# Exhaust fan CFM calculator

> Free exhaust fan calculator for grow tents and rooms: tent volume, air exchange, heat removal, carbon filter, duct and bend losses, and the inline fan size (4 to 12 inch) to buy.

Interactive: https://genetixbay.com/knowledge/tools/exhaust-fan.html

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.

## How it works

### 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.

## Constants

| Constant | Value | Unit | 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

1. 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
2. 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
3. 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
4. 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
5. 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/
6. GenetixBay survey of published free-air ratings for mixed-flow inline fans sold for grow rooms, 2026 (ranges only; no brands).