Tool — Form GL-4

Raised Bed Soil Calculator

A 4 ft × 8 ft bed filled 12 inches deep takes 32 cubic feet of soil — about 1.19 cubic yards, or 22 standard 1.5 cubic foot bags. Change any dimension below for your own beds, and order roughly 10% extra: fresh mix settles noticeably in its first season.

Raised bed soil calculator

Form GL‑4
10–12 in suits most vegetables; 18 in+ for carrots and parsnips.

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Buying options
FormatQuantity needed
Or mix your own (Mel's-mix style blend, by volume)
IngredientShareVolume

Common bed sizes at a glance

Soil volume for standard raised beds
Bed size6 in deep12 in deep18 in deep
2 × 4 ft 4.0 cu ft 8.0 cu ft 12.0 cu ft
3 × 6 ft 9.0 cu ft 18.0 cu ft 27.0 cu ft
4 × 4 ft 8.0 cu ft 16.0 cu ft 24.0 cu ft
4 × 8 ft 16.0 cu ft 32.0 cu ft 48.0 cu ft
4 × 12 ft 24.0 cu ft 48.0 cu ft 72.0 cu ft
5 × 10 ft 25.0 cu ft 50.0 cu ft 75.0 cu ft

Once the bed is filled, work out how many plants fit it or lay the whole bed out square by square.

Where to get the mix

Requisition

The three components a bed mix needs. Buying these separately and blending is cheaper and better than any bagged "raised bed soil" for beds above about 16 cubic feet.

Compost

The fertility third of the mix. Blend several sources if you can — mushroom, municipal and composted manure all have different nutrient profiles.

Skip anything that still smells of ammonia; it is not finished.

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Coarse vermiculite

The aeration third. This is the component people skip to save money, and skipping it is exactly why beds turn to brick by year two.

Coarse grade, not the fine horticultural stuff sold for seed starting.

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Coco coir

The moisture-retention third. Raised beds drain faster than open ground and dry from the sides as well as the top. Coir is pH neutral, unlike peat.

Compressed blocks expand enormously — check the expanded volume, not the block size.

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Perlite

An alternative to vermiculite for the aeration third. Lighter and cheaper by volume; holds less water, which suits wetter climates.

Wear a mask when pouring — the dust is unpleasant.

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Common questions

How much soil does a 4x8 raised bed need?
A 4 ft × 8 ft bed filled 12 inches deep needs 32 cubic feet of soil, which is about 1.19 cubic yards, or 22 of the standard 1.5 cubic foot bags. At 6 inches deep it halves to 16 cubic feet.
How deep should a raised bed be?
Ten to twelve inches suits nearly every vegetable, because most feeder roots sit in the top foot of soil. Go to 18 inches or more if you want long carrots or parsnips. If the bed sits on open ground rather than a hard surface, roots will grow down into the native soil below, so a shallower bed goes further than you'd think.
Is it cheaper to buy soil in bags or in bulk?
Bulk delivery is usually much cheaper per cubic yard, but most suppliers have a minimum order of one to three yards plus a delivery fee. Below about one cubic yard — roughly 27 cubic feet, or one 4×8 bed at 10 inches — bags generally win once delivery is priced in.
What should I fill a new raised bed with?
Not bagged "topsoil" alone, which compacts, and not pure compost, which is too rich and slumps as it breaks down. A durable blend is roughly one third compost, one third aeration (coarse vermiculite, perlite, or pumice) and one third moisture retention (peat or coco coir) — the calculator gives you volumes for that mix. See the full soil guide for cheaper large-bed approaches.
Why does my soil level drop after the first season?
Because the organic fraction is decomposing, which is what makes it useful. Expect to lose one to three inches a year and to top up with compost each spring. Order about 10% more than the calculator says to allow for initial settling.