How to Calculate Compost
Compost volume uses the same formula as any other soil amendment: area times depth. Multiply your bed length by its width for square footage, then multiply by the depth in feet (divide inches by 12). That gives cubic feet. Divide by 27 for cubic yards.
Volume (cu yd) = Volume (cu ft) ÷ 27
The key difference between compost and other materials: the depth you apply is not a freestanding layer. For garden bed amendment, you spread 2 to 4 inches on the surface and work it into the existing soil. The volume calculation is the same regardless — you need to move that amount of material to the bed.
Compost Application Rates by Project
- Garden bed amendment — 2 to 4 inches spread across the bed surface, then tilled or forked into the top 6 to 8 inches of soil. This is the primary annual maintenance application for most vegetable and perennial beds. University extension services consistently recommend at least 2 inches per season.
- Lawn topdressing — 0.25 to 0.5 inches, applied after aerating. Compost is one of the best materials for improving lawn soil over time — it improves drainage in clay soils and moisture retention in sandy ones. More than half an inch risks smothering the existing grass and should be avoided.
- New planting area — 3 to 4 inches spread before planting and incorporated with a fork or tiller. This is the highest application rate and is appropriate when establishing a bed in poor soil for the first time.
- Raised bed top-up — 1 to 2 inches per growing season. Compost breaks down over time and raised beds slowly lose volume. An annual top-up maintains the soil structure and nutrient levels.
Bags vs Bulk Compost
Bagged compost at garden centres typically runs $6 to $10 per 1 cubic foot bag — $162 to $270 per cubic yard. Bulk screened compost from a landscape or municipal composting facility costs $25 to $50 per cubic yard delivered, or less if you pick it up yourself. Once you need more than half a cubic yard (about 14 bags), calling for bulk delivery almost always makes economic sense. Many municipalities also offer free or subsidised compost pickup — worth checking before buying bagged.
Screened vs Unscreened Compost
Screened compost has been passed through a mesh to remove large chunks and produce a fine, uniform texture — it spreads more easily and works well for topdressing and fine garden work. Unscreened compost is coarser and less expensive; it is fine for working into beds where appearance and texture matter less than nutrient content. For lawn topdressing, always use screened compost — unscreened material will mat down and look messy.