Raised bed soil volume calculator
Enter your raised bed dimensions. Get total soil volume needed plus the right mix ratio of topsoil, compost, and a soilless component — the Mel's Mix-style approach the University of Maryland Extension recommends for new raised beds.
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Recommended mix (1/3 each by volume, per University of Maryland Extension and Cornell Cooperative Extension raised-bed guidance):
- 0.4 cu yd topsoil or screened garden loam — the base substrate
- 0.4 cu yd finished compost — nutrients and microbial life
- 0.4 cu yd coarse aggregate (perlite, vermiculite, or coarse sand) — drainage and aeration
Why a 1/3 mix instead of pure topsoil
Pure topsoil — even good screened topsoil — compacts in a raised bed over time, losing drainage and oxygen for roots. Pure compost is too rich, holds too much water, and shrinks dramatically as the organic matter breaks down. The 1/3-1/3-1/3 mix balances long-term structure (topsoil), fertility and microbial activity (compost), and drainage (perlite or sand).
The Mel Bartholomew "Square Foot Gardening" book popularized this mix. University of Maryland Extension's raised-bed guidance recommends a similar approach — "a mix of topsoil, compost, and a soilless component for drainage."
You don't have to buy three separate ingredients every year. After the initial fill, you only need to top off with 1-2 inches of fresh compost annually as the existing soil settles and the organic content depletes.
Sources: University of Maryland Extension Home & Garden Information Center, Cornell Cooperative Extension vegetable gardening guidance.