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Calculation Machine

Greenhouse Ventilation Calculator

Exhaust fan sizing for a greenhouse.

Reviewed by the Calculation Machine team · How we check our maths

Your values

ⓘ Live results

Results
Summer exhaust
2,304 CFM
Winter minimum (2 CFM/sq ft)
576 CFM
Floor area
288 sq ft
Intake louvre area
4.6 sq ft
Suggested fans
1 unit(s)

Quick answer

A greenhouse typically needs 8-12 CFM per square foot of floor in summer, corrected for altitude and shading.

The formula

CFM = area x 8 x altitude factor / shade factor

Worked example

Worked example

Using the values the calculator loads with.

Inputs

  • Length24 ft
  • Width12 ft
  • Altitude0 ft
  • ShadingNo shade cloth

Result

  • Summer exhaust2,304 CFM
  • Winter minimum (2 CFM/sq ft)576 CFM
  • Floor area288 sq ft
  • Intake louvre area4.6 sq ft
  • Suggested fans1 unit(s)

How to use the Greenhouse Ventilation Calculator

  1. 1Measure the greenhouse footprint.
  2. 2Add altitude and any shade cloth already fitted.
  3. 3Size intake louvres at roughly 1.25 times the fan area so the fans are not starved.

Frequently asked questions

Two-speed fans?

Yes — a low speed near 2 CFM per sq ft handles spring and autumn without chilling plants.

Where do fans go?

Exhaust at one gable end, intake louvres at the opposite end for end-to-end sweep.

About this calculator