Compost Calculator
Work out how much compost you need for a garden bed. Enter length, width and depth to get cubic feet, cubic yards, liters and bags, with the volume formula shown.
Updated 2026-06-14 · Free · No sign-up · Runs privately in your browser
Show the formula & steps
How the Compost Calculator Works
Buying too little compost means a second trip to the garden centre; buying too much wastes money. This calculator tells you exactly how much compost you need to cover a bed at your chosen depth. Enter the length, width and layer depth, and it returns the volume in cubic feet, cubic yards and liters, plus the number of bags to buy.
The Formula
Compost coverage is a volume problem: the area of the bed times how deep you want the layer.
Volume (ft³) = length (ft) × width (ft) × depth (ft)
Because depth is usually given in inches, the calculator first converts it to feet by dividing by 12. The volume is then expressed in other units:
Cubic yards = ft³ ÷ 27
Liters = ft³ × 28.317
Bags = ft³ ÷ bag size, rounded up
Worked Example
For a raised bed 10 ft long and 4 ft wide, topped with a 3-inch layer of compost, using 1.5 ft³ bags:
- Depth = 3 ÷ 12 = 0.25 ft
- Volume = 10 × 4 × 0.25 = 10 ft³
- Cubic yards = 10 ÷ 27 ≈ 0.37 yd³
- Liters = 10 × 28.317 ≈ 283 L
- Bags = 10 ÷ 1.5 = 6.7 → 7 bags
Compost Depth Guide
| Use | Suggested depth |
|---|---|
| Topdressing a lawn | 0.25-0.5 in |
| Mulching around plants | 1-2 in |
| Amending a vegetable bed | 2-4 in |
| Filling a new raised bed | mix with topsoil to fill |
Tips for Buying Compost
Bulk compost by the cubic yard is usually cheaper than bagged compost for beds larger than a few square metres, while bags are convenient for small jobs and topdressing. Always round up to the next bag or part-yard, and order a little extra to allow for settling, since fresh compost compacts as it breaks down and rain works it into the soil.
Frequently asked questions
How much compost do I need for my garden bed?+
Multiply the bed length by its width by the layer depth, all in feet, to get cubic feet. A 10 by 4 ft bed topped with 3 inches (0.25 ft) of compost needs 10 x 4 x 0.25 = 10 cubic feet.
How many bags of compost do I need?+
Divide the volume in cubic feet by the size of one bag, then round up. If you need 10 cubic feet and each bag holds 1.5 cubic feet, that is 10 / 1.5 = 6.7, so buy 7 bags.
How deep should a layer of compost be?+
For topdressing an established bed, 1 to 2 inches is typical. For amending soil before planting, 2 to 4 inches dug in is common. New raised beds often use a thicker layer mixed with topsoil.
How do I convert cubic feet of compost to cubic yards?+
Divide the cubic feet by 27, because one cubic yard contains 27 cubic feet. Bulk compost is usually sold by the cubic yard, so 10 cubic feet is about 0.37 cubic yards.
How much area does a cubic yard of compost cover?+
One cubic yard equals 27 cubic feet. Spread 1 inch deep it covers about 324 square feet, at 2 inches about 162 square feet, and at 3 inches about 108 square feet.