Most guidance assumes 20–33%.
How deep you will work it into existing soil.
Area 1
For tree rings, measure to the drip line.
Advanced options
Raised bed mode only. Pure compost is too rich.
Raised bed mode only. Compost keeps decomposing.
Varies 800–1,350 with moisture.
This compost calculator works out cubic yards and bags for lawn topdressing, bed amendment and raised-bed blends — and can work backwards from a target percentage by volume, which is how horticultural guidance is actually written.
The short answer
Compost volume is area × depth ÷ 324 for cubic yards. A vegetable garden takes 2 inches, a new bed 3, and a lawn only ¼ inch.
1,000 ft² of lawn at ¼ inch needs 0.77 cubic yards. The same area at the garden rate of 2 inches needs 6.2 — eight times as much. Depth matters far more than area here.
On this page
How to use this compost calculator
Pick what you’re doing from the dropdown — it sets the right depth — then enter the area. Four shapes:
- Rectangle — lawns, garden beds, plots
- Circle — tree rings, measured to the drip line
- Raised bed — length, width and fill depth, with a compost blend percentage
- Known area — you’ve already measured
The dropdown also has “Amend to a target percentage by volume”, which works backwards: tell it you want 25% compost in the top 6 inches and it tells you to spread 2 inches. That’s the mode covered below, and it’s the one no other compost calculator offers.
The compost formula
Cubic yards = (Area in ft² × Depth in inches) ÷ 324
Cubic feet = Area × Depth ÷ 12
A 1 ft³ bag is the retail standard and weighs about 40 lb
27 bags = 1 cubic yard
Worked example — 20 × 10 ft vegetable garden at 2 inches
Tilled into the top 6 inches, those 2 inches give a mixed layer of 8 inches at 25% compost.
Application rates
| Job | Depth | yd³ per 1,000 ft² | 1 yd³ covers |
|---|---|---|---|
| Lawn topdressing | ¼ in | 0.77 | 1,296 ft² |
| Lawn overseeding | ½ in | 1.54 | 648 ft² |
| New lawn, tilled in | 1 in | 3.09 | 324 ft² |
| Established bed, annual | 1½ in | 4.63 | 216 ft² |
| Vegetable garden, spring | 2 in | 6.17 | 162 ft² |
| New bed or poor soil | 3 in | 9.26 | 108 ft² |
| Tree and shrub mulch | 2 in | 6.17 | 162 ft² |
Note the spread: the same 1,000 ft² takes 0.77 yards as a lawn topdressing or 9.26 as a new bed. That’s a twelvefold difference, entirely from depth. Getting the depth right matters far more than measuring the area precisely.
Compost is specified by percentage, not depth
Read enough horticultural guidance and you’ll notice it rarely says “apply 2 inches.” It says something like “incorporate 2 to 4 inches into the top 6 to 8 inches of soil” — two numbers, not one. That’s because what matters to the plant isn’t the layer you spread; it’s how much organic matter ends up in the root zone as a proportion of the whole.
Compost % = Spread depth ÷ (Till depth + Spread depth)
Spread depth = Till depth × % ÷ (100 − %)
Spread 2 in, till into 6 in → mixed layer is 8 in → 2 ÷ 8 = 25%
The mixed layer ends up deeper than the tilling depth, because you added material to it. Six inches of soil plus two inches of compost is eight inches of amended bed — which is where the percentage comes from, and the step most people skip.
Run the standard rates through that and something satisfying happens:
| Spread depth | Tilled into 6 in | Tilled into 8 in |
|---|---|---|
| 1 in | 14% | 11% |
| 1½ in | 20% | 16% |
| 2 in | 25% | 20% |
| 3 in | 33% | 27% |
| 4 in | 40% | 33% |
Every combination in the standard advice — 2 into 6, 2 into 8, 3 into 6, 4 into 8 — lands between 20 and 33%. That’s the real target, and the depth recommendations are just convenient ways of hitting it.
If you’re tilling shallower or deeper than the guidance assumed, the depth figure gives you the wrong answer. Working compost into only 4 inches? Two inches gives you 33%, not 25%. Going down 10 inches with a broadfork? You’d need 3.3 inches for the same result.
Set the calculator to percentage mode, enter your actual tilling depth, and it works out the spread depth for you.
Lawn topdressing — a quarter inch, and that’s it
Lawn topdressing goes on at ¼ inch, up to ½ inch maximum. Beyond that you bury the crowns of the grass plants and smother the lawn you were trying to feed. The turf thins instead of thickening, which is the exact opposite of the intended result.
If you want more organic matter into a lawn, apply ¼ inch twice a year rather than ½ inch once. Spring and autumn, ideally straight after core aeration so it drops into the holes.
At ¼ inch, one cubic yard covers 1,296 ft² — so a typical 5,000 ft² lawn takes under 4 yards. Use screened, fine-textured compost; anything coarse sits on the surface and looks like debris rather than working in.
Raised beds — compost is not a growing medium
Filling a raised bed with pure compost is a common and expensive mistake. Compost on its own is too rich, holds too much water, and slumps dramatically as it continues to break down.
Blend
25–40% compost
The rest topsoil, with something for drainage — coarse sand, perlite or bark fines.
Allow for settling
About 25%
Compost keeps decomposing after it’s in the bed. Order 25% over the calculated fill volume.
Top up annually
1–2 in
Beds drop noticeably in the first year. Add compost each spring rather than rebuilding.
The raised bed mode handles this: enter the bed dimensions and fill depth, set your compost share, and it returns both the compost quantity and the total mix volume, with settling included.
You can use too much compost
Compost is beneficial, which makes it easy to assume more is better. It isn’t.
- Phosphorus accumulation. Compost carries phosphorus that plants use slowly and soil holds tightly. Heavy annual applications build it up over years, and excess phosphorus interferes with micronutrient uptake — iron, zinc and manganese in particular.
- Salt build-up. Manure-based composts especially can raise soluble salts to levels that stress roots.
- Nutrient imbalance. A soil dominated by organic matter can end up with plenty of nitrogen and phosphorus but skewed ratios elsewhere.
- Waterlogging. Very high organic content holds water, which is helpful in sandy soil and a problem in anything that already drains poorly.
The practical rule: 3 to 4 inches when establishing a new bed on poor soil, then 1 to 2 inches annually. The heavy rate is a one-off correction, not a yearly habit. After a few seasons of good management, a soil test is worth more than another delivery.
Weight and density
Compost density is quoted anywhere from 800 to 1,350 lb per cubic yard — a 70% spread, much wider than for mineral materials like sand or gravel.
The reason is moisture. Compost is organic and porous, so it absorbs and holds water to a degree that sand simply doesn’t. The same screened compost can genuinely weigh 800 lb per yard after a dry spell and approach 1,350 after a week of rain.
| Condition | lb per yd³ | lb per ft³ |
|---|---|---|
| Dry, screened, fine | 800 | 30 |
| Typical bagged — the checkable anchor | 1,080 | 40 |
| Planning figure used here | 1,100 | 41 |
| Wet or unscreened | 1,350 | 50 |
The useful anchor is the bag: a standard 1 ft³ retail bag weighs about 40 lb, which works out to 1,080 lb per cubic yard. That sits neatly inside the published range and you can verify it on any bag in a garden centre. It’s why 1,100 is the default here.
Practically, this matters mainly if your supplier sells by weight. Most compost is sold by volume, and for volume the density is irrelevant.
A published error worth knowing about
A 4 × 8 ft bed is the most common raised bed size there is, so it’s worth flagging that a widely-read gardening article gets its figures badly wrong.
| 4 × 8 ft bed (32 ft²) | Published | Correct | Error |
|---|---|---|---|
| 2 inch layer | 14 ft³ / 0.5 yd³ | 5.3 ft³ / 0.20 yd³ | 2.6× too high |
| 4 inch layer | 27 ft³ / 1.0 yd³ | 10.7 ft³ / 0.40 yd³ | 2.5× too high |
The arithmetic is easy to check: 32 ft² × 2 in ÷ 324 = 0.20 cubic yards. Fourteen cubic feet spread over 32 ft² would be 5¼ inches deep, not two.
What makes it notable is that the same article gives the correct formula — area × depth ÷ 324 — and a correct worked example for a 100 ft² bed a few paragraphs later. The formula is right; that one example isn’t. Follow it and you’d buy two and a half times the compost you need, and if you actually spread it all on a raised bed you’d be well into the over-application territory described above.
Finished vs unfinished compost
Compost that hasn’t finished breaking down still has active microbial decomposition running. Those microbes need nitrogen, and they take it from the surrounding soil — so instead of feeding your plants, fresh compost temporarily starves them. The effect reverses once decomposition completes, but that can take a full season.
Finished compost is dark and crumbly, smells of earth rather than anything sour or ammoniac, and you can’t identify what went into it. If you can still see recognisable food scraps or bright green material, give it longer.
Unfinished compost can also carry viable weed seeds, if the pile never reached the temperature needed to kill them. Commercially produced compost is generally hot-composted and screened; home piles vary enormously.
Compost ordering mistakes to avoid
- Topdressing a lawn thicker than ½ inch. It smothers the turf.
- Filling raised beds with pure compost. Too rich, too wet, and it slumps.
- Ignoring the tilling depth. The same 2 inches gives 25% or 33% depending on how deep you work it.
- Applying the new-bed rate every year. 3 inches is a one-off correction, not annual maintenance.
- Using unfinished compost. It ties up nitrogen and can carry weed seeds.
- Forgetting settling in raised beds. Order about 25% over.
- Leaving compost on the surface of a bed. It dries out and does little; work it in.
Frequently asked questions
How much compost do I need?
Multiply area in square feet by depth in inches and divide by 324 for cubic yards. A 20 × 10 ft vegetable garden at 2 inches needs 1.23 cubic yards, or about 35 one-cubic-foot bags including waste.
How deep should compost be?
A quarter inch for lawn topdressing, 1½ to 2 inches for established garden beds, 2 inches for vegetable gardens in spring, and 3 to 4 inches when establishing a new bed on poor soil. Drop back to 1 to 2 inches annually after the first year.
How much does a cubic yard of compost cover?
1,296 square feet at a quarter inch, 648 at half an inch, 324 at one inch, 162 at two inches and 108 at three inches. A single cubic yard goes a very long way as a lawn topdressing and not far at all as a new-bed amendment.
How much compost for a 4×8 raised bed?
A 2 inch layer over a 4 × 8 ft bed (32 ft²) is 5.3 cubic feet, or 0.20 cubic yards — about 6 bags. For filling the bed rather than topping it, use a blend of roughly 30% compost with topsoil and a drainage amendment, and allow around 25% extra for settling.
How much compost should I mix into soil?
Aim for 20 to 33% compost by volume in the amended layer. Spreading 2 inches and tilling into the top 6 inches gives 25%, because the mixed layer ends up 8 inches deep. If you till to a different depth, the spread depth changes — use the percentage mode in the calculator.
How much compost to topdress a lawn?
A quarter inch, which is 0.77 cubic yards per 1,000 square feet. Half an inch is the absolute maximum — beyond that you bury the grass crowns and thin the turf. Apply a quarter inch twice a year rather than a heavier single application.
Can you use too much compost?
Yes. Repeated heavy applications build up phosphorus, which soil holds tightly and which interferes with iron, zinc and manganese uptake. Manure-based composts can also raise soluble salts. Use 3 to 4 inches once when establishing a bed, then 1 to 2 inches annually.
How much does a cubic yard of compost weigh?
Around 1,100 lb as a planning figure, but it ranges from 800 to 1,350 depending on moisture and screening. The reliable anchor is the bag — a standard 1 cubic foot retail bag weighs about 40 lb, which is 1,080 lb per cubic yard.
How many bags of compost in a cubic yard?
27 bags at the standard 1 cubic foot size. For larger bags it’s 18 at 1.5 ft³, 13.5 at 2 ft³ and 9 at 3 ft³. Bulk becomes cheaper than bags at around one cubic yard, though delivery minimums vary.
Can I fill a raised bed with just compost?
No. Compost is an amendment rather than a growing medium — on its own it’s too rich, holds too much water, and slumps badly as it continues to decompose. Blend 25 to 40% compost with topsoil plus something for drainage, and allow about 25% for settling.
What’s the difference between compost and mulch?
Compost is decomposed organic matter worked into soil to improve structure and fertility. Mulch sits on the surface to retain moisture, suppress weeds and moderate temperature. Compost can be used as a mulch, but bark or wood chips are better at it — and mulch that hasn’t broken down shouldn’t be dug in.
When should I apply compost?
Spring before planting for vegetable beds, and autumn after harvest to work over winter. Lawns are best topdressed in spring or early autumn, ideally straight after core aeration so the compost falls into the holes. Finished compost can go on at any time without burning roots.
Sources and method
- Application rates cross-checked against six published horticultural and calculator sources
- Percentage-by-volume relationships derived geometrically and verified in both directions against standard “X inches into the top Y inches” guidance
- Density anchored to standard US retail bag weight of approximately 40 lb per cubic foot
- Phosphorus accumulation and salt build-up from over-application are documented in extension-service guidance; a soil test is the reliable way to check
Related calculators
How this compost calculator works out its numbers
Volume is area times depth, divided by 27 for cubic yards, with a waste allowance on top. The application dropdown sets the depth from published rates; custom mode takes your own figure.
Percentage mode inverts the relationship. Because tilling compost into soil produces a mixed layer deeper than the original tilling depth, the compost fraction is spread depth divided by the sum of both. Solving for spread depth gives till depth × percentage ÷ (100 − percentage). The calculator shows the resulting mixed-layer thickness so you can check it.
Raised bed mode calculates the full fill volume, applies the settling allowance, then takes your compost share of that total — reporting both the compost quantity and the total mix volume, since you need to source the rest as well.
Compost varies more than most materials. Density, nutrient content and maturity differ substantially between suppliers and between batches from the same supplier. These are planning figures. For anything beyond routine garden use — and certainly before applying heavily year after year — a soil test tells you what your soil actually needs, which no calculator can.