Raised Bed Material Calculator

How this is calculated

Boards are sold by NOMINAL size and cut to a smaller ACTUAL size. A 2x8 is not eight inches wide; it is 7.25 inches, because the nominal figure describes the rough board before it was dried and planed smooth on all four sides. The dropdown lists actual widths for that reason - 2x6 is 5.5 inches, 2x8 is 7.25, 2x10 is 9.25 and 2x12 is 11.25. That is why the bed you build is rarely the height you asked for. The number of courses - rows of board stacked up the side - is your target height divided by the real board width, rounded UP, because you cannot buy part of a board. The actual height is then that number of courses times the real width. Ask for 12 inches with 2x8 boards and you get two courses at 7.25, which is 14.5 inches. The soil figure follows the ACTUAL height, not the one you typed. That is the part worth catching before you order: a bed built to 14.5 inches instead of 12 needs about a fifth more soil, and soil is usually the largest cost in the whole project. Ordering to the height you designed leaves you short. Boards are counted by working out how many pieces each side needs and how many pieces fit in one board. Two long sides and two short sides, each as many pieces high as there are courses. Where a side is shorter than the board, several pieces come out of one board - a 4 foot side takes half an 8 foot board, so two of them use one board. Where a side is LONGER than the board, the calculator allows a whole board for each part rather than assuming offcuts will fit, which is the conservative and usually correct assumption on a job with few cuts. The corner post figure is the total length of post you need, assuming four corners each the full height of the bed. It does not include stakes along a long side, which a bed over about six feet wants to stop the boards bowing outward once the soil is wet. Nothing here accounts for waste, mistakes or the board that turns out to be warped. Buy one more than the number says.

Common questions

Why is a 2x8 only 7.25 inches wide?

Because the nominal name describes the rough sawn board, not the one you take home. Lumber is cut at roughly the nominal size, then dried and planed smooth on all four sides, and both processes remove material. The convention is that 2x4 and 2x6 lose half an inch of width while 2x8, 2x10 and 2x12 lose three quarters - so 5.5, 7.25, 9.25 and 11.25 inches respectively. Every one is 1.5 inches thick.

Why is my bed taller than I asked for?

Because you can only stack whole boards. Asking for 12 inches with 2x8 boards means two courses, and two courses of 7.25 inches is 14.5. You cannot get 12 inches from 7.25-inch boards at all - the options are 7.25 or 14.5. The calculator rounds up rather than down because a shorter bed than you planned is usually the worse outcome, but it is worth knowing that a single course of 2x12 gives you 11.25 inches, which may be near enough and is much less work.

Why does the soil figure look higher than I expected?

Because it uses the height you will actually build, not the height you typed. A bed asked for at 12 inches and built at 14.5 needs about a fifth more soil - and soil is usually the biggest cost in the whole project, so this is the number worth catching before you order rather than after. Ordering against your design height rather than your real height is the most common way a raised bed project runs short.

How does it count boards?

By pieces, not by total footage. Each side needs one piece per course, so a two-course bed needs two pieces per side. Where a side is shorter than the board, several pieces come from one - two 4 foot sides use one 8 foot board between them. Where a side is longer than the board, it allows a full board per part rather than assuming an offcut will fit, which is conservative but usually right on a job with few cuts to salvage from.

Do I need corner posts?

For anything more than one course, yes - they are what hold the corners square and stop the boards separating once wet soil is pushing outward. The figure given is the total length for four corners at the full height of the bed. A bed longer than about six feet also wants a stake or two along the long sides, or the middle will bow outward over a season; that is not included here because it depends on the timber and the soil.

What wood should I use?

That is a judgement rather than a calculation, and it is the question worth doing your own reading on. Cedar and redwood resist rot naturally and cost more; untreated pine or fir is cheap and will need replacing sooner. Modern pressure-treated lumber uses different preservatives from the older CCA formulations, and whether to use it around food crops is a decision people reach differently - it is worth looking into rather than taking a calculator's word for.

Take it further with AI

Copy this into ChatGPT or Claude with your own numbers filled in. It hands over the figures this calculator worked out, so the answer is built on real arithmetic instead of a guess.

I used the Raised Bed Material Calculator at https://www.bfcbrilliance.com/tools/raised-bed-material-calculator.

What I entered:
- Bed length (ft): ___
- Bed width (ft): ___
- Height you want (in): ___
- Board size: ___
- Board length you can buy: ___

What it calculated:
- Boards to buy: ___
- Rows of board high: ___
- Height you will ACTUALLY get: ___
- Soil to fill it: ___
- Which is: ___

Use those figures as given — they are already worked out, so please don't recalculate or estimate your own. Help me turn them into a plan: what to buy or do, in what order, roughly what it should cost, and the mistakes people most often make with this job.

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