Compost Calculator
Work out how much compost a bed or border needs — from the area and spread depth, in cubic yards and bags, with your own bag size.
Compost is spread thin — an inch or two — but over a whole bed that is still cubic yards. This calculator turns the bed size and the depth into a volume and a bag count.
Feed the soil every year: a thin dressing beats a thick one every few years.
Your results
Enter your measurements and press Calculate.
Quantities are rounded up to whole tiles, planks, boxes or bags, after the wastage allowance. Cost appears only when you enter a price.
Formula
All lengths are converted to a single unit before the calculation: 1 ft = 0.3048 m, 1 in = 25.4 mm, 1 ft² = 0.09290304 m², 1 yd² = 9 ft². Material quantities are rounded up — you cannot buy part of a tile, plank or bag.
How to use this calculator
- Choose the shape of the area (rectangle, circle, or a known area).
- Enter the depth — 1–2 in is normal for a top dressing.
- Enter the bag volume from the packaging.
- Add a price per bag if you want a cost estimate.
Worked example
| Bed | 10 ft × 20 ft |
|---|---|
| Area | 10 × 20 = 200 ft² |
| Volume (2 in deep) | 200 × ⅙ ft = 33.33 ft³ = 1.23 yd³ |
| With 10% wastage | 33.33 × 1.10 = 36.67 ft³ = 1.36 yd³ (1.04 m³) |
| Bags (40 L bags) | ⌈1.04 ÷ 0.04⌉ = 26 bags |
Check it by hand: every step above uses the formula in the box. If your numbers match the example inputs but the calculator gives a different answer, please report it via the contact page.
Frequently asked questions
How much compost do I need?
Multiply the area by the depth. A 10 × 20 ft bed top-dressed 2 in deep needs 1.36 yd³ — 26 bags of 40 L. This calculator does the same maths for your bed.
How thick should compost be spread?
1–2 in (25–50 mm) as a top dressing each year, or mixed into the top 6 in when preparing a new bed. More than 2 in as a dressing can smother small plants.
What is the difference between compost and mulch?
Compost feeds the soil — it is dug in or spread thin and breaks down. Mulch sits on top to keep moisture in and weeds down. Both help, and they are different jobs: use the mulch calculator for mulch.
Can I make my own compost?
Yes — kitchen scraps and garden waste, layered green and brown, turned regularly, ready in 6–12 months. Homemade compost is free, but you still need the volume: this calculator tells you how much.
Related calculators
Estimates only. Results are for planning. Measure twice, confirm box coverage and tile sizes on the packaging you buy, and follow the manufacturer's instructions. See the disclaimer.