How do I calculate cubic feet of a greenhouse?

For a peaked-roof greenhouse, treat it as a rectangular base + triangular prism on top. Approximate as length × width × average height (where average height = wall height + half the peak rise).

How many BTUs does my greenhouse need?

Required BTU = cubic feet × 0.4 × temperature differential (°F). For a 1,000 ft³ greenhouse keeping 60 °F when outside is 20 °F: 1,000 × 0.4 × 40 = 16,000 BTU/hr.

How big a heater do I need for a 10 × 12 ft greenhouse?

At 8 ft average height, that's 960 ft³. For a 40 °F temperature differential (e.g., keeping 50 °F when outside is 10 °F): 960 × 0.4 × 40 ≈ 15,400 BTU/hr.

Why is the BTU formula 0.4 per cubic foot per °F?

The constant accounts for typical greenhouse glazing (single-pane glass or double-wall polycarbonate) and assumes roughly 1 air change per hour. Better-insulated greenhouses can use 0.3; single-poly or older glass houses use 0.5.

Should I use BTU or cubic feet to compare greenhouse heaters?

Both. BTU rates the heater's output; cubic feet sizes the greenhouse. Match heater BTU to your worst-case nighttime temperature differential, with 20% headroom.

Greenhouse Cubic Feet Calculator — Heater & Vent Sizing for Hoophouse Builds

How to calculate cubic feet for greenhouse

Greenhouses lose heat through every surface — glass, polycarbonate, even insulated walls. To size a heater, you need the air volume in cubic feet and the worst-case temperature differential between inside and outside.

BTU/hr = cubic feet × 0.4 × ΔT (°F)

For peaked-roof greenhouses, approximate the volume as: length × width × (wall height + half the peak rise above the wall).

Worked examples

Example 1: A 10 × 12 ft greenhouse, 8 ft average height, 40 °F ΔT

10 × 12 × 8 = 960 ft³. BTU = 960 × 0.4 × 40 = 15,360 BTU/hr.

Example 2: A 20 × 30 ft commercial hobby greenhouse, 10 ft avg height, 50 °F ΔT

20 × 30 × 10 = 6,000 ft³. BTU = 6,000 × 0.4 × 50 = 120,000 BTU/hr.

Example 3: A 6 × 8 ft hobby greenhouse, 7 ft avg, 30 °F ΔT

6 × 8 × 7 = 336 ft³. BTU = 336 × 0.4 × 30 = 4,032 BTU/hr.

Common greenhouse sizes

Greenhouse	Conversion factor	Worked example
6 × 8 × 7 ft	336 ft³	~4,000 BTU @ 30 °F ΔT
8 × 10 × 7 ft	560 ft³	~6,720 BTU @ 30 °F ΔT
10 × 12 × 8 ft	960 ft³	~15,360 BTU @ 40 °F ΔT
12 × 16 × 8 ft	1,536 ft³	~24,576 BTU @ 40 °F ΔT
16 × 24 × 9 ft	3,456 ft³	~55,300 BTU @ 40 °F ΔT
20 × 30 × 10 ft	6,000 ft³	~120,000 BTU @ 50 °F ΔT

Tips and considerations

Insulate the north wall

The north-facing wall gets minimal sun. Replacing glass with insulated panel cuts heater requirements by 15–25% with no reduction in plant light.

Plan for ventilation too

Greenhouses also need cooling — pick exhaust fans rated for at least 1 air change per minute on a sunny day. For a 1,000 ft³ greenhouse, that\'s 1,000 CFM minimum.

Common mistakes to avoid

Using outside dimensions for heating calcs. Heat loss is based on internal volume — subtract wall thickness before sizing the heater.
Forgetting peak ceiling for hoophouse designs. A 10×20 hoophouse with a 10-foot peak is not a 2,000 ft³ box — it averages closer to 1,200 ft³ because of the curved roof.
Sizing ventilation by area instead of volume. Greenhouse ventilation is rated in air changes per hour — that is volume-based, not floor area.
Ignoring polyfilm vs glass losses. Polyethylene loses heat at about 1.15 W/m²/°C; single-pane glass loses 5.8. Same volume, very different heater needs.