How wide, how long, how deep, and what to build it from - the one dimension that actually matters, why the rest is up to you, and how to fill it without wasting money.
A raised bed is just a frame that holds a mound of good soil above the ground you already have. It lets you bring in the soil you want instead of fighting the soil you've got, it drains and warms faster in spring, and it draws a hard line between the growing ground and the paths - so the growing ground never gets walked on and stays loose. If you're still deciding whether a raised bed is the right choice at all, the garden types guide weighs it against in-ground and containers. This one assumes you've decided, and answers: how wide, how long, how deep, and what to build it from.
The one dimension that actually matters: width
Length is up to you and depth is a range. Width is the single measurement the design turns on, and the planner enforces it as a rule:
PromisingA bed whose shorter side runs well past ~1.2 m has interior ground that can't be reached from its edges.
suboptimal if ignored → advise
Mechanism: A comfortable arm's reach is ~0.6 m, so ground more than twice that across can only be tended by standing on the bed — which compacts the soil and breaks the structure the bed exists to protect.
Evidence: confirmed against 3 sources in the research-based literature.
the 3 sources
Iowa State Univ. Extension, Yard & Garden FAQ, raised bed size (read 2026-07-21; read by the maintainer 2026-07-25): 'Beds accessible on both sides can be 3 to 4 feet wide'; one-sided, 'the maximum width should be 1 1/2 to 2 feet (i.e., approximately arm's reach)'.
Univ. of Minnesota Extension, Raised bed gardens (read 2026-07-21; read by the maintainer 2026-07-25): 'the reach of your arm is generally a good metric ... If you can access both sides, the bed can be up to five feet wide.'
The two sources DISAGREE on the ceiling - Iowa State's 3-4 ft against Minnesota's 5 ft - and this rule takes the tighter one. Which source to take is a judgement, which is why this is recorded as promising rather than well established. AMENDED 2026-08-21: the threshold was first recorded as 120 cm, a metric rounding that fell 2 cm short of Iowa State's own 4-ft ceiling - so the single most common raised-bed size (a 4x4, 121.9 cm) tripped the reach warning and the another rule here strip split. The maintainer's 4x4 report caught it. Now 122 cm, exactly the tighter source's stated ceiling: a number the source does state.
remedy
Make a wide area into several beds with a path between them, not one bed you step into. Beds are individual entities here — there is no sub-bed to carve, so add another bed instead.
Read the full rule → · open in the planner ↗
Keep the bed no more than about 1.2 m4 feet across. That number isn't arbitrary - it's twice a comfortable arm's reach, so you can tend the middle from either edge without ever stepping onto the soil. A bed you have to step into defeats its own purpose: your weight compacts the soil the frame exists to keep loose. If you can only reach the bed from one side - it's against a wall or a fence - halve it to about 0.6 m.
Once you know the width, the same reach principle governs how you'll plant it:
PromisingA planted bed keeps every plant within arm's reach of a path, edge, or walkable row.
suboptimal if ignored → advise
Mechanism: A comfortable arm's reach is ~0.6 m (the same basis the draw-time reach nudge uses). Ground past that from any access line can only be tended by standing on the soil, compacting it. So a reach-from-edge bed wider than reach-from-both-sides (~1.2 m) is laid out as <=1.2 m planting strips separated by ~0.5 m paths; a field is worked from walkable rows ~0.75 m apart, not one solid block. Which model applies is read from the bed's declared structure: raised / in_ground / container are edge-reached (strips), field is walked (rows).
Evidence: confirmed against 4 sources in the research-based literature.
the 4 sources
Iowa State Univ. Extension, Yard & Garden FAQ, raised bed size (read 2026-07-21): 'Beds accessible on both sides can be 3 to 4 feet wide'; one-sided 'the maximum width should be 1 1/2 to 2 feet (i.e., approximately arm's reach)'; equipment pathways '~3' wide'.
Univ. of Minnesota Extension, Raised bed gardens (read 2026-07-21): 'the reach of your arm is generally a good metric ... If you can access both sides, the bed can be up to five feet wide.'
Field/row model: sweet-corn row spacing '30 to 36 inches ... so you can hoe, mulch, and harvest without breaking brace roots' (Univ. of Maryland / UMN / Illinois Extension, read 2026-07-21). Path 18-24 in foot-traffic minimum (garden extension consensus).
Container upper bound: the largest common vegetable container is a half-whiskey-barrel / 15-20 gal grow bag ~0.66 m across (Harvest to Table, harvesttotable.com, and the Old Farmer's Almanac container-size chart; read 2026-07-21). 75 cm is that with a small margin; past it the ground is a raised or in-ground bed.
Read the full rule → · open in the planner ↗
Length and depth: pick from a range
Length is free. Make it as long as your space and your lumber allow. The only practical limits are that a very long bed is a long way to walk around, so most people cap a single run around 2.4–3.6 m8–12 feet and leave a path.
Depth is a range, not a fixed number. Extension guidance converges on roughly 20–30 cm8–12 inches of bed for most vegetables. If the native soil underneath is decent, the bed's soil and the ground below act as one deeper column and shallower sides are fine. If you're building over a hard surface - a patio, compacted clay, or a spot you can't dig - go deeper, 30 cm12 inches or more, so root crops have room. Carrots and other roots are the classic reason to build up: a deep, loose bed gives them the straight run they can't get in packed ground.
What to build the sides from
The frame is not the hard part, and it does not need to be fancy or expensive. Common, sound choices:
- Untreated rot-resistant lumber - cedar, redwood, or larch. It weathers grey and lasts years.
- Ordinary construction lumber - cheaper, shorter-lived; fine for a first bed you may move.
- Concrete block, brick, or stone - permanent, no rot, more work to lay level.
Avoid old railroad ties and other timbers treated with creosote. Modern pressure-treated lumber uses copper-based preservatives that extension services generally consider low-risk for garden use, but if that gives you pause, line the inside face with plastic or simply use untreated wood. Whatever you use, build the frame level and square - the walls hold the weight of wet soil, so corners want to be well fastened.
Filling it - and where the money goes
This is where a raised bed quietly costs more than people expect, because a bed holds a surprising volume of soil and you're buying all of it. Fill with a bulk mix of topsoil and compost, roughly half and half, ordered by the cubic yard from a landscape supplier - not bags of potting mix, which are formulated for containers and cost several times as much by volume. Multiply length by width by depth to get the volume before you order.
Get the soil right the first time and you rarely touch it again - a raised bed's soil is not used up at the end of a season, it just settles. The what to do with last year's soil guide covers refreshing it, and why replacing it is almost never the answer.
One thing the engine will not do is invent a fertilizer number for you. If you want to know what your new mix actually needs, the honest answer is a soil test, not a bag of numbers guessed from a chart.
When it's built: don't overfill it
The reason to build loose, reachable soil is to grow more in less space - but "more" has a ceiling, and crowding past it costs you the yield you were reaching for. Each plant still needs its mature footprint, and the planner holds that line:
Well establishedA plant's mature spread must fit its allocated area.
fatal if ignored → block
Mechanism: Geometry.
Evidence: settled without a trial - arithmetic.
remedy
Reduce plant count, or choose a bush cultivar group.
Read the full rule → · open in the planner ↗
Plant to the mature size on the label, not the size of the seedling in your hand. A bed packed to look full in May is a bed of stunted, mildew-prone plants in July. Then plant your first season with the step-by-step first bed guide, which counts the timing from your frost dates.
