How to Calculate Cubic Feet

Published 2026-09-04

Multiply length by width by height, all in feet. If your measurements are in inches, multiply the three inch figures and divide by 1,728. A 30 × 24 × 18 inch box is 12,960 cubic inches, or 7.5 cubic feet.

Cubic feet is the unit almost every material and appliance in a house is sold or rated in — mulch, soil, concrete bags, freezers, moving boxes, air purifiers. The formula never changes. What trips people up is the units they measured in.

The formula

cubic feet = length ft × width ft × height ft

All three numbers must be in feet before you multiply. That single rule is behind almost every wrong answer.

Converting before you multiply

These are exact, not rounded: NIST Handbook 44’s general tables of units of measurement defines a cubic yard as 27 cubic feet exactly and a foot as 0.3048 metre exactly. | Centimetres | Multiply all three, then divide by 28,317 | | Metres | Multiply the result by 35.31 |

The 1,728 comes from 12 × 12 × 12 — the number of cubic inches in a cubic foot. It is not 12, and that mistake makes an answer 144 times too large.

Worked example: a moving box

A box measuring 30 × 24 × 18 inches:

1. Multiply: 30 × 24 × 18 = 12,960 cubic inches
2. Divide by 1,728: 12,960 ÷ 1,728 = 7.5 cubic feet

The same box in feet: 2.5 × 2 × 1.5 = 7.5 ft³. Same answer, fewer zeros — which is why converting each dimension first is usually the safer habit.

Worked example: a room

A 12 by 14 foot room with an 8-foot ceiling:

12 × 14 × 8 = 1,344 cubic feet

This is the figure that sizes heating, cooling and air purifiers. Floor area alone misses it — a room with a 12-foot ceiling holds half again as much air as the same floor plan at 8 feet, and a purifier rated by square feet will be undersized in it.

Worked example: a refrigerator

Appliance capacity is measured inside, not outside. Measure the interior height, width and depth of each compartment in inches, multiply, divide by 1,728, and add the compartments together:

Shelves and drawers are included in the rating, so do not subtract them.

Where cubic feet meets cubic yards

Bulk landscape material — mulch, topsoil, gravel, concrete — is sold by the cubic yard, and 1 cubic yard is 27 cubic feet. Bagged versions of the same material are sold by the cubic foot, usually 2 ft³ per bag for mulch and 0.6 ft³ per 80 lb bag of concrete mix. Working out the cubic feet first lets you price bags against bulk directly.

For a bed 20 by 4 feet at a 3-inch depth:

1. Depth in feet: 3 ÷ 12 = 0.25
2. Cubic feet: 20 × 4 × 0.25 = 20 ft³
3. Cubic yards: 20 ÷ 27 = 0.74 yd³
4. Bags at 2 ft³: 20 ÷ 2 = 10 bags

Other conversions worth knowing

• 1 cubic foot = 7.48 US gallons
• 1 cubic foot = 28.32 litres
• 1 cubic foot of water weighs about 62.4 lb
• 1 cubic foot of concrete weighs about 150 lb — see the concrete calculator to turn a slab into yards and bags

The weight figures are the reason a small volume can still be an ordering problem: a single cubic yard of concrete is 27 × 150, or roughly 4,050 lb.