Calculate attic volume for gable, hip, or flat roofs from floor length, width, and peak height, with results in ft³, m³, and US gallons.
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Attic Volume Formula
The attic volume calculator uses a different volume formula depending on the roof or space shape you select.
Gable attic
V = 1/2 * L * W * H
Hip roof attic
V = 1/3 * L * W * H
Flat or box-shaped attic
V = L * W * H
Unit conversions used for the result
m^3 = ft^3 * 0.0283168
US\ gallons = ft^3 * 7.48052
- V = attic volume
- L = attic floor length
- W = attic floor width
- H = peak height or ceiling height
- ft³ = cubic feet
- m³ = cubic meters
For a gable attic, the calculator treats the attic cross-section as a triangle, so the volume is half of the full rectangular box.
For a hip roof attic, the calculator uses a pyramid-style approximation, so the volume is about one third of the length times width times height.
For a flat or box-shaped attic, the calculator uses the full rectangular volume because the ceiling height is assumed to be consistent across the space.
If you enter inches or meters, the calculator first converts each dimension to feet before calculating cubic feet. It then converts the result to cubic meters and US gallons. For gable and hip attics, it also gives a rough attic-fan sizing estimate based on a 2-minute air change.
Common Attic Shape Multipliers
| Attic type | Multiplier | When to use it |
|---|---|---|
| Gable | 0.5 | Use for a triangular attic under a simple two-slope roof. |
| Hip roof | 0.333 | Use for a roof that slopes down on all sides. This is an approximation. |
| Flat / box | 1.0 | Use for a rectangular attic, storage space, or room with a mostly even height. |
Attic Volume Reference Values
| Floor footprint | Height | Gable volume | Hip roof volume | Flat volume |
|---|---|---|---|---|
| 30 ft × 20 ft | 6 ft | 1,800 ft³ | 1,200 ft³ | 3,600 ft³ |
| 40 ft × 28 ft | 7 ft | 3,920 ft³ | 2,613 ft³ | 7,840 ft³ |
| 50 ft × 32 ft | 8 ft | 6,400 ft³ | 4,267 ft³ | 12,800 ft³ |
Example Calculations
Example 1: Gable attic
You have a gable attic with a floor length of 40 ft, a floor width of 28 ft, and a peak height of 7 ft.
V = 1/2 * 40 * 28 * 7
V = 3920\ ft^3
The attic volume is 3,920 ft³.
Example 2: Flat attic space
You have a flat or box-shaped attic space with a length of 30 ft, a width of 20 ft, and a ceiling height of 5 ft.
V = 30 * 20 * 5
V = 3000\ ft^3
The attic volume is 3,000 ft³.
FAQ
How do you measure attic height?
For a gable or hip attic, measure from the attic floor to the highest point of the roof framing, usually the ridge or peak. For a flat or box-shaped attic, use the ceiling height. If the height changes across the space, use an average height or divide the attic into smaller sections and add the volumes together.
Why is gable attic volume half of length times width times height?
A simple gable attic has a triangular cross-section. A triangle has half the area of a rectangle with the same base and height, so the attic volume is half of the full box volume.
Is the hip roof result exact?
The hip roof result is an approximation. It works best when the attic shape is close to a pyramid or a simple hip roof form. More complex roofs with dormers, multiple ridges, valleys, or uneven slopes should be broken into simpler sections for a better estimate.
