Wood-Core Bed Calculator
How much wood, brush, compost, and topsoil a wood-core raised bed actually takes, layer by layer, in cubic yards for the delivery and wheelbarrow loads for the haul, how much to overbuild so it does not sink, and the water the buried wood banks for you.
Set your place for the best week to build it.
Which wood to use
The buried wood is the engine. You want wood that rots and drinks, not wood built to last.
Best to bury
- Alder, birch, poplar, aspen
- Maple, oak (oak is slow but fine)
- Apple and most fruit woods
- Cottonwood, willow (dry it first so it does not resprout)
Use with care
- Pine, fir, spruce: resinous and acidic, bury them deep in the core
- Eucalyptus: let it age a season first
- Very fresh, sappy logs: age them or expect slow year one
Keep out
- Black walnut: its juglone poisons many vegetables
- Black locust, cedar, redwood: built to resist rot, so they never break down or hold water
- Pressure-treated, painted, or stained wood: chemicals in the bed
How a wood-core bed works
A wood-core raised bed, the method many gardeners call hügelkultur, is built over a core of logs and branches. As the wood rots it acts like a sponge, soaking up rain and releasing it slowly through dry spells, and it feeds the bed for years as it breaks down, so you water and fertilize less over time.
How the math works
Volume is footprint times height. A framed bed is a box, so it takes the full length by width by height. A mound has sloped sides, so it holds about half of that for the same footprint and peak. We split the total into the usual layers, roughly forty-five percent wood, fifteen percent brush, twenty percent compost and greens, and a twenty percent topsoil cap. One cubic yard is twenty-seven cubic feet and a wheelbarrow holds about six. Build the pile about thirty percent taller than the height you want, because the wood compresses and decays hard in the first year and the bed settles. Keep the topsoil cap at least four inches deep so there is something to plant into, more for root crops.
About the banked water
The banked-water figure assumes the decayed wood eventually holds about a quarter of its own volume in water, at seven and a half gallons per cubic foot, against a bed that drinks about an inch a week in summer. It is a planning estimate that grows as the wood rots, not a guarantee. Fresh logs hold far less than punky, well-rotted wood, so a wood-core bed gets more drought-proof every year for the first several.