Compost Calculator

How it's calculated

The calculator first finds the volume of the area in cubic feet: length (ft) × width (ft) × depth (in ÷ 12). That's converted to cubic yards by dividing by 27, since a cubic yard is 3 ft × 3 ft × 3 ft. A waste percentage is added on top — the same 15% default as topsoil, since compost settles and keeps breaking down after it's spread — and the adjusted cubic yardage is multiplied by compost's density (tons per cubic yard) to get the weight in tons. Bag counts divide the adjusted cubic feet by each bag's yield; truckloads divide the adjusted cubic yards by the truck's capacity.

Worked example

A 10 ft × 10 ft bed amended 2 inches deep: volume = 10 × 10 × (2 ÷ 12) = 16.67 cubic feet. In cubic yards, that's 16.67 ÷ 27 = 0.62 yd³. Adding a 15% waste factor: 0.62 × 1.15 = 0.71 yd³. At 0.50 tons per cubic yard, that's 0.71 × 0.50 = 0.35 tons — or 0.28–0.43 tons across the full 0.40–0.60 density range. For comparison, the same 0.71 cubic yards of topsoil would weigh roughly 0.78 tons at topsoil's 1.10 tons/yd³ figure — more than double compost's weight for the identical volume, since compost is organic matter rather than mineral soil. In 1 cu ft bags, the adjusted 19.17 cu ft (16.67 × 1.15) needs 20 bags.

How deep should it be?

Compost, topsoil, and garden soil blends get confused constantly, but they do different jobs. Compost is fully broken-down organic matter — a soil amendment, not a growing medium on its own — added to improve structure, drainage, and nutrient content in soil that's already there. Topsoil is mineral-based soil, used to build up grade or fill a bed from scratch. Garden soil blends start with topsoil and mix in compost and other amendments, sold pre-mixed for planting beds. Amending an existing bed calls for compost worked into soil that's already there, usually a shallow 1–2" layer; building a brand-new bed from bare ground usually needs topsoil or a garden soil blend as the bulk of the fill, with compost mixed in rather than standing in for the whole bed. Compost is also dramatically lighter than topsoil for the same volume, and it's worth understanding why before ordering by weight: topsoil is mineral particles — sand, silt, clay — which are dense, while compost is broken-down organic matter, full of air pockets and far less dense material by volume. Order the same weight of each and the compost delivery will fill a noticeably bigger pile, which matters if you're ordering by the ton rather than the yard. Whatever depth you spread, compost keeps settling and breaking down after it's applied — more than topsoil does, since it's still actively decomposing — so order based on the settled depth you actually want, not the loose depth at delivery, and expect to see it shrink further over the following weeks. Finally, buy bulk compost expecting real batch-to-batch variation: UNH Extension notes that no two batches are likely to be uniform, even from the same supplier, because the organic feedstocks that go into any given batch change over time — worth knowing before you order a second load expecting it to match the first exactly.

Common mistakes

• –Confusing compost with topsoil or a garden soil blend. Compost is an amendment for soil that's already there, not a standalone fill — building a new bed from bare ground needs topsoil or a blend as the bulk of the fill, with compost mixed in.
• –Ordering compost by weight without accounting for how light it is. Compost is organic matter, not mineral soil, so the same weight delivers dramatically more volume than topsoil — a ton of compost is a much bigger pile than a ton of soil.
• –Measuring the loose delivered depth instead of the settled depth. Compost keeps breaking down and settling after it's spread, more than topsoil does, so what you spread today isn't what remains in a few weeks.
• –Assuming a second bulk order will match the first exactly. Compost varies batch to batch, even from the same supplier, because the organic feedstocks going into it change over time — check color, texture, and smell against what you're used to rather than assuming uniformity.
• –Applying too thick a layer over an established bed in one pass. A shallow 1–2" amendment layer worked into existing soil does more good than a thick pile that takes longer to integrate.

Where the density figure comes from

This is the first density on this site sourced to university extension services rather than suppliers, and that's deliberate: extension services publish compost bulk density because farmers need it to calculate application rates, so these figures are measured rather than estimated. UNH Extension states compost should generally run about 1,000 lb per cubic yard, give or take a few hundred pounds, and notes that no two batches are likely to be uniform, even from the same supplier, because of the varying organic materials that go into any given batch. Oregon State University Extension's rule of thumb puts screened compost at 50% moisture at about the same figure, 1,000 lb per cubic yard, and explains that bulk density is what lets a lab's weight-based analysis convert into the volume a field application actually uses. Both sources converge on roughly 1,000 lb (0.50 tons) per cubic yard — the published 0.40–0.60 ton/yd³ range reflects that compost varies by feedstock, screening, and moisture more than any mineral material on this site, not disagreement between the two sources. We use 0.5 tons/yd³ as the display figure, drawn from:

• UNH Extension — Purchasing Compost (tip sheet)
• Oregon State University Extension — Interpreting Compost Analyses (EM 9217)

Read more in our density methodology.

FAQ

What's the difference between compost, topsoil, and garden soil?

Compost is fully broken-down organic matter — an amendment for soil that's already there, not a standalone growing medium. Topsoil is mineral-based soil used to build up grade or fill a bed from scratch. Garden soil blends start with topsoil and mix in compost and other amendments, sold pre-mixed for planting. Use compost to amend, topsoil or a blend to build.

Why is compost so much lighter than topsoil?

Topsoil is mineral particles — sand, silt, clay — which are dense. Compost is broken-down organic matter, full of air pockets and far less dense material by volume. Order the same weight of each and compost fills a noticeably bigger pile, which matters if you're buying by the ton rather than the yard.

Should I use compost to build a new garden bed?

Not on its own. Compost is an amendment, meant to improve soil that's already there — a new bed built from bare ground needs topsoil or a garden soil blend as the bulk of the fill, with compost mixed in rather than standing in for the whole bed.

Should I order based on the loose or settled depth?

Settled. Compost keeps breaking down and settling after it's applied, even more than topsoil does, since it's still actively decomposing. Order based on the depth you want after it settles, and let the waste factor cover the difference.

Why does compost density vary so much?

More than any mineral material on this site — feedstock, screening, and moisture all shift the weight, and UNH Extension notes that no two batches are likely to be uniform, even from the same supplier, because the organic materials going into any given batch change over time. Confirm the specific batch's figure with your supplier for a large order.

Is bagged or bulk compost cheaper?

Bulk wins past about a cubic yard, the same threshold as topsoil. Bagged compost carries packaging and handling costs that add up fast once you're buying more than a few dozen bags — a small bed amendment is usually the ceiling for where bags still make sense over a bulk delivery.

How many truckloads of compost will I need?

Divide your adjusted cubic yardage by the truck's capacity — most standard dump trucks carry about 10-14 cubic yards. The calculator above uses 12 cubic yards as a default and rounds up, since a partial truckload still requires a full trip.