Compost Calculator

The calculator starts with the finished layer you want in the bed:

finished cu ft = length ft x width ft x depth in / 12

It then increases that amount for settling:

starting cu ft = finished cu ft / (1 - shrinkage percent / 100)

That starting volume is useful when you are buying loose compost, opening bags, or blending raw compost materials. Cubic yards are calculated by dividing cubic feet by 27, and bag count is rounded up so you do not run short because of a partial bag.

Worked example

A 12 ft x 4 ft raised bed with a 2 inch compost layer has:

• Area: 12 x 4 = 48 sq ft
• Finished compost: 48 x (2 / 12) = 8 cu ft
• With 25% shrinkage: 8 / 0.75 = 10.67 cu ft starting material
• Cubic yards: 10.67 / 27 = 0.395 cu yd
• 1 cu ft bags: ceil(10.67 / 1) = 11 bags
• At 45 per cubic yard: 0.395 x 45 = 17.78
• With 35% greens: 3.73 cu ft greens and 6.93 cu ft browns

Typical planning ranges

Greens and browns

Greens are nitrogen-rich materials such as fresh grass clippings, coffee grounds, fruit scraps and vegetable trimmings. Browns are carbon-rich materials such as dry leaves, straw, wood chips, sawdust in small amounts and plain shredded cardboard. Many gardeners aim for roughly one part greens to two parts browns by volume, then adjust by smell and texture. If the pile smells sour or wet, add browns. If it stays dry and slow, add water and a little more greens.

For purchased finished compost, the greens and browns split is only a planning aid. Finished compost is already decomposed enough to use as a soil amendment, so you normally order by finished volume, texture and quality rather than by raw ingredient ratio. Compost is not a complete raised-bed soil by itself; blend it with mineral soil or a planting mix so roots get structure, drainage and nutrients in balance.