Raised Bed Size Calculator
Seven ways to answer "how big should my raised garden bed be" — from crop depth needs and the reach rule to soil volume, accessible beds and yard fit.
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The reach rule: why 4 feet is the magic width
Use this raised bed planner to pick your raised garden bed size before you buy lumber. The core rule is simple: a bed should never be wider than you can comfortably reach.
Most adults can reach about 2 feet without straining, so a bed reachable from both sides tops out at 4 feet wide — you work the middle from either side and never compact the soil by stepping into it. Against a wall or fence, where only one side is reachable, the limit drops to 2–2.5 feet. Children's beds are capped at 3 feet, and wheelchair-accessible beds at 3 feet wide with a 24–30 inch work height.
Proven raised bed dimensions
| Size (L × W × H) | Best for | Why it works |
|---|---|---|
| 4 × 8 ft × 12 in | Lettuces, herbs, brassicas — the all-purpose bed | Full 8-ft lumber boards, zero cutting waste |
| 2 × 10 ft × 6 in | Green beans, trellised tomatoes | Narrow trellis row; shallow soil is enough |
| 3 × 12 ft × 18 in | Root crops (carrots, potatoes) | Deep soil for straight roots, more moisture |
| 4 × 4 ft × 24 in | Diverse small-space bed | Deep, compact, reachable from all sides |
| 1.2 × 2.4 m × 300 mm | UK standard all-purpose bed | 2.88 m² of growing space; matches sleeper courses |
How the soil math works
Raised bed depth and soil volume are simple geometry: volume (cu ft) = length (ft) × width (ft) × depth (ft). A 4 × 8 ft bed at 12 inches deep holds 32 cubic feet (32 ÷ 27 ≈ 1.19 cubic yards). Divide by your bag size and round up — soil settles after the first watering, so a little extra is normal. A proven fill is the 1:1:1 mix: one part topsoil, one part compost, one part peat moss or coconut coir.
Related tools
Computing fill for a non-garden project? Try the concrete calculator. Converting soil volume to water capacity? See cubic feet to gallons. Tuning the compost fraction of your mix? Use the compost ratio calculator.
Raised bed size FAQs
How wide should a raised bed be?
No wider than 4 feet if you can reach it from both sides — most adults can comfortably reach about 2 feet, so a 4-foot bed lets you work the middle without stepping on the soil. If the bed sits against a fence or wall and you can only reach it from one side, keep it 2 to 2.5 feet wide. Children's beds should be no wider than 3 feet.
How deep should a raised garden bed be?
At least 6 inches, ideally 12 inches. Six inches is enough for shallow-rooted crops like lettuce, herbs and radishes; most vegetables — tomatoes, peppers, beans — want 12 inches. Grow root crops like carrots and potatoes in 18 to 24 inches, and build at least 18 inches tall if your native soil drains poorly.
What is the best size for a raised garden bed?
The classic all-purpose bed is a 4x8 raised bed, 12 inches deep: big enough for lettuces, herbs and brassicas, and it uses full 8-foot lumber boards with no cutting waste. Other proven combinations are 2 x 10 ft x 6 in for beans or trellised tomatoes, 3 x 12 ft x 18 in for root crops, and 4 x 4 ft x 24 in for a diverse small-space bed.
How much soil does a 4x8 raised bed need?
A 4 x 8 ft bed that is 12 inches deep holds 32 cubic feet of soil (about 1.19 cubic yards). To work out how much soil for a raised bed you need, multiply length x width x depth in feet — then divide by your bag size and round up. With 1.5-cubic-foot bags that is 22 bags; with 2-cubic-foot bags it is 16 bags.
What size raised bed works for a wheelchair user?
For a wheelchair accessible raised bed, build it no wider than 3 feet so it is reachable from a seated position, and 24 to 30 inches tall so the work surface is at a comfortable height. Published guides vary slightly on height (24 versus 30 inches), so plan within the 24-30 inch range and confirm against the gardener's actual reach and mobility. Children using wheelchairs do best with beds about 2 feet wide.
What are the standard raised bed sizes in metric (UK)?
The UK guideline for a metric raised bed is a maximum of 1.2 m wide when accessible from both sides (about 4 ft), or 600 mm against a wall or fence. Practical lengths run 2.4 to 3.6 m, and heights of 200 to 300 mm suit most vegetables, with 400 mm or more recommended for gardeners with mobility needs. A classic 1.2 x 2.4 m bed gives 2.88 m² of growing space.
What soil mix should I put in a raised bed?
A popular, proven mix is 1:1:1 — one part topsoil, one part compost, one part peat moss (or coconut coir as a peat-free alternative). For a 4 x 8 ft bed that is 12 inches deep (32 cubic feet), that means about 10.67 cubic feet of each component — roughly 8 bags of each if you buy 1.5-cubic-foot bags.
How far apart should raised beds be?
Leave at least 2 feet between beds so a wheelbarrow fits through the paths; 3 feet is more comfortable for kneeling, turning and carrying harvests. The multi-bed raised bed layout mode above flags any path narrower than the 2-foot wheelbarrow minimum.
Can a raised bed be too long?
Length is flexible, but very long beds are awkward to walk around and work. Eight feet is the common standard because it matches lumber lengths with no waste, 10 to 12 feet is still manageable, and most guides advise against beds longer than 20 feet — use several shorter beds instead.
How deep do carrots and potatoes need in a raised bed?
At least 18 inches of soil. Deeper beds of 18 to 24 inches give root crops room to develop straight roots and hold moisture better than shallow beds. A 3 x 12 ft bed at 18 inches deep is the textbook root-crop size.