Compost Calculator
Work out how many 1 cubic foot bags of compost you need to top-dress a bed at your depth, plus total cost at your price per bag.
How this calculator works
Compost is most often bought bagged for topping off beds and lawns, so this calculator converts your area and depth directly into bag count using the standard 1 cubic foot bag size.
Formula: Cubic yards = (Length ft × Width ft × Depth ft) ÷ 27. Bags = (Cubic yards × 27) ÷ 1.
Worked example
A 10 ft × 10 ft bed at a 2-inch top-dress depth:
- Depth in feet: 2 ÷ 12 = 0.167 ft
- Cubic yards: (10 × 10 × 0.167) ÷ 27 ≈ 0.617 yd³
- Bags: (0.617 × 27) ÷ 1 ≈ 16.67 bags
- At $5/bag, that’s $83.33.
Coverage reference
| Bed size | Depth | Bags needed (1 cu ft) |
|---|---|---|
| 4 ft × 8 ft | 2 in | ~5.3 bags |
| 10 ft × 10 ft | 2 in | ~16.7 bags |
| 20 ft × 10 ft | 3 in | ~50.0 bags |
How to measure your beds
Measure each bed’s length and width at ground level, not around individual plants, and use the top-dress thickness you’re aiming for — usually 1-2 inches for maintenance, more if you’re building up new soil before planting. Measure each bed separately if depths vary across your yard, since a thin lawn top-dress and a thick new-bed fill need very different amounts.
Frequently asked questions
How many bags of compost do I need per bed?
This calculator assumes a standard 1 cubic foot bag, the most common size sold at garden centers. Enter your bed's area and target depth, and it converts the resulting cubic yardage directly into bag count.
How deep should a compost top-dressing be?
1-2 inches is typical for an annual top-dress applied to an existing bed or lawn. If you're building new soil or heavily amending a bed before planting, working in a thicker 3-4 inch layer is more common.
Should I till compost into the soil or leave it on top?
For established beds and lawns, top-dressing without tilling is usually preferred — it protects soil structure and existing root systems while microbes and worms work the compost down naturally. For new beds being prepared before planting, tilling it into the top 6-8 inches of soil is standard.
Is bulk or bagged compost cheaper?
Bulk compost delivered by the cubic yard is typically much cheaper per unit once you need more than 1-2 cubic yards (roughly 27-54 bags), but bagged compost is more practical for small beds or container gardening where you don't want a full truckload delivered.
How often should I add compost to a garden bed?
Once or twice a year is standard for most vegetable and ornamental beds — commonly in spring before planting and again in fall. Heavy feeders or depleted soil may benefit from more frequent, lighter applications instead.
Why isn't waste included on this calculator?
Compost is a top-dressing material applied by eye more than to a precise structural depth, and bags are easy to buy one extra of if you come up short, so this calculator skips the waste toggle rather than padding a number most people round up manually anyway.
Estimates only. Verify quantities with your supplier before purchasing.