Compost Calculator
Cubic yards & bags of compost for beds and lawns
Amending soil, topdressing a lawn or filling a raised bed? Enter the area and depth to get the cubic yards and bags of compost you need. Below: how deep to go for each job, a raised bed chart, what a yard of compost weighs, and how much is too much.
How much compost do I need?
The right depth depends entirely on the job, and the range is wide — from a quarter inch on a lawn to ten inches in a raised bed.
| Job | Depth | Method |
|---|---|---|
| Topdressing an established lawn | ¼ to ½ in | Spread and rake into the canopy |
| Overseeding cover | ¼ in | Over the seed, not buried |
| Amending an existing bed | 1 to 2 in | Fork into the top 6 in |
| New bed from poor soil | 2 to 3 in | Work into the top 6 to 8 in |
| Mulching around plants | 1 to 2 in | Surface, kept off stems |
| Raised bed mix component | 30 to 50% of fill | Blended with topsoil |
Compost is an amendment, not a growing medium. Filling a bed with 100% compost is a common and expensive mistake: it holds too much water, slumps by a third within a season, and over-supplies nutrients. A blend of roughly half screened topsoil and half compost outperforms pure compost in almost every raised bed.
How much compost for lawn topdressing
Per 1,000 ft² of lawn, which is the unit lawn products are sold in. Compost coverage is driven entirely by depth, so a quarter inch goes four times further than an inch:
| Depth | Cubic yards | 1.5 ft³ bags | Notes |
|---|---|---|---|
| ¼ in | 0.77 yd³ | 14 bags | Standard annual topdress |
| ½ in | 1.54 yd³ | 28 bags | Maximum in one pass |
| 1 in | 3.09 yd³ | 56 bags | Too deep — will smother grass |
Half an inch is the ceiling for a single application. Any deeper and you bury the crowns, block light and suffocate the lawn you are trying to feed. If your soil badly needs improving, two half-inch applications a season apart work; one inch at once does not. Rake it in so the compost reaches the soil surface rather than sitting on the blades, and topdress in the growing season when the grass can knit through it.
How much compost to fill a raised bed
These figures are for filling to a 10-inch depth, which suits most beds once you allow a couple of inches of freeboard below the rim. Halve them if you are blending 50/50 with topsoil, which is what we would recommend.
| Bed size | Area | Total fill | 1.5 ft³ bags | If 50% compost |
|---|---|---|---|---|
| 3 × 6 ft | 18 ft² | 0.56 yd³ | 10 bags | 5 bags compost |
| 4 × 4 ft | 16 ft² | 0.49 yd³ | 9 bags | 5 bags compost |
| 4 × 8 ft | 32 ft² | 0.99 yd³ | 18 bags | 9 bags compost |
| 4 × 12 ft | 48 ft² | 1.48 yd³ | 27 bags | 14 bags compost |
A 4 × 8 bed taking almost exactly one cubic yard is a handy landmark, and it is also the point where bulk delivery starts to beat bags. For a dedicated fill blend, the raised bed soil calculator handles the whole mix rather than the compost alone.
Compost weight per yard, and bags versus bulk
Compost is light — this is the thing that surprises people who have ordered topsoil before. One cubic yard weighs roughly 1,080 lb, about half a ton, against 2,430 lb for screened topsoil. A yard of compost will fit in a half-ton pickup where a yard of soil will not.
| Quantity | Equals |
|---|---|
| 1 cubic yard | 27 ft³, about 1,080 lb (0.54 tons) |
| 1 cubic yard in bags | 18 × 1.5 ft³ bags |
| 1 cubic yard at ¼ in deep | covers 1,296 ft² |
| 1 cubic yard at ½ in deep | covers 648 ft² |
| 1 cubic yard at 2 in deep | covers 162 ft² |
Bags run roughly $4 to $8 each, so a cubic yard in bags is $70 to $145. Bulk compost is commonly $30 to $60 per cubic yard plus delivery. Past about half a yard — nine bags — bulk wins on price, and it wins on your back well before that.
Can you use too much compost?
Yes, and it is more common than under-applying. The failure modes are specific:
- Phosphorus build-up. Repeated heavy applications accumulate phosphorus that plants cannot use and rain carries into waterways.
- Waterlogging. High-organic soil holds moisture well past what most vegetables want.
- Slumping. Compost keeps decomposing. A bed filled with pure compost drops noticeably in one season.
- Smothered lawns. More than half an inch at once buries the crowns.
- Salt and immaturity. Uncomposted or hot material can burn roots. It should smell like forest floor, not ammonia.
For most beds, 1 to 3 inches worked in annually is plenty, and mature gardens often need less each year as organic matter builds. A soil test every few years is cheaper than guessing.
How this compost calculator works out its numbers
Volume is area × depth at 27 ft³ per cubic yard. Bag counts assume the standard US retail bag of 1.5 ft³, giving 18 bags to the cubic yard.
- Density is 40 lb/ft³, so a cubic yard is about 1,080 lb. Real compost ranges 30 to 60 lb/ft³ depending on moisture and how finished it is; wet, freshly turned material sits at the top of that range.
- Depth guidance reflects extension-service practice: ¼ to ½ in for lawn topdressing, 1 to 3 in worked in for beds. The ½ in ceiling exists because deeper applications smother turf crowns.
- Raised bed figures use a 10 in fill depth, allowing freeboard below the rim. Compost settles roughly 20 to 30% in the first season, so expect to top up.
- Bag sizes vary by brand from 1 to 2 ft³. Check the bag before scaling the counts here.
- Prices are US planning ranges gathered September 2026.
Reviewed September 2026.
Figures reviewed . Every worked example on this page is checked against the calculator above.
Compost Calculator: frequently asked questions
How thick should compost be on a garden bed?
2 to 3 inches worked into the top 6 to 8 inches for a new bed from poor soil, or 1 to 2 inches to top up an existing bed. Established beds usually need less each year as organic matter accumulates.
How many bags of compost in a cubic yard?
18 standard 1.5 cubic foot bags, since a cubic yard is 27 cubic feet. At $4 to $8 a bag that is $70 to $145 per yard, against $30 to $60 for bulk.
How much compost do I need for a 4x8 raised bed?
Filling a 4 × 8 ft bed to 10 inches takes 0.99 cubic yards, which is 18 bags. If you blend 50/50 with topsoil, as we would suggest, that is 9 bags of compost and 9 of soil.
How much compost for a lawn?
A quarter inch per application, which is 0.77 cubic yards or 14 bags per 1,000 ft². Half an inch is the absolute maximum in one pass; deeper smothers the crowns and kills the grass.
How much does a yard of compost weigh?
About 1,080 lb, roughly half a ton, at a typical 40 lb per cubic foot. That is less than half the weight of a cubic yard of screened topsoil, so it is much easier to haul yourself.
Can you use too much compost?
Yes. Excess builds up phosphorus, holds too much water and slumps as it continues decomposing. Keep beds to 1 to 3 inches worked in and lawn topdressing to half an inch at a time.
Can I fill a raised bed with only compost?
It is not a good idea. Pure compost holds too much water, over-supplies nutrients and drops by around a third in the first season as it breaks down. Blend it roughly half and half with screened topsoil.