Compost Calculator
Area to cover
Check the size printed on your bag.
Optional. Loads the soil USDA maps at a US address to prefill and tailor the result.
How much compost to use
Colorado State University’s rates for new lawns, new gardens and beds you dig every year[1], and the top-dressing ceiling for beds you don’t dig and lawns[2][3]:
| Use | Compost depth | Work it into | Cubic yards per 1,000 sq ft |
|---|---|---|---|
| New lawn (one-time) | 1–2 in | the top 6 in | 3.1–6.2 |
| New vegetable or flower garden | 3–4 in | the top 8–12 in | 9.3–12.3 |
| Beds you dig every year | 0.25 in a year | the top 8–12 in | 0.8 |
| Established beds and lawns (top-dress) | up to 1 in a year | nothing: leave it on top | 3.1 at most |
CSU’s rates assume plant-derived compost, or compost a soil test shows is low in salts; compost made with manure or biosolids needs substantially lower rates. If you work it in deeper or shallower than listed, adjust the rate to match, and let an annual soil test tell you whether you need compost at all[1].
3 cubic yards (81 cubic feet) covers about 1,000 square feet 1 inch deep[1].
What compost does for your soil
Organic matter improves how much water and nutrients a sandy soil holds; in clay, it helps bind the tiny clay particles into larger chunks[1].
Choosing compost for your soil type
For sandy soils, Colorado State University recommends well-decomposed material such as finished compost or aged manure; for clay soils, fibrous material such as composted wood chips or straw. In a new clay bed, work the compost in to the full depth: CSU warns that shallower mixing can leave roots shallow[1]. Enter an address in the calculator to highlight the line for the soil USDA maps there.
Don’t overdo it
Adding a lot of amendment in one season can backfire: Colorado State University lists high salts, high nitrogen, soil that holds too much water, and ammonia that burns roots and leaves. Continual high rates add excess nitrogen, phosphorus and potassium, groundwater contamination and micronutrient imbalances[1]. Oregon State adds that too much compost stunts plant growth and may pollute water[4].
Checking compost quality
CSU’s guidelines for organic amendments: dark brown to black, earthy with no ammonia smell, particles under 1 to 2 inches (under 3/8 inch for lawn top-dressing), less than 1% foreign material, and if it’s lab-tested, a carbon-to-nitrogen ratio under 20:1, with 12:1 to 15:1 desirable[1].
Worked example
A new 10 × 20 ft vegetable garden (200 sq ft) at 3 to 4 inches: 50 to 66.7 cu ft, about 1.9 to 2.5 cubic yards, or 34 to 45 bags of 1.5 cu ft[1][5]. Oregon State University Extension’s own example, 7.5 cubic yards covering 800 sq ft about 3 inches deep, is the same arithmetic[4].
Ways people use this calculator
- Preparing a new lawn.
- Building a new vegetable or flower bed.
- The yearly top-up for beds you dig, or a top-dressing for beds you don’t.
- Improving clay or sandy soil before planting.
How we calculate this
Volume is area × compost depth, at the low and high ends of the rate you pick, with unit factors from NIST[5]. Bags are rounded up. Conversions: 1 yd³ = 27 ft³ = 0.764555 m³; 1 ft³ = 28.3168 L. Our texture groups for the advice line: sandy = sand, loamy sand, sandy loam; clay = clay loam, silty clay loam, sandy clay, silty clay, clay; loamy = loam, silt loam, silt, sandy clay loam.
Frequently asked questions
How much compost do I need per square foot?
At 1 inch deep, 1 cu ft covers 12 sq ft[4] and 1 cubic yard covers 324 sq ft[5].
How many bags of compost make a cubic yard?
It depends on bag size: 27 bags of 1 cu ft, 18 of 1.5 cu ft, 13.5 of 2 cu ft, or 36 of 0.75 cu ft[5].
How much compost should I add to clay soil?
In a new bed, use the same rates, choose fibrous compost and work it in to the full depth: CSU warns that shallower mixing in clay can leave roots shallow[1]. On an established bed or a lawn you can’t dig, top-dress up to an inch a year and let it work in[2][3].
Can I add too much compost?
Yes. In one season, Colorado State University lists high salts, high nitrogen, soil that holds too much water, and ammonia that burns roots and leaves; continual high rates bring nutrient excesses and groundwater contamination[1].
Is manure compost different?
Yes. Compost made with manure or biosolids, or tested high in salts, needs substantially lower rates; let a soil test guide you[1].
Sources
- Colorado State University Extension, Soil Amendments (GardenNotes #241; reviewed Sept 2024)
- Ask Extension, Should I top dress with compost or a combo of compost & soil? (2024)
- UC Master Gardeners (Tulare/Kings), Lasagna Gardening aka No-till Gardening (K. Dressel; 2025)
- Oregon State University Extension, How to use compost in gardens and landscapes (EM 9308, 2021; reviewed 2025)
- NIST, Guide for the Use of the International System of Units (SP 811), Appendix B.8: Factors for Units Listed Alphabetically
Marko Visic · MPharm, BSc Physics · Last reviewed November 13, 2026
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