How this compost calculator works
Compost math is thin-layer volume math — the depths are small, so the bag count and the rounding matter more than usual:
order = yd³ × (1 + allowance%) → rounded up to ¼ yd³ · 27 × 1 ft³ bags per yard
At the defaults — a 20 × 10 ft bed spread 2 in deep — the order is 1.5 yd³, about $53 bulk or 35 bags; the full chain is in the worked example below. Because a compost layer is thin, the depth in inches swings the volume hard — an inch either way is a different order — so set it deliberately from the spread guide.
Compost vs topsoil vs mulch — which layer is which
Compost, topsoil and mulch get ordered together and confused constantly, but they do three different jobs and go on in a set order — feed, grow, protect:
- Compost — the amendment: decomposed organic matter, worked into the soil to feed it. 1–2 in a season, never a standalone growing medium. About 1,000 lb/yd³.
- Topsoil — the growing medium: the mineral soil roots live in; build beds with it and amend with compost. Priced on the topsoil calculator.
- Mulch — the blanket: a coarse layer on top that holds moisture and blocks weeds; it feeds the soil only slowly. The mulch calculator handles it.
- Order for the whole yard: tally beds of any one of them on the landscape calculator for a single delivery.
Timing matters as much as depth. A fall topdressing has all winter for worms and frost to work it in, so it is ready by spring; compost spread at planting wants forking in first, because a fresh surface layer ties up nitrogen as it finishes breaking down. Either way, finished, dark, crumbly compost is what you want — half-rotted material still competes with plants for the nitrogen it needs to cure.
How deep should you spread compost?
Compost is an amendment, so thin and worked-in beats thick and piled — a heavy surface layer crusts over and sheds water instead of feeding the soil:
- Lawn topdressing: ½ in raked in after aeration — one yard covers 648 ft².
- Annual bed feed: 1–2 in forked into vegetable and flower beds each season.
- New bed into poor soil: 2–3 in tilled deep to build structure the first year.
- Raised beds: a compost-and-topsoil mix, not pure compost, which settles and burns roots.
- Never planted straight: seedlings set into 100% compost struggle — it holds too much water and too much salt.
How much compost per square foot?
Table 1 is how far one cubic yard stretches at each spread depth; Table 2 is the depth a given job wants. Read them together: a ½ in lawn topdressing covers four times the ground a 2 in bed feed does, off the very same yard — which is why compost goes further than gardeners expect.
| Depth | 1 yd³ covers |
|---|---|
| ½″ (lawn topdress) | 648 ft² |
| 1″ | 324 ft² |
| 2″ | 162 ft² |
| 3″ | 108 ft² |
| Job | Depth |
|---|---|
| Lawn topdressing | ½″ |
| Annual bed feed | 1–2″ |
| New bed into poor soil | 2–3″ |
How to measure and spread compost
- Measure each bed’s length and width; total odd shapes on the square footage calculator first.
- Set the spread depth thin — ½ in for lawns, 1–2 in for beds — from the depth guide, not by eye.
- Leave the allowance at 5% for the little that clings to the wheelbarrow and rake.
- Compare bulk against bags — one cubic yard is 27 of the 1 ft³ bags, so bulk wins past about a yard.
- Spread and work it in — fork or till compost into the top few inches so it feeds rather than crusts.
Worked example: compost for a 20 × 10 bed
The default, worked line by line — a 20 × 10 ft bed, 2 in spread, 5% allowance, $35 a yard against $5 bags:
volume = 200 × (2 ÷ 12) = 33.3 ft³ = 1.23 yd³
order = 1.23 × 1.05 = 1.30 → 1.5 yd³
bulk = 1.5 × $35 = $53 · bags: (33.3 × 1.05) ÷ 1 = 35 ≈ $175 · covers 162 ft²
One yard covers 162 ft² at this depth, so a single scoop handles most gardens — and the $53-versus-$175 gap is the whole case for bulk. Compost bags are smaller than mulch bags, just 1 ft³, so the count climbs fast; past a yard the delivery beats hauling three dozen bags home.
What this compost estimate doesn’t include
This sizes the compost and nothing else. A bed build usually needs a few more things it doesn’t count:
- Topsoil. Compost amends soil; it doesn’t replace it. New beds need soil under the amendment.
- Mulch. The protective top layer over the finished bed — a separate order.
- Delivery. A flat bulk haul fee, with short-load minimums that make small drops uneconomic.
- Bed prep. Edging, sod removal and tilling to work the compost in.
- Moisture weight. Fresh, wet compost runs heavier than the 1,000 lb/yd³ finished average.
Compost FAQ
How much compost do I need?
Area × depth ÷ 12 ÷ 27: spreading 2 in over the default 20 × 10 ft bed is 1.23 yd³ — order 1.5 cubic yards with 5% allowance, or 35 one-cubic-foot bags. Bulk runs about $53 against $175 bagged at the defaults.
How many bags of compost are in a cubic yard?
27 of the common 1 ft³ bags — compost bags run smaller than mulch bags. That is why bagged compost turns expensive fast: the default job is 35 bags, already past the point where a bulk drop wins.
How deep should I spread compost?
1–2 inches worked into vegetable and flower beds each season, half an inch raked over lawns as topdressing, and up to 2–3 inches when building a new bed into poor soil. More is not better — compost is an amendment, not a growing medium on its own.
How much does a yard of compost weigh?
Roughly 1,000 lb — half a ton — finished and moderately moist, so the default 1.5-yard order is about 0.75 tons. Fresh, wet compost can push 1,400 lb a yard; a pickup handles a yard comfortably.
How much does compost cost?
Bulk finished compost typically runs $30–60 per cubic yard in 2026, with municipal programs often cheaper or free-load days. Bags work out to 3–4× bulk per yard; the two cost lines above make the comparison for your numbers.
How much compost for a 4x8 raised bed?
A 4 × 8 ft raised bed is 32 ft²; topped 2 in that is 5.3 ft³ — about 6 of the 1 ft³ bags, or 0.25 yd³ bulk with the allowance. At that size bags win, since a quarter-yard is below most delivery minimums. To fill a new raised bed from empty, plan a mix — compost plus topsoil — not pure compost, which settles and burns roots.
Sources and standards
The volume math is exact; the weight and the spread depths are horticultural rules of thumb:
- Exact conversions: 27 ft³ per cubic yard, so 27 of the 1 ft³ bags fill a yard.
- Estimating weight: ~1,000 lb/yd³ for finished, moderately moist compost — fresh or wet runs heavier.
- Cooperative-extension spread rates: ½ in lawn topdressing, 1–2 in bed amendment, worked in rather than piled.
- The scale and the season: compost weight swings with moisture, and beds want feeding yearly — this sizes one application.
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