Compost Calculator
Find out exactly how much compost you need — for a topdressing, a new bed, or blending into existing soil — in cubic yards, bags and cost.
Your area
2–3 in topdressing, 3–4 in garden bed, 6–12 in new raised bed fill.
20–30% compost is typical for amending existing soil.
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Volume breakdown
11.0 cu ft needed
Bagged vs bulk delivery
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Coverage by depth
The short answer
Multiply your area in square feet by the compost depth in feet to get cubic feet, add 5–10% waste, then divide by your bag size or by 27 for cubic yards. If you’re amending existing soil rather than filling from empty, use a tilling depth and a compost percentage (commonly 20–30%) instead of a full-depth fill.
Compost depth by project type
| Project | Typical depth | Approach |
|---|---|---|
| Lawn topdressing | 0.25–0.5 in | Full-depth fill |
| Amending an existing bed | 6–12 in tilled, 20–30% compost | Blend percentage |
| Established garden bed topdressing | 2–4 in | Full-depth fill |
| New raised bed fill | 6–12 in | Full-depth fill (blended with other media) |
| Potting mix component | Varies by recipe | Blend percentage |
A brand-new raised bed is usually filled with a full blend of compost and other media rather than tilled into existing ground, so it’s treated as a full-depth fill in this calculator.
How to calculate compost needed
The volume depends on whether you’re filling an empty space or blending compost into soil that’s already there.
- Find the area. Length × width for a rectangle, or π × (diameter ÷ 2)² for a circle, in square feet.
- Choose your method. Full-depth fill for topdressing or new beds, or a blend percentage for amending existing soil.
- Multiply to get volume. Area × effective compost depth gives cubic feet.
- Add waste. 5–10% covers settling and uneven ground.
- Convert to your order unit. Divide by your bag’s cubic feet and round up, or by 27 for a bulk cubic yard order.
Worked example
A 10 ft × 4 ft garden bed, topdressed with 3 inches of compost, using 1 cu ft bags:
- Area10 × 4 = 40 sq ft
- Volume40 × 0.25 = 10.0 cu ft
- Plus 10% waste11.0 cu ft
- Divide by bag size11.0 ÷ 1 = 11.0
- Round up11 × 1 cu ft bags
That’s about 0.4 cubic yards — small enough that bags are the practical choice over a bulk delivery.
Bagged compost vs bulk delivery
| Bagged compost | Bulk delivery | |
|---|---|---|
| Best for | Small beds, containers, under ~1 cu yd | Larger beds and lawns, 1 cu yd and up |
| Handling | Carry and pour bag by bag | Dumped in one spot, often needs a wheelbarrow to move |
| Consistency | Often finer-screened and consistent | Can vary more between loads |
| Minimum order | None — buy what you need | Often around 1 cu yd, may include delivery fees |
| Typical cost per yard | Usually higher per yard | Usually lower per yard at volume |
For a container bed or small planter, bags are simple and there’s no minimum order. For a lawn topdressing or a large bed, get a bulk delivery quote once you’re approaching about a cubic yard.
Applying compost: a quick checklist
- Clear the areaRemove debris and, for topdressing, mow the lawn short first so compost reaches the soil.
- Spread evenlyUse a rake or spreader to distribute compost in a consistent layer across the area.
- Work it in if amendingTill or fork compost into the top layer of soil rather than leaving it sitting on the surface.
- Water it inA light watering helps settle compost into the soil and kick-starts microbial activity.
- Avoid over-applicationStick to a recommended depth or blend percentage rather than piling on extra “to be safe.”
- Reassess seasonallyMost beds benefit from a fresh application once or twice a year rather than one large dose.
Common mistakes
- Filling entirely with compost when amending soilFor in-ground beds, 100% compost holds too much moisture and can throw off nutrient balance — blend it into existing soil instead.
- Confusing compost with topsoilCompost is a soil amendment, not a bulk fill medium — for filling large volumes cheaply, topsoil or a blended mix is usually more appropriate.
- Skipping the waste allowanceSettling after watering and spreading can leave you short if you order exactly to the calculated volume.
- Applying too thick on lawnsA layer thicker than about half an inch on turf can smother grass rather than feed it.
- Buying bagged for a large jobBagged compost is convenient for small areas but gets expensive fast — price a bulk delivery once you’re near a cubic yard.
- Not accounting for delivery minimumsSome suppliers charge extra below their minimum order — check before finalizing a bulk order.
What this calculator assumes
Adjust your result if your project differs from these.
- Compost density of about 40 lb per cubic foot (roughly 1,080 lb per cubic yard)
- Rectangular area treated as length × width; circular area treated as a true circle
- Blend mode multiplies area × tilling depth × compost percentage, not a full-depth fill
- Bagged vs bulk guidance uses a roughly 1 cubic yard rule of thumb, not a supplier quote
- Prices are whatever you enter; they are not live market rates
Common questions
How much compost do I need for a garden bed?
Multiply the bed’s area in square feet by the depth in feet to get cubic feet, add 5 to 10 percent waste, then divide by your bag size or by 27 for cubic yards. A 2 to 4 inch layer is typical for an established bed.
What is a good compost to soil ratio for raised beds?
For amending existing soil, 20 to 30 percent compost by volume mixed into the top 6 to 12 inches is a common ratio. A brand-new raised bed is often filled with 100 percent compost blend rather than a percentage mix.
How deep should I topdress my lawn with compost?
A thin layer of about a quarter to half an inch is standard for lawn topdressing, applied once or twice a year and worked in lightly.
How much does a cubic yard of compost weigh?
Compost commonly weighs around 1,000 to 1,100 lb per cubic yard when moist, roughly 40 lb per cubic foot, though exact weight varies with moisture content and material.
Should I buy compost in bags or bulk?
Bags are convenient for small beds or containers, generally under about a cubic yard. Beyond that, bulk delivery from a local supplier is usually far more cost effective.
What is the difference between compost and topsoil?
Compost is decomposed organic matter rich in nutrients, used to amend and feed soil. Topsoil is a bulk growing medium, often a blend of native soil, sand and some organic matter, used to fill or build up an area.
How often should I add compost to my garden?
Once or twice a year is common for most garden beds, typically at the start of a growing season and again in fall, depending on how heavily the bed is planted.
Can I use too much compost?
Yes. Very high compost ratios can hold excess moisture, alter soil pH, or over-supply certain nutrients, so sticking to a recommended blend percentage is generally safer than filling entirely with compost for in-ground beds.
How many 1 cubic foot bags are in a cubic yard?
There are 27 cubic feet in a cubic yard, so it takes 27 bags of 1 cubic foot compost to equal one cubic yard.
Is bagged compost as good as bulk compost?
Quality depends more on the producer and feedstock than on packaging. Bagged compost is often screened finer and more consistent, while bulk compost can vary more but costs less per cubic yard.
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Compost Estimate
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Project
| Area | — |
| Method | — |
| Waste allowance | — |
Total shopping list
| Cubic feet | — |
| Cubic yards | — |
| Bags (1 cu ft) | — |
| Weight | — |
| Estimated cost | — |
Notes
Estimate generated by materialcalculate.com. Assumes a compost density of about 40 lb per cubic foot. Confirm quantities and pricing with your local supplier before buying.