads (3 h or more, which every grow light is) may use only 80% of the breaker | Established (code) |
PF | 0.95 power factor | Drivers and ballasts draw slightly more amps than watts ÷ volts; we divide by 0.95 to be safe | Estimate |
Sources
Show all 32 sources
- ASHRAE. 2021. ASHRAE Handbook: Fundamentals. Ch. 1 (psychrometrics: sensible heat of air, q = 1.08 × cfm × ΔT at standard air), Ch. 18 (heat gain from people, lighting and equipment), Ch. 21 (duct design: fitting and friction losses), Ch. 26–27 (thermal resistance of building assemblies). ASHRAE, Atlanta. https://www.ashrae.org/technical-resources/ashrae-handbook
- ASHRAE. 2019. ASHRAE Handbook: HVAC Applications, chapter 'Control of gaseous indoor air contaminants' (sorbent bed depth and residence time govern removal). https://www.ashrae.org/technical-resources/ashrae-handbook
- NIST. Guide for the Use of the International System of Units (SP 811), Appendix B: conversion factors (1 W = 3.412 142 BTU/h; 1 ft³ = 0.028 316 8 m³; 1 US gal = 3.785 412 L). https://www.nist.gov/pml/special-publication-811
- AMCA/ASHRAE Standard 210-16: Laboratory methods of testing fans for certified aerodynamic performance rating (why 'free air' CFM falls as static pressure rises). https://www.amca.org/
- GenetixBay Knowledge Base. Airflow: air exchange, canopy air speed and fan placement (one tent volume every 1–3 min; +25–50% for a carbon filter and ducting; passive intake ≈ 2–3× exhaust duct area). https://genetixbay.com/knowledge/environment/airflow.html
- GenetixBay Knowledge Base. LED vs HPS: the heat and power-bill hype, and an LED buyer-beware guide (fixture µmol/J tiers, 85% canopy capture, watt-for-watt house advice, 1 W = 3.412 BTU/h). https://genetixbay.com/knowledge/equipment/led-vs-hps-hype-and-buyer-beware.html
- GenetixBay Knowledge Base. PPFD and DLI by stage, with the evidence (stage targets; DLI = PPFD × hours × 0.0036). https://genetixbay.com/knowledge/environment/ppfd-dli-by-stage.html
- GenetixBay Knowledge Base. CO₂ enrichment (targets 800–1,200 ppm at lights-on only; worth it only sealed and at high light). https://genetixbay.com/knowledge/environment/co2-enrichment.html
- Rodriguez-Morrison V, Llewellyn D, Zheng Y. 2021. Cannabis yield, potency, and leaf photosynthesis respond differently to increasing light levels in an indoor environment. Frontiers in Plant Science 12:646020. https://doi.org/10.3389/fpls.2021.646020
- Kusuma P, Pattison PM, Bugbee B. 2020. From physics to fixtures to food: current and potential LED efficacy. Horticulture Research 7:56. https://doi.org/10.1038/s41438-020-0283-7
- Nelson JA, Bugbee B. 2014. Economic analysis of greenhouse lighting: light emitting diodes vs. high intensity discharge fixtures. PLoS ONE 9(6):e99010 (independent fixture efficacy tests). https://doi.org/10.1371/journal.pone.0099010
- Chandra S, Lata H, Khan IA, ElSohly MA. 2008. Photosynthetic response of Cannabis sativa L. to variations in photosynthetic photon flux densities, temperature and CO₂ conditions. Physiology and Molecular Biology of Plants 14(4):299–306. https://doi.org/10.1007/s12298-008-0027-x
- ACCA. Manual S: Residential Equipment Selection (cooling equipment kept close to the calculated load; oversizing short-cycles and dehumidifies poorly). https://www.acca.org/standards/technical-manuals/manual-s
- US DOE. 10 CFR 430 Subpart B, Appendix CC: Uniform test method for portable air conditioners (seasonally adjusted cooling capacity, which counts the infiltration a single-hose unit causes). https://www.ecfr.gov/current/title-10/chapter-II/subchapter-D/part-430/subpart-B/appendix-Appendix%20CC%20to%20Subpart%20B%20of%20Part%20430
- US DOE. 10 CFR 430 Subpart B, Appendix X1 (dehumidifier test at 65 °F and 60% RH, required from June 2019; the older AHAM DH-1 rating used 80 °F and 60% RH) and 10 CFR 430.32(v) (minimum integrated energy factor, L/kWh). https://www.ecfr.gov/current/title-10/chapter-II/subchapter-D/part-430/subpart-B/appendix-Appendix%20X1%20to%20Subpart%20B%20of%20Part%20430
- ENERGY STAR. Dehumidifiers: key product criteria and the 2019 capacity re-rating (a unit once sold as 70 pints/day is about 50 pints/day on the new test). https://www.energystar.gov/products/dehumidifiers
- Taiz L, Zeiger E, Møller IM, Murphy A. 2015. Plant Physiology and Development, 6th ed., Ch. 4 (about 97% of the water a plant absorbs is lost by transpiration). Sinauer. https://global.oup.com/academic/product/plant-physiology-and-development-9781605353531
- Caird MA, Richards JH, Donovan LA. 2007. Nighttime stomatal conductance and transpiration in C3 and C4 plants. Plant Physiology 143(1):4–10 (night transpiration commonly 5–15% of daytime, up to ~30%). https://doi.org/10.1104/pp.106.092940
- Alduchov OA, Eskridge RE. 1996. Improved Magnus form approximation of saturation vapor pressure. Journal of Applied Meteorology 35(4):601–609. https://doi.org/10.1175/1520-0450(1996)035%3C0601:IMFAOS%3E2.0.CO;2
- NOAA Global Monitoring Laboratory. Trends in atmospheric carbon dioxide (global mean above 420 ppm since 2023). https://gml.noaa.gov/ccgg/trends/
- OSHA. Carbon dioxide, chemical data: PEL 5,000 ppm 8-hour TWA (29 CFR 1910.1000, Table Z-1). https://www.osha.gov/chemicaldata/183
- NIOSH Pocket Guide to Chemical Hazards: carbon dioxide (REL 5,000 ppm TWA; short-term 30,000 ppm; IDLH 40,000 ppm). ACGIH lists the same TLV-TWA 5,000 ppm and TLV-STEL 30,000 ppm. https://www.cdc.gov/niosh/npg/npgd0103.html
- NFPA 70, National Electrical Code (2023): Art. 100 (continuous load = 3 hours or more), 210.19(A)(1) and 210.20(A) (conductors and breakers at 125% of continuous load, i.e. load ≤ 80%), 210.8 (GFCI locations), 400.12 (flexible cords not a substitute for fixed wiring). https://www.nfpa.org/codes-and-standards/nfpa-70-standard-development/70
- CSA C22.1, Canadian Electrical Code, Part I: Rule 8-104 (continuous loads limited to 80% of the breaker rating) and Rule 26-700 series (receptacles and GFCI). https://www.csagroup.org/store/product/CSA%20C22.1:24/
- OSHA. 29 CFR 1910.303(b)(2): listed equipment must be used per its listing and labelling (power strips are not to be chained together). https://www.osha.gov/laws-regs/regulations/standardnumber/1910/1910.303
- Combustion stoichiometry and heating values of propane and methane: NIST Chemistry WebBook (enthalpies of combustion) and US EIA, 'Units and calculators: British thermal units' (propane ≈ 91,500 BTU per gallon at 4.24 lb per gallon). https://webbook.nist.gov/chemistry/
- Mattson NS, Lieth JH. 2019. Liquid culture hydroponic system operation. In: Raviv M, Lieth JH, Bar-Tal A (eds), Soilless Culture: Theory and Practice, 2nd ed. Elsevier, pp. 567–585 (EC monitoring, replenishment and replacement). https://doi.org/10.1016/B978-0-444-63696-6.00012-8
- NIST. Atomic weights and isotopic compositions (P 30.974, K 39.098, O 15.999), for the P₂O₅ and K₂O conversion factors. https://www.nist.gov/pml/atomic-weights-and-isotopic-compositions-relative-atomic-masses
- Allen RG, Pereira LS, Raes D, Smith M. 1998. Crop evapotranspiration (FAO Irrigation and Drainage Paper 56), Ch. 1 (1 mm of water over 1 m² = 1 litre). https://www.fao.org/4/x0490e/x0490e04.htm
- GenetixBay Knowledge Base. EC vs PPM: the 500 and 700 scales. https://genetixbay.com/knowledge/ph-ec/ec-ppm-scales.html
- GenetixBay house position (founder's field experience, decades of indoor rooms): passive intake for most growers; oversize lights and dim down; replace HPS watt for watt unless the LED is top tier. https://genetixbay.com/editorial-policy
- GenetixBay survey of published free-air ratings for mixed-flow inline fans sold for grow rooms, 2026 (ranges only; no brands).
