The golden rule for greenhouse fans is one air change per minute: pick a fan whose CFM rating roughly equals your greenhouse’s cubic-foot volume.
For the full breakdown, see our best Small Greenhouse Fan guide.
The fastest way to ruin a summer crop is an exhaust fan that’s too small, and the fix starts with one number. To work out how to size a greenhouse exhaust fan, start with one ratio: one full air change per minute, meaning a fan rated in CFM — cubic feet per minute — roughly equal to the greenhouse’s volume in cubic feet. Nail that ratio and the fan clears trapped heat about as fast as a sunny afternoon builds it.
What CFM Do You Actually Need?
Target one air change per minute in summer. That means a fan whose CFM rating matches the greenhouse’s volume — the length × width × average interior height, all measured in feet. A fan this size replaces the entire air mass every 60 seconds, so heat gain at the roof gets pushed outside before it accumulates.
Average height matters more than most people expect. On a sloped or gable roof, measure halfway up a roof rafter and straight down to the floor; for a gable, (sidewall + peak) ÷ 2 is a close shortcut. If your hoop house has a curved roof, take the measurement at the middle of the arch. Multiply the three dimensions and you get your cubic feet, which converts directly to the CFM number you shop for.
On the small side, a compact 54-square-foot greenhouse with a 7-foot average height comes to about 378 cubic feet, so a 400 CFM fan covers it. UMass’s ventilation fact sheet walks through the same calculation for full-size houses.
Two practical notes before you buy. Fan ratings are free-air numbers, measured with no restrictions attached; louvers, shutters, and insect screen all cut what actually moves, so the rating is your ceiling, not your guarantee. And if the math lands between sizes, round up — the one-change-per-minute target is a minimum, not a maximum.
Adjustments That Change The Number
The one-change-per-minute baseline assumes light crops, some shade, and low elevation. Your conditions may demand more — or, in winter, a whole lot less.
| Condition | How It Affects Fan Sizing |
|---|---|
| Summer baseline | 1 air change per minute — fan CFM ≈ greenhouse volume |
| No shade cloth | Multiply the target by 1.2 |
| Dense crops, grow lights, bench heating | 1.5–2 air changes per minute |
| Winter ventilation | ¼ air change per minute is usually enough |
| High elevation (E feet) | Elevation factor: 1 + 0.04 × (E ÷ 1,000) |
| Air changes per hour (ACH) sizing | CFM = volume × ACH ÷ 60 |
The intake matters as much as the fan. Openings on the opposite end should be at least as large as the fan opening, and most guidance calls for more: roughly 1.25× the fan opening area, with 1.5× a common rule of thumb. Shutter sizing follows the same logic — When intake lags behind the fan, the fan starves, airflow drops, and the house runs hotter than the rating suggests.
The usual failures are all variations on undersizing:
- Using floor area instead of volume, which badly under-sizes tall and gutter-connected houses.
- Ignoring the extra height of a sloped roof.
- Skipping the shade-cloth, crop-density, and altitude adjustments.
In a large house, two or three smaller fans beat one monster: run one on mild days, all of them on hot afternoons. That staged setup also matches the width-based sizing charts most fan makers publish, which group houses into 15-foot, 22/26-foot, and 30/34-foot classes. Winter is the one place to shrink the number — a quarter air change per minute moves moisture without blasting young plants with cold drafts.