Measure the inside of the bed
Measure the length and width between the inside faces of the boards. Then measure from the bottom to the intended soil level. Use several readings if the bed or ground is uneven. The board height is not automatically the soil depth: room at the rim and existing material both reduce the space you need to fill.
Example: an 8 × 4 foot bed, filled 12 inches deep
Twelve inches is one foot, so 8 × 4 × 1 = 32 cubic feet. Divide by 27 to get about 1.185 cubic yards. The same volume is about 906.1 liters. In the calculator, choose feet and enter 8, 4 and 1; alternatively choose inches and enter 96, 48 and 12. All three dimensions must use the selected unit.
Subtract space already occupied
If a uniformly filled layer occupies 4 inches of a planned 12-inch fill, calculate the remaining 8 inches. Uneven layers make this an approximation. The calculator does not decide whether a filler is suitable for your plants or predict how organic material will settle.
Compare bags with bulk soil
For 32 cubic feet, 2-cubic-foot bags give exactly 16 bags before any optional allowance. Bulk suppliers may quote cubic yards: use 32 ÷ 27 rather than treating cubic feet and cubic yards as equal. Delivery and minimum-order charges need to be checked separately.
Choose the growing medium separately
Volume describes space, not the right growing mix. University of Maryland Extension discusses soil and compost combinations for raised beds. Check your existing soil, intended crops and product instructions instead of assuming the entire bed should be filled with pure compost.
Frequently asked questions
Can I enter feet and inches together?
Choose one measurement unit for all dimensions. Convert 12 inches to 1 foot, for example, or enter everything in inches.
Is an allowance required?
No. It is an optional planning choice, not a prediction of settling.