Compost Calculator
A compost calculator multiplies length × width × depth ÷ 12 to get cubic feet, one 1 cu ft bag each. A 10 × 10 ft bed at 2 in needs 16.67 cu ft, so 17 bags.
What your order looks like
A few bags or one pickup trip will cover it.
0.62 yd³
5 wheelbarrow trips (4 cu ft each)
1 pickup load (2 cubic yards each)
Rules of thumb: A wheelbarrow trip of light material is 4 cubic feet. A pickup load of light material is 2 cubic yards.
How much compost do I need?
A compost calculator turns your bed’s size and the depth you want into cubic feet, then into bags. Cubic feet = length × width × (depth in inches ÷ 12). This one assumes 1 cubic foot bags, so the cubic feet are the bag count, rounded up to a whole bag. A 10 × 10 ft bed covered 2 in deep needs 16.67 cu ft, so you buy 17 bags. At $5 a bag that is $85.
Compost is light and fluffy, and a little goes a long way on top of a bed. Getting the depth right matters more than the exact bag count. Too thin and it barely feeds the soil. Too thick on a lawn and it smothers the grass.
The formula in plain words
- Area = length × width, in square feet.
- Depth in feet = depth in inches ÷ 12.
- Cubic feet = area × depth in feet. The calculator gets there through cubic yards (÷ 27) and back again (× 27), which gives the same number.
- Bags = cubic feet ÷ 1 cu ft per bag, rounded up.
- Cost = bags × price per bag.
Rounding happens before pricing, because a store will not sell you 0.67 of a bag.
Worked examples
- Mulching a vegetable bed: 10 × 10 ft, 2 in deep (calculator defaults)
- Area: 10 × 10 = 100 sq ft
- Depth: 2 ÷ 12 = 0.167 ft
- Volume: 100 × 0.167 = 16.67 cu ft (0.62 yd³)
- Bags: 16.67 → 17 bags
- Cost: 17 × $5 = $85.00
- Topping up a 4 × 8 ft raised bed by 3 in
- Area: 4 × 8 = 32 sq ft
- Depth: 3 ÷ 12 = 0.25 ft
- Volume: 32 × 0.25 = 8 cu ft
- Bags: 8 bags, $40.00 at $5
Three beds that size need 24 bags, which is 24 ÷ 27 = 0.89 cubic yard. At that point a single bulk yard is worth pricing.
Compost needed per 100 sq ft
| Depth | Use | Cu ft per 100 sq ft | 1 cu ft bags |
|---|---|---|---|
| ¼ in | Lawn top-dressing | 2.08 | 3 |
| ½ in | Heavy lawn top-dress, seeding | 4.17 | 5 |
| 1 in | Light bed top-up | 8.33 | 9 |
| 2 in | Annual bed top-dress | 16.67 | 17 |
| 3 in | Amending a new bed | 25.00 | 25 |
A cubic yard (27 cu ft) covers about 324 sq ft at 1 in, 162 sq ft at 2 in, and 108 sq ft at 3 in.
How to measure your beds
- Rectangles: measure length and width at soil level.
- Curved or island beds: take the longest length and an average of three widths.
- Several beds: run each one and add the bags, or add the areas first if they share a depth.
- Depth: decide what you are doing. Top-dressing an established bed takes 1 to 2 in. Amending a new bed takes 2 to 3 in, worked in.
- Raised beds: a fresh fill is a different job. Use the raised bed soil calculator for the full depth.
Common mistakes
- Assuming every bag is 1 cu ft. Bags of 0.75, 1.5, and 2 cu ft are common. Divide the cubic feet by your bag size.
- Piling it against stems and trunks. Keep compost a couple of inches back from plant crowns and tree bark.
- Using compost in place of topsoil for a deep fill. Pure compost shrinks as it breaks down. For filling, blend it with soil from the topsoil calculator or buy a premixed garden soil.
- Spreading too thick on turf. More than about ½ in at once can bury grass blades. The lawn topdressing calculator is set up for those thin layers.
Bags or bulk?
Bags are clean and easy to carry, which suits a few beds. Bulk compost is sold by the cubic yard, and one yard replaces 27 bags. Divide your cubic feet by 27 to see how many yards you need. Then compare the bag cost with a delivered bulk price.
Frequently asked questions
How many bags of compost do I need per bed?
Multiply bed length × width in feet by the depth in inches ÷ 12 to get cubic feet. With 1 cu ft bags, that is the bag count, rounded up. A 10 × 10 ft bed at 2 in is 16.67 cu ft, so 17 bags; a 4 × 8 ft raised bed at 3 in is 8 cu ft, so 8 bags.
How deep should a compost top-dressing be?
About 1 to 2 in spread on top of an existing bed once or twice a year, and about ¼ in on a lawn. When building a new bed, people often work 2 to 3 in into the top several inches of soil. Each inch over 100 sq ft is 8.33 cu ft.
Should I till compost into the soil or leave it on top?
For established beds and lawns, leaving it on top is gentler on roots and soil structure; worms and rain carry it down over the season. For a new bed with poor soil, mixing it into the top 6 to 8 in gets organic matter into the root zone straight away.
Is bulk or bagged compost cheaper?
One cubic yard is 27 cu ft, which is 27 one-cubic-foot bags. At $5 a bag that is $135 of bags for one yard, so once a job passes about 1 yard, bulk delivery usually wins even after the delivery fee. Bags make sense for small beds and pots.
What if my compost bags aren't 1 cubic foot?
Read the volume on the bag and divide. The calculator assumes 1 cu ft per bag, so 16.67 cu ft becomes 17 bags. With 0.75 cu ft bags it is 16.67 ÷ 0.75 = 22.2, so 23 bags; with 1.5 cu ft bags it is 11.1, so 12 bags.
Why isn't waste included on this calculator?
Compost is spread by eye and does not get cut or compacted into a set structure, so a waste percentage adds little. The bag count is already rounded up to a whole bag, which gives a small buffer on every order.
Estimates only. Verify quantities with your supplier before purchasing.