Planter Box Calculator: The Boards and Cut List
A raised planter box is a simple build with one annoying habit: it always seems to need one more board than the pile you brought home. This planter box calculator works out the lumber for the box itself — the long boards, the short boards, the corner posts and the screws — and turns it into a cut list you can hand to a saw. Pick your box size, your board size and how many boards high you want it, and the list is done.
This tool builds the box; filling it is a soil question, and soil is not sold by the board. The Soil Calculator works out bags and yards from the same length, width and depth, so run your finished box dimensions through it once the frame is planned. Keeping the two jobs on two pages keeps both honest: lumber is a count of boards, soil is a volume, and mixing them is how people end up with three spare bags and one unfilled corner.
Planter Box Calculator
Enter the outside size of the box, choose a board and how many boards high it stands. Results update as you type, or press Calculate.
Cut list — long boards: —
Cut list — short boards: —
Total boards: —
Total linear feet: —
8 ft stock boards to buy: —
Finished box height: —
Corner posts: —
Screws estimate: —
The short boards fit between the long boards, so each one is cut 3 inches shorter than the box width — the thickness of the two long boards it sits between. Corner posts are 4 pieces of 4×4, each the height of the box; assume all four are cut from one 8 ft length of 4×4.
Choosing lumber for a planter box
Wood in ground contact lives a hard life, so species and treatment matter more here than in almost any other garden build. Cedar and pressure-treated lumber resist rot and are the two sensible defaults: cedar weathers to a soft grey and needs no treatment near food plants, while treated lumber is cheaper and lasts for years in damp soil. Whichever you choose, line the inside with landscape fabric or plastic to slow soil contact — it keeps the boards drier, keeps soil from washing out of the joints, and can add seasons to the box.
Untreated pine is the cheapest option and the honest one to choose with your eyes open: it builds fast, looks fine for a few seasons, and then softens at the soil line. For a temporary bed, a renter's garden or a first try at growing, that trade can be exactly right. Just do not price a pine box as if it were cedar — the replacement build is part of its real cost.
Build notes that make a stronger box
- Put the corner posts inside the corners. They make the box strong, hold the courses in line as you build, and give the screws something solid to bite into instead of end grain alone.
- Pre-drill near board ends. Screws close to the end of a board split it, especially in dry cedar. A small pilot hole takes seconds and saves the board you just cut.
- Mind what the box sits on. A box on paving needs a base and drainage holes so water can escape; a box on soil does not — it drains straight down, and a layer of fabric under it is enough.
- Cut everything before you assemble. The cut list above is short on purpose: two lengths, repeated. Cutting all of it first turns the build into assembly, and assembly is where the fun is.
How deep should a planter box be?
Depth is set by roots, not by looks. One board high — about 5.5 to 7 inches depending on the board — suits herbs and flowers, whose roots stay shallow. Two boards, roughly 11 to 14 inches, suits most vegetables and is the size most growers settle on; the default in this calculator is two 2×6 boards, a finished height of 11 inches. Three boards gives deep-root crops such as carrots and parsnips the run they need, and it raises the working height, which means less bending at harvest. Taller is kinder on the back but hungrier for soil, so settle the height before you order either material.
What to calculate next
- Soil Calculator — fill the box you just built, in bags and cubic yards.
- Board Foot Calculator — lumber volume and cost when wood is priced by the board foot.
- Deck Calculator — boards, joists and screws for a full deck build.
Frequently asked questions
What lumber is best for a planter box?
Cedar or pressure-treated lumber, because both resist rot in constant contact with damp soil. Line the inside with landscape fabric or plastic to slow that contact further. Untreated pine is cheapest but lasts only a few seasons.
How deep should a planter box be?
About 11 inches — two 2×6 boards — suits most vegetables. One board high suits herbs and flowers, and three boards suit deep-root crops and anyone who would rather bend less.
How much soil do I need to fill it?
That is a volume question, and this page deliberately does not guess it. The Soil Calculator works it out from the same length, width and depth and gives you bags and cubic yards.
Does a planter box need a bottom?
On soil, no — the box drains downward and open ground is the simplest base. On paving or a deck, yes, with drainage holes, so water escapes instead of pooling under the boards.
Estimates only — check your lumber's actual dimensions; nominal sizes are not real sizes.