Compost Calculator

Enter your area and depth to get compost volume, weight, bag counts, and load estimates for lawns, garden beds, and raised beds.

Lawn: 0.25–0.5 in, beds: 1–3 in

Covers settling, spillage, and uneven surfaces.

Typical compost: 400–800 kg/m³

Common: 1.5–3.0 ft³ in US retail

How Much Compost Do You Need?

Compost volume is area multiplied by depth. The challenge is knowing the right depth for your application, because too little compost applied to a lawn or garden bed won't deliver the soil improvement you're after, and too much can actually harm plant roots by creating an anaerobic layer. The application presets in the calculator give you a sensible starting point for each use case, and you can adjust the depth from there.

Weight matters when you're ordering bulk delivery or loading a trailer. Compost density varies considerably depending on moisture content and material composition. Fresh screened compost typically runs around 400 to 500 kg/m³ when dry and can approach 800 kg/m³ when wet. If your supplier gives you a specific density for their product, enter it in the density field for a more accurate weight estimate.

Recommended Depths by Application

Application Depth (in) Depth (mm) Notes
Lawn topdressing 0.25–0.5 in 6–12 mm Rake in after core aerating for best results
Flower / garden bed 1–3 in 25–75 mm Incorporated into top 6–8 in of soil
Vegetable bed 2–4 in 50–100 mm Higher organic matter improves drainage and nutrient retention
Raised bed fill (pure compost) 4–8 in 100–200 mm Often blended 50/50 with topsoil or potting mix
Tree / shrub planting 2–3 in 50–75 mm Applied as mulch ring, kept away from trunk

Bag Coverage Quick Reference

Use this as a cross-check against the calculator output. Coverage per bag at a given depth is calculated as bag volume divided by depth, both in consistent units.

Bag Size Coverage @ 1 in Coverage @ 2 in Coverage @ 3 in
1.5 ft³ 18 ft² 9 ft² 6 ft²
2.0 ft³ 24 ft² 12 ft² 8 ft²
3.0 ft³ 36 ft² 18 ft² 12 ft²
40 L (metric) 4 m² @ 10 mm 2 m² @ 20 mm 1.3 m² @ 30 mm

Compost vs Topsoil: Which One Do You Need?

Compost and topsoil are often confused but serve different purposes. Compost is a finished organic material that improves soil structure, adds nutrients, and supports microbial activity. It breaks down over time and is not a structural fill material. Topsoil is a broader term covering the upper layer of native or screened soil and is used when you need to raise grade, fill low spots, or establish a new planting area from scratch. For most garden improvement work, compost incorporated into existing soil is the right choice. For new beds, a 50/50 mix of topsoil and compost typically gives the best balance of drainage, structure, and fertility.

Bags vs Bulk Delivery

Bagged compost works well for small areas up to about 50 square feet at 2 inches deep (roughly 8 to 10 cubic feet). Beyond that, bulk delivery by the cubic yard becomes more cost-effective. A cubic yard of compost delivered covers about 160 square feet at 2 inches deep, and bulk pricing is typically 30 to 60% cheaper per unit volume than bagged product. If the calculator shows more than 15 to 20 bags, get a bulk delivery quote before ordering bags.

Frequently Asked Questions

For lawn topdressing, 1/4 to 1/2 inch (6 to 12 mm) is the standard depth. More than that can smother grass and cause yellowing. Apply after core aerating so the compost works into the holes and makes direct contact with the soil. A thin, even layer raked into the turf gives the best results.
Enter your raised bed dimensions and a depth of 6 to 8 inches in the calculator to get the exact figure. As a rough guide, a 4 x 8 foot bed at 6 inches deep needs about 0.6 cubic yards of material. Most raised bed fills are a blend of compost, topsoil, and potting mix rather than pure compost, which would be too rich and may cause nutrient burn.
Moisture content is the main reason. Dry compost can weigh as little as 400 kg/m³, while freshly screened, wet compost can approach 900 kg/m³. The default density of 600 kg/m³ is a reasonable mid-point for typical screened garden compost. If you're ordering bulk and need to know trailer capacity, ask your supplier for the specific density of their product.
Once your order exceeds about 1 cubic yard (roughly 10 to 15 bags), bulk delivery almost always works out cheaper. The crossover point depends on local pricing, but bulk compost typically costs 30 to 60% less per cubic foot than bagged product. The trade-off is that bulk delivery requires vehicle access and somewhere to pile the material close to where you need it.

Compost Project Planning Checklist

Before ordering, confirm delivery access, whether you need screened or unscreened compost, and check that the application timing suits your garden schedule.

Download Checklist (PDF)

Planning use only. See Methodology and Data Sources. View all project checklists →

Related Calculators

Topsoil Calculator Mulch Calculator Gravel Calculator Aggregate Calculator Stone Calculator Sand Calculator

Disclaimer

These calculations are general planning estimates only. Actual compost quantities may vary due to moisture content, material density, settling, and application method.

Depth recommendations are general guidelines. Specific soil conditions, plant types, and site requirements may alter the amount of compost needed. Always confirm quantities with your supplier before ordering.

See our Methodology and Data Sources for details.