How is soil volume calculated?

Volume is simply length times width times depth. The calculator converts every dimension to feet (or metres), multiplies them, and reports the result. One cubic yard equals 27 cubic feet, and one cubic foot equals about 28.3 litres, so the same volume is shown in all three units for convenience.

How deep should I fill a raised bed?

Most vegetables thrive with 10-12 inches (25-30 cm) of soil. Root crops like carrots and parsnips prefer 12 inches or more. Shallow beds of 6 inches work for lettuce and herbs. Deeper is generally better for drainage and root room, but it also means more soil to buy, which is exactly what this calculator quantifies.

Why does the bag count round up?

You cannot buy a fraction of a bag, and beds always settle and compact, so the calculator rounds up to the next whole bag. It is normal to have a little soil left over; gardeners use the surplus to top up beds after the soil settles a few weeks later.

What soil mix should I use?

A common raised-bed recipe is roughly one third compost, one third topsoil and one third aeration material such as coarse sand or perlite, though ratios vary. The calculator gives total volume; multiply by your chosen fraction to work out how much of each component to buy. Bagged raised-bed mixes are pre-blended if you prefer simplicity.

Does soil settle over time?

Yes. Fresh soil and compost are fluffy and compact noticeably in the first few weeks, dropping the level by an inch or more. Many gardeners fill slightly proud of the top, or hold back a bag or two to top up once the bed settles. Allow for this if you want the bed level with the frame long-term.

Raised Bed Soil Volume Calculator

The Raised Bed Soil Volume Calculator works out exactly how much soil or compost you need to fill a raised garden bed, and translates that into the number of bags to buy. It is built for gardeners filling a new bed who want to avoid the classic mistakes of ordering far too little or wasting money on excess.

How it works

Volume is the product of the three dimensions. The only subtlety is unit conversion — you usually measure a bed in feet and inches but buy soil by the cubic foot, cubic yard or litre:

volume (cu ft) = length(ft) × width(ft) × depth(ft)
cubic yards    = cubic feet / 27
litres         = cubic feet × 28.3168
bags needed    = ceil(cubic feet / bag size in cu ft)

The calculator converts inches to feet (divide by 12) or centimetres to metres internally, so you can enter mixed units naturally. Because you cannot buy part of a bag and soil settles, the bag count always rounds up.

Worked example and tips

A bed 8 ft long, 4 ft wide, filled 12 inches (1 ft) deep:

Volume = 8 × 4 × 1 = 32 cubic feet
= 32 / 27 ≈ 1.19 cubic yards
= 32 × 28.32 ≈ 906 litres
Using 2 cu ft bags: ceil(32 / 2) = 16 bags

Tips:

Fill 10-12 inches deep for most vegetables; root crops like 12 inches or more.
Buy a touch extra. Fresh soil settles and compacts in the first weeks, so a bed filled exactly level will drop below the frame. Hold back a bag to top up later.
For a custom mix, multiply the total volume by your chosen fractions of compost, topsoil and aeration material.
Ordering by the cubic yard in bulk is cheaper than bags for large beds — use the cubic-yard figure to compare quotes.