How much compost you need
Area × depth gives the volume, 10% covers spill and settling, and the bags are that volume over what one bag holds, rounded up. A vegetable garden 10 feet by 20 feet with 2 inches of compost worked in is 10 × 20 × 2/12 = 33.3 cubic feet; with 10% that is 36.7 cubic feet, or 1.36 cubic yards. In the 1 cubic foot bags most stores stock that is 37 bags, or 19 of 2 cubic feet.
Two inches is Virginia Tech's figure for a garden bed: spread about 2 inches over the garden each year, work it 6 to 8 inches into the soil, and no more at one time, since more may encourage grubs. Washington State gives a range, 1 to 3 inches worked in at least 6 to 8 inches. The page warns past 3 inches, the most either of them gives for one application.
How deep, by what you are doing
| Use | Compost | Source |
|---|---|---|
| Working into a garden bed | about 2 in, tilled 6 to 8 in deep | Virginia Tech 426-704 |
| Working into a garden bed, WSU's range | 1 to 3 in, tilled 6 to 8 in deep | WSU FS123E |
| A bed already in good shape | about ½ in a year | WSU FS123E |
| Around shrubs and trees | about 1 in, under 2 to 3 in of mulch | Virginia Tech 426-704 |
| Before seeding or sodding a lawn | about 2 in, tilled in | Virginia Tech 426-704 |
| Top-dressing a lawn | ¼ to ½ in | Virginia Tech, WSU |
Top-dressing a lawn is a thin layer over a large area: Washington State says no more than ½ inch, and this page's 3 inch ceiling is no guide to it.
Bags per cubic yard
| Bag | Bags per cubic yard | Seen as |
|---|---|---|
| 0.75 ft³ | 36 | Scotts Premium Humus & Manure, 43 lb |
| 1 ft³ | 27 | Black Kow, Espoma, Coast of Maine |
| 1.5 ft³ | 18 | G&B Organics soil builder |
| 2 ft³ | 13.5 | Kellogg Amend |
Every size above is worked out on its own row of the bag table, each rounded up to whole bags, with what is left over. Pick the one your store has; the shopping list, the weight and the cost follow the row you pick.
What a yard of compost weighs
About 800 lb, which is the figure California's organic-waste regulation sets for counting compost (14 CCR §18993.1: a short ton of organic waste counts as 0.58 short ton or 1.45 cubic yards of compost, and 1,160 lb ÷ 1.45 = 800). It is a conversion, not a weighing: wet compost is heavier, dry compost lighter. The page uses it only for the load you will haul; it is a box under Assumptions.
Bags or a delivered load
A landscape yard sells compost by the cubic yard, delivered. From 2 cubic yards — about 54 of the 1 cubic foot bags — the page compares the two and buys the cheaper; below that it buys bags, because a delivery charged by the trip rarely pays for a few bags' worth. That threshold is our choice, not a store's, and it is a box under Assumptions. The reference prices are estimates: type your store's and your yard's. We are not connected with any brand or store named here.