Common sizes at a glance
Each row is one bed, measured inside. Plant counts are for a single crop filling the bed at its block spacing (lettuce 12 in, carrot 3 in, pepper 18 in, supported tomato 24 in). Soil is the volume for a full fill and the number of 1.5 ft³ bags, rounded up.
| Inside size | Area | Lettuce | Carrot | Pepper | Tomato | Soil, 10 in · bags | Soil, 12 in · bags |
|---|---|---|---|---|---|---|---|
| 4 × 2 ft | 8 ft² | 8 | 128 | 2 | 2 | 6.67 ft³ · 5 | 8 ft³ · 6 |
| 3 × 3 ft | 9 ft² | 9 | 144 | 4 | 1 | 7.5 ft³ · 5 | 9 ft³ · 6 |
| 6 × 3 ft | 18 ft² | 18 | 288 | 8 | 3 | 15 ft³ · 10 | 18 ft³ · 12 |
| 4 × 4 ft | 16 ft² | 16 | 256 | 4 | 4 | 13.33 ft³ · 9 | 16 ft³ · 11 |
| 8 × 4 ft | 32 ft² | 32 | 512 | 10 | 8 | 26.67 ft³ · 18 | 32 ft³ · 22 |
| 12 × 4 ft | 48 ft² | 48 | 768 | 16 | 12 | 40 ft³ · 27 | 48 ft³ · 32 |
Width is about reach
The most useful rule for width is not about plants at all. NC State Extension recommends beds no more than 4 ft wide so you can reach the middle, and University of Delaware Cooperative Extension calls 4 ft “a convenient width” because the centre is reachable from either side. Reaching in from the path keeps you off the soil, which is the main reason to have a bed.
Against a fence or wall, where you can only reach from one side, 2–3 ft wide is easier to work. Length is limited mainly by materials and by how far you want to walk around the end.
Some sizes suit some crops better
Because each plant keeps half a spacing from the frame, the count does not scale smoothly with area. A 3 × 3 ft bed is 9 ft², bigger than a 4 × 2 ft bed at 8 ft², but it holds 1 supported tomato against 2: 36 in fits only one 24 in plant in each direction. Peppers at 18 in go the other way: 4 in the 3 × 3 ft bed and 2 in the 4 × 2 ft bed.
Splitting a bed can cost plants too. One 8 × 4 ft bed holds 10 peppers; two separate 4 × 4 ft beds hold 8 between them, because each extra pair of frame edges takes an edge allowance. For lettuce and carrots, which divide evenly into 4 ft, the totals are the same (32 lettuce and 512 carrots either way).
If one crop matters most, check its spacing against the bed size before you build. The plant spacing calculator does this for any size.
Depth and soil
University of Delaware Cooperative Extension suggests at least 1 ft of depth for a productive bed. Soil volume rises in a straight line with depth, so a 12 in fill needs 20% more material than a 10 in fill of the same bed. For a 4 × 8 ft bed that is 32 ft³ against 26.67 ft³, or 22 bags against 18.
If the bed sits on dug, workable ground, the new layer can be shallower than the frame; measure only the depth you are adding. The soil chart lists more sizes and depths, and the soil calculator handles any size.
Leave room for paths
Beds are only half of the layout. University of Delaware Cooperative Extension suggests paths of at least 4 ft where you need a wheelbarrow or garden cart. Narrower paths work for foot traffic, but plan for kneeling: a path that feels wide when the bed is empty can feel tight once plants spill over the edge.
The planner does not model paths. Enter only the growing area inside each frame.
Choosing a size
- First bed: 4 × 4 ft or 4 × 8 ft is easy to reach, uses standard lumber lengths, and holds a useful mix.
- Against a wall: 2–3 ft wide, so the back is within reach.
- Big crops: squash, cucumber on the ground and tomatoes need a bed planned around them. Check the counts in the table first.
- Several beds: keep sizes the same where you can. It makes plans, soil orders and crop rotation simpler.