What size exhaust fan do I need?
Your fan has two jobs: swap the air about once a minute, and carry the lights' heat out. Work out both, take the bigger one, then add for the carbon filter, the duct and every bend. Pick a fan that can do that with some speed left over.
Your result
How it works, in plain words
Air exchange
CFM = tent volume in ft³ ÷ minutes per exchange. A 4 × 4 × 6.5 ft tent is 104 ft³, so 104 CFM for one change a minute.
Heat removal
CFM = watts × 3.412 ÷ (1.08 × allowed rise in °F). 480 W with a 7 °F rise (75 °F in, 82 °F max) needs about 217 CFM. In hot rooms this is the bigger number.
Losses
Fans are rated in free air, with nothing attached. A carbon filter costs about 25%, each 90° bend about 15% and duct about 10% per 10 ft (3 m). These are rules of thumb: the fan's own pressure curve is the real answer.
Headroom
Add about 25% so the fan runs near three-quarters speed on a controller: quieter, cooler and with room for a heatwave.
Every number this calculator uses
The same constants feed the Calculate it properly guides, so the tools and the articles agree. Rules of thumb are labelled as such.
| Constant | Value | What it is | Evidence |
|---|---|---|---|
EXCHANGE_MIN | 1 minutes | Default: replace the whole tent's air once a minute (the KB range is every 1–3 minutes) | Grower practice |
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 |
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 |
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) |
FT3_PER_M3 | 35.3147 ft³ per m³ | Volume conversion | Established |
M3H_PER_CFM | 1.699 m³/h per CFM | Airflow conversion | Established |
Sources
- 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
- 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
- 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
- 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
- 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 survey of published free-air ratings for mixed-flow inline fans sold for grow rooms, 2026 (ranges only; no brands).
