Garden Raised bed soil calculator
Raised beds swallow far more soil than people expect: a standard 4×8 foot bed at 12 inches deep needs 32 cubic feet, which is sixteen of the large bagged mixes. Filling with pure bagged potting mix is both expensive and, for a bed this size, not the best growing medium.
How this is calculated
Cubic feet = length (ft) × width (ft) × depth (in) ÷ 12, times the number of beds. Cubic yards = cubic feet ÷ 27.
Source: Volume conversion; mix ratio follows extension raised-bed guidance
Worth knowing: The suggested 50/30/20 mix of topsoil, compost and an aeration material is a common starting point, not a standard — good raised-bed soil varies with what you are growing and what is available locally. Do not fill a deep bed entirely with bagged potting mix: it is formulated for containers, compacts over a season, and at this volume the cost is punishing.
Common questions
How deep does a raised bed need to be?
Six inches works for lettuce and herbs. Ten to twelve inches suits most vegetables. Root crops like carrots and parsnips, and anything long-lived like asparagus, want eighteen inches or more of loose soil to work into.
Can I fill the bottom with branches or logs?
You can — the technique is often called hugelkultur — and it reduces how much soil you buy. Be aware the bed will settle noticeably over the first year or two as the wood breaks down, and decomposing wood ties up nitrogen at the interface, so plan to top up and feed.
Is bagged garden soil the same as potting mix?
No. Potting mix is a soilless blend designed to drain fast in a container and is priced accordingly. Garden or raised-bed soil is heavier and intended for volume. Filling a 32 cubic foot bed with potting mix costs several times more and gives you a medium that slumps.
Affiliate Disclosure
Some links on this page are affiliate links. As an Amazon Associate, we earn from qualifying purchases — at no extra cost to you. We do not buy, test, or physically handle the products we cover; our rankings come from aggregated owner reviews, ratings and expert consensus. Commissions never change our verdicts. How we rank · full disclosure.