How wide, how long, how deep, and what to build it from. Width is the one dimension that matters, the rest is up to you, and here's how to fill it without wasting money.
Photo: Dereckson · CC BY 4.0, via Wikimedia Commons.
What you need
A reasonably level, sunny spot with six to eight hours of sun a day
Boards or blocks for the sides, and a way to cut and fasten them
Soil to fill it, a bulk mix of topsoil and compost (bagged potting mix costs far more)
A tape measure, and a few hours
A raised bed is 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, and it drains and warms faster in spring. It also 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 covers how wide, how long, how deep, and what to build it from.
How wide should I make the bed?
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.
A bed whose shorter side runs well past ~1.2 m (4 ft) has interior ground that can't be reached from its edges.
If you skip this, it still grows, just less well. The planner suggests it.
How it works: 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. Standing on it compacts the soil and breaks the structure the bed exists to protect.
Evidence: checked against 2 sources in the research-based literature.
The 2 sources
Iowa State Univ. Extension, Yard & Garden FAQ, raised bed size
Beds accessible on both sides can be 3 to 4 feet wide
Univ. of Minnesota Extension, Raised bed gardens
… 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.
Plus one working note on how the sources were read.
Remedy
Make a wide area into several beds with a path between them, so you never step into a bed. In the planner each bed stands alone and can't be split into parts, so add another bed instead.
Keep the bed no more than about 1.2 m4 feet across. That’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, because your weight compacts the soil the frame exists to keep loose. If you can only reach the bed from one side, because it’s against a wall or a fence, halve it to about 0.6 m2 feet.
Once you know the width, the same reach principle governs how you’ll plant it.
A planted bed keeps every plant within arm's reach of a path, edge, or walkable row.
If you skip this, it still grows, just less well. The planner suggests it.
How it works: You can comfortably reach about 2 feet (0.6 m) into a bed. Soil farther than that from a path or edge can only be tended by stepping on it, which packs it down. So a bed worked from its edges and wider than about 4 feet (1.2 m) is laid out as planting strips no wider than 4 feet, with paths about 20 inches (0.5 m) between them. A field is worked from rows you walk, about 2.5 feet (0.75 m) apart. Raised beds, in-ground beds and containers are worked from their edges; a field is walked.
Evidence: checked against 2 sources in the research-based literature.
The 2 sources
Iowa State Univ. Extension, Yard & Garden FAQ, raised bed size
Beds accessible on both sides can be 3 to 4 feet wide
Univ. of Minnesota Extension, Raised bed gardens
… 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.
Plus 2 working notes on how the sources were read.
Length is free. Make it as long as your space and your lumber allow. The only practical limit is 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. 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, such as 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 should I build the sides out of?
The frame is the easy part, and it doesn’t need to be fancy or expensive. Common, sound choices:
Untreated rot-resistant lumber - cedar, redwood, or larch. It weathers gray 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 use untreated wood. Whatever you use, build the frame level and square. The walls hold the weight of wet soil, so fasten the corners well.
What do I fill it with, and what will it cost?
This is where a raised bed 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. Bags of potting mix 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 isn’t used up at the end of a season. It settles. The what to do with last year’s soil guide covers refreshing it, and why replacing it is almost never the answer.
The engine won’t make up a fertilizer number for you. If you want to know what your new mix needs, get a soil test. A chart can only guess.
Now that it's built, is there anything I should avoid?
You build loose, reachable soil 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.
A plant's mature spread must fit its allocated area.
Plant to the mature size on the label. The seedling in your hand is much smaller than the plant it becomes. 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.
Further reading
Good places to read more about the technique on this page. The grade on each card comes from that card’s own sources.
