How this instrument works
A raised garden bed is, geometrically, an open-topped box, and the soil needed to fill it is exactly the box's volume: length times width times depth. Because bulk soil, compost and garden-blend mixes are sold by the cubic yard rather than the cubic foot, this instrument carries the extra step of dividing that raw volume by 27 — the number of cubic feet in one cubic yard, since a yard measures 3 feet on each side.
Fill depth is not one-size-fits-all. University of Maryland Extension's guidance on raised beds recommends at least 8 inches of depth for shallow-rooted crops like leafy greens, beans and cucumbers, and 12 to 24 inches for deeper-rooted crops like tomatoes, peppers and squash — a range wide enough that entering the wrong depth changes the required volume by two to three times over for the same footprint. Deeper beds generally support healthier root systems and hold moisture more evenly, at the cost of needing proportionally more soil to fill.
This calculation fills an empty box from the bottom up, which is a different job from either of this site's other soil instruments. The mulch calculator top-dresses an already-planted bed with a shallow 2-to-4-inch surface layer, and the potting soil calculator fills individual containers from bagged mix measured in quarts — both shallower, smaller-scale jobs than bulk-filling a raised bed's full depth from delivered soil measured in cubic yards.
- Measure the bed's inside dimensions and enter them into Bed length (ft) and Bed width (ft).
- Enter the depth you plan to fill into Soil depth (ft) — convert inches first if needed, since 12 inches equals 1 foot.
- Read Soil needed (cubic yards) beneath all three fields; it recalculates instantly as any dimension changes.
- Filling several identical beds? Multiply the single-bed result by the bed count, or add each bed's volume separately if depths differ.
- Order slightly more than the exact figure shown — bulk suppliers commonly round deliveries to the nearest half yard, and settling after the first watering can reduce the fill level.
Worked example — an 8 ft x 4 ft bed filled 1 ft deep
An 8-foot by 4-foot raised bed, a common size for a first vegetable garden, is filled a full 1 foot deep. Enter 8 into Bed length (ft), 4 into Bed width (ft), and 1 into Soil depth (ft). The box's volume is 8 x 4 x 1 = 32 cubic feet, and dividing by 27 cubic feet per cubic yard gives Soil needed (cubic yards) of 1.1852.
That comes out to just over a full cubic yard for one standard-size bed — a useful anchor figure, since two identical 8 ft x 4 ft beds at the same 1-foot depth would need roughly 2.37 cubic yards combined, close to two and a third yards rather than a clean double number because of the repeating decimal in the single-bed figure.
Questions
How deep should a raised bed be filled with soil?
It depends on what you're growing. University of Maryland Extension recommends at least 8 inches for shallow-rooted crops like leafy greens, beans and cucumbers, and 12 to 24 inches for deeper-rooted crops like tomatoes, peppers and squash. Entering a shallower depth than your crops need will understate the soil you actually should buy, even though the volume math itself is exact for whatever depth you enter.
Why does the formula divide by 27?
Because one cubic yard equals 27 cubic feet — a cubic yard measures 3 feet on every side, and 3 x 3 x 3 is 27. Bulk soil suppliers price and deliver by the cubic yard, so this division converts the box's raw cubic-foot volume into the unit you'll actually be quoted and billed in.
My bed height is given in inches — do I need to convert it first?
Yes, this field expects feet, so convert before entering. Divide the inch measurement by 12 — a 6-inch fill depth becomes 0.5 ft, an 18-inch depth becomes 1.5 ft. Entering 6 directly instead of 0.5 would compute the volume for a 6-foot-deep bed, twelve times too much soil.
How is this different from the mulch calculator on this site?
The mulch calculator spreads a shallow 2-to-4-inch layer over the surface of a bed that's already planted, for weed suppression and moisture retention. This calculator fills an empty box from the bottom up, typically 6 to 24 inches deep, before anything is planted — a much larger volume for a fundamentally different stage of the same garden.
Should I fill a raised bed with pure topsoil, or a blended mix?
Most raised-bed guidance recommends a blend — commonly topsoil combined with compost and sometimes a soilless component — rather than pure topsoil alone, since blends drain and hold nutrients better in a contained space than dense topsoil by itself. This calculator only computes total volume; the mix ratio is a separate decision worth checking against extension guidance for your specific crops.