Amendment, by volume
Compost calculator
Top-dressing a lawn, amending a bed, or filling a new one. Compost is applied far thinner than most people assume.
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How deep, by project
| Project | Depth | Notes |
|---|---|---|
| Lawn top-dressing | 1/4–1/2 in | Raked in so grass blades still show through. Anything thicker smothers the turf. |
| Amending an existing bed | 2–3 in | Spread and forked into the top 6 inches, or left for worms to take down. |
| Filling a new raised bed | Bed depth, blended | Around 30–40 percent of the total volume, with topsoil making up the rest. |
| Mulching vegetable beds | 1–2 in | Compost as surface mulch feeds and covers at the same time. |
| Planting hole backfill | About 25% | Mixed with native soil. All-compost holes create a bathtub that roots refuse to leave. |
Thin layers, repeated
Compost applied to lawns is measured in fractions of an inch. A quarter to a half inch, twice a year, does more for soil than a single deep application — and a deep application will kill grass by burying the crowns.
Do the sum before you order. Half an inch over 2,000 square feet is 83 cubic feet, just over 3 cubic yards. It is easy to under-order for lawns and wildly over-order for a small bed.
It is light for its volume
At roughly 1,200 pounds per cubic yard, compost weighs half what topsoil does. That makes it the one bulk material a half-ton pickup can genuinely take a yard of.
It is also why bagged compost feels reasonable in a way bagged topsoil does not — a cubic foot bag is only 45 pounds.
Finished compost only
Compost that is still working will rob nitrogen from your plants as it finishes decomposing, and hot compost dug into a bed can cook roots. Finished compost is dark, crumbly, smells of earth, and you cannot identify what went into it.
If a bulk supplier's pile is steaming, it is not ready. That is not a subtle test — it is visible from the car.
Common questions
How much compost do I need for a raised bed?
Usually 30 to 40 percent of the bed volume, with topsoil making up the rest. A 4 by 8 foot bed at 12 inches is 32 cubic feet, so about 11 cubic feet of compost.
How thick should compost be on a lawn?
A quarter to a half inch, raked in so the grass still shows. One cubic yard covers about 1,300 square feet at a quarter inch, or 650 at a half inch.
How many bags of compost in a cubic yard?
Twenty-seven bags of 1 cubic foot, or 18 bags of 1.5 cubic feet.
Can you use too much compost?
Yes. Repeated heavy applications build up phosphorus and salts, and a thick layer on a lawn smothers it. Thin and regular beats deep and occasional.
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