Compost Calculator
Work out compost two ways — a thin topdressing layer over a lawn or bed, or a share mixed into the soil you're amending. Get the volume in cubic yards and cubic feet plus the bag count.
Thin for lawns (¼–½ in), thicker for beds (1–2 in).
How deep you're working the soil, and what fraction of that layer is compost by volume.
Bagged compost is often 1 or 1.5 ft³ — confirm on yours. Leave blank if buying bulk.
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Formula & how it works
Topdress: volume = area × (layer depth ÷ 12) cubic feet.
Mix in: volume = area × (amend depth ÷ 12) × compost share. Cubic yards = ÷ 27; bags = cubic feet ÷ bag volume, rounded up.
Worked example
Topdressing a 500 ft² lawn with a ½ in layer is 500 × 0.042 ≈ 21 ft³, about 0.8 cubic yards or 14 bags at 1.5 ft³. To amend a 100 ft² bed, mixing compost into the top 8 in at a 25% share, that's 100 × 0.667 × 0.25 ≈ 17 ft³ — roughly 0.6 cubic yards.
Using compost well
Topdressing is a thin-layer game
Over a lawn, compost works as a thin quarter- to half-inch dusting that filters down between the blades and feeds the soil life — pile it on too thick and you smother the grass. On beds you can go heavier, an inch or two, and let worms and weather work it in. Because the layer is thin and the area large, small depth changes swing the volume a lot.
Mixing in is about the share, not just the depth
When you're building or improving a bed, what matters is how much of the worked layer is compost. A quarter to a third by volume is a common amendment — enough to add organic matter and structure without turning the bed into pure compost, which drains and settles too much. Set the depth you'll cultivate and the fraction, and the mix-in mode does the rest.
Order a little extra
Compost keeps decomposing, so a pile shrinks over time, and screened compost settles as it's worked in. Rounding up slightly means you finish the job in one go. If you're covering a large area, compare bulk by the yard against bags — bulk usually wins past about a cubic yard.