Soil Amendment Calculator
Work out compost, peat moss, perlite, sand, or biochar for any bed.
Enter length, width, and spread depth to get volume, bags, and weight to move.
How much amendment a bed actually takes
Amendments are sold and spread as a layer, so the sum is the same one you would use for mulch or topsoil. You are buying the volume of a slab the size of the bed.
Volume = Bed Length × Bed Width × Spread Depth
Every quantity below comes out of that single figure. The only trick is the depth: it goes in inches or centimetres while the bed is in feet or metres, so one of the three numbers has to be divided before you multiply.
Worked example, and this one matches the calculator exactly. A 20 ft × 4 ft bed getting 3 inches of compost:
- Area = 20 × 4 = 80 sq ft
- Depth = 3 ÷ 12 = 0.25 ft
- Volume = 80 × 0.25 = 20 cubic feet, which is 0.57 m³ or 566 litres
- Bags at 1.5 cu ft = 20 ÷ 1.5 = 13.3, so 14 bags
- Compost runs about 40 lb per cubic foot, so that is 800 lb to shift
The same bed in metric is 6.1 m × 1.22 m, and 7.6 cm of compost over it comes to the same 0.57 m³. Use whichever set of units your tape measure is marked in.
Rough bulk densities, because weight decides how you move it
| Amendment | Weight per cu ft | Typical layer |
|---|---|---|
| Compost, aged manure | 40 to 45 lb | 2 to 3 in |
| Worm castings | 45 lb | half an inch, no more |
| Coarse sand or grit | 100 lb | 2 to 3 in, never alone into clay |
| Peat moss | 10 lb | 2 to 3 in |
| Coconut coir | 12 lb | 2 to 3 in |
| Perlite, vermiculite | 7 lb | 1 to 2 in |
| Biochar | 15 lb | half to one inch |
Sand is the one that catches people out. A cubic yard of it weighs well over a ton, and it is also the amendment most likely to make things worse: sand into heavy clay with no organic matter in the same pass sets like screed. Add the compost at the same time or skip the sand entirely.
Peat and coir are the other trap, in the opposite direction. Both are sold compressed. A 3.8 cu ft bale of peat fluffs out to roughly 7 cu ft once it is broken up and wetted, so ordering by the number on the bale means paying for about twice what you need.
How we build and check this calculator
This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
SuperGlobalCalculator is independently built and maintained. See how we build and verify our calculators.