Raised Bed Materials Calculator – Boards and Posts to Build Any Bed

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What you need before you buy timber

A raised bed looks simple, four sides and some soil, but buying the wrong timber is a common and expensive mistake. Buy too little and you make a second trip mid-build, buy too much and you waste money and store offcuts for years. This calculator counts what you need from five numbers: bed length, bed width, bed height, board height, and the board length you can actually buy. It gives you stacked courses, total boards, corner posts, post lengths, and a screw count.

Start with the bed size. A bed 240 cm long and 120 cm wide is a popular size because you can reach the middle from either side without stepping on the soil. Stepping on raised bed soil compacts it and undoes the main benefit of the bed, which is loose, deep soil for roots. Keep beds no wider than about 120 cm if you can only reach from one side, and keep paths wide enough for a wheelbarrow.

A worked example

Take a bed 240 cm long, 120 cm wide, and 30 cm high, built from 15 cm boards that come in 240 cm lengths. The calculator gives two courses. Each course needs two long boards and two short boards, but the short boards must be cut from full-length boards, so the count is eight boards in total: four long sides across two courses and four short sides cut from full boards. Corner posts number four, each at least 60 cm long, so 30 cm sits below the bed height inside the ground or as a stake. Screws come to about 64, or eight per board.

Change one number and the count changes fast. A 300 cm bed from 240 cm boards needs two boards per long side per course, because one board cannot span 300 cm, so the total jumps. That is why the calculator asks for the board length you can buy, not an ideal length. Check your timber yard’s actual stock before you buy: boards sold as 240 cm may be slightly shorter after trimming, and a board that is a few centimetres short forces a join mid-side.

Posts, screws, and joints

Corner posts do more than hold corners together. They anchor the bed against the outward push of wet soil, which is heavier than most people expect. Drive posts into the ground or stake them firmly, and screw each board into the post from both sides. A post that only sits on the surface will lean outward within a season as the soil settles and swells with rain. For beds longer than about 240 cm, add a middle post on each long side to stop bowing.

Screws beat nails for raised beds because they hold tighter and let you dismantle a bed later. Use exterior-grade screws long enough to pass through the board and well into the post, and pre-drill near board ends to stop splitting. The calculator’s screw count is a planning number: buy a box with a margin, because screws drop, snap, and get used on the next job too. One spare board is just as wise, since a board with a big knot or a twist can ruin a course.

Choosing timber and lining

Untreated softwood is cheap but rots in a few years in constant contact with damp soil. Naturally durable timber or treated timber rated for ground contact lasts far longer, but check that the treatment is safe for food growing if you plan to grow vegetables. Some gardeners line the inside with plastic or landscape fabric to slow rot, but lining must not trap water: leave the bottom open for drainage, and do not seal the bed into a bath.

Remember that these garden results are general guidance estimates only. Your local climate, soil, weather, and timber supply govern the real build, so measure your site, check your timber yard’s real board lengths, and adjust the plan before cutting. A bed built from a counted list is a one-trip job, while a bed built from a guess is a weekend of trips.

Frequently asked questions

Can I join boards mid-side? Yes, if the join lands on a post. Never join boards in mid-air between posts, because soil pressure will push the join apart. Add a post wherever a join falls.

How deep should posts go? At least 30 cm into firm ground for a normal bed, and deeper for tall beds. The post length the calculator shows includes the bed height plus 30 cm for this reason.

Do I need a base? No, in most cases. An open base lets worms in and water out, which is what you want. Only add mesh if burrowing pests are a real problem in your garden, and use mesh with holes large enough for worms.

Use the calculator above with your real bed size and board length, buy the list plus one spare board, and build on level ground. Measure twice, cut once, and your bed will last for years.

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