Raised bed garden plan: sizing, soil math and a first-year layout
A raised bed garden plan that works: how to size and space your beds, calculate soil volume and cost, and lay out a first year that will not overwhelm you.
A good raised bed garden plan comes down to four decisions: how big the beds are, how far apart, how much soil goes in them, and what you actually plant the first year. Get the sizing right and the bed works for a decade. Get the first-year planting list wrong and you quit in July.
Here are the numbers, the soil math, and a layout for a single 4 by 8 foot bed that produces well without becoming a second job.
Size the bed around your arms, not the lumber
Every raised bed dimension traces back to one rule: you must never step into the bed. Stepping in compacts the soil, which is the entire thing you built the bed to avoid.
- Width: 4 feet maximum with access from both sides. Most adults reach comfortably about 2 feet, so 4 feet lets you work the middle from either edge.
- Width: 3 feet if the bed is against a wall or fence with access from one side only. Same reason. Use 2.5 to 3 feet if children will garden it or your reach is limited.
- Length: 8 feet is the sweet spot, mostly because lumber comes in 8 foot lengths and you avoid cutting. Longer beds are fine, but past about 12 feet people start stepping across instead of walking around.
- Paths: 18 to 24 inches minimum between beds. Go to 36 inches if you need to fit a wheelbarrow or a wheelchair. Skimping here is the regret people mention most.
Sun beats every other siting factor. Fruiting crops such as tomatoes, peppers, cucumbers and squash want 6 to 8 hours of direct sun. Leafy greens will accept 4. Watch the spot across a day before you build, because you cannot fix a shady bed later.
Depth: how deep is deep enough
Depth depends almost entirely on what is underneath the bed.
| Situation | Depth | Why |
|---|---|---|
| On existing soil, general vegetables | 10 to 12 in | Roots continue below the frame, so the box is a head start, not the whole root zone |
| Salad greens, herbs only | 6 in | Shallow roots, minimal need |
| Long carrots, parsnips, big root crops | 16 to 18 in | The whole root has to fit inside loose soil |
| On concrete, patio, or hardpan | 18 in or more | The box is the entire root zone, with nowhere to go below |
If you are on decent ground, loosen the soil underneath with a fork before filling. Do not lay landscape fabric on the bottom: it blocks the roots and worms you want moving between bed and earth. To kill grass beneath, lay plain cardboard, which smothers it and then rots away. Use hardware cloth only if you have gophers or voles.
The soil math, done properly
The formula is the whole trick:
length (ft) x width (ft) x depth (ft) / 27 = cubic yards
You divide by 27 because a cubic yard is 3 x 3 x 3 feet. Depth has to be converted to feet first: 12 inches is 1.0, 6 inches is 0.5, 18 inches is 1.5.
Worked for a 4 x 8 bed at 12 inches deep:
- 4 x 8 x 1 = 32 cubic feet
- 32 / 27 = about 1.2 cubic yards
- In bags at 1.5 cubic feet each: 32 / 1.5 = 22 bags
Same bed at other depths:
| Depth | Cubic feet | Cubic yards | 1.5 cu ft bags |
|---|---|---|---|
| 6 in | 16 | 0.6 | 11 |
| 12 in | 32 | 1.2 | 22 |
| 18 in | 48 | 1.8 | 32 |
Two things this math does not tell you, and they cost real money:
Order 10 to 20 percent extra. The mix settles substantially over the first season as organic matter breaks down. Everyone underestimates this and ends up making a second trip.
Past roughly one cubic yard, buy in bulk. Twenty-two bags is a lot of plastic, a lot of lifting and a lot more money than the same volume delivered loose from a landscape supplier. Bags make sense for topping up, not for filling.
What to fill it with
You are building soil, not buying dirt. Two workable approaches:
Bulk mix (cheaper, better for multiple beds): roughly 50 to 60 percent screened topsoil, 30 to 40 percent compost, 10 to 20 percent something coarse for aeration such as bark fines, pumice or perlite. Many suppliers sell a pre-blended “raised bed mix” that lands in this range.
Soilless mix (the classic square foot recipe): equal thirds compost, peat moss or coconut coir, and coarse vermiculite. Light and weed-free, but pricier and it dries out faster.
Three fills to avoid: pure compost, which slumps by a third and is too rich for many crops; bagged potting mix in a large bed, which is built for containers and costs a fortune at this volume; and native clay straight out of the yard, which sets like a brick inside a frame.
Then top up 1 to 2 inches of compost every spring. The level will drop every year. That is normal and it is how the bed keeps working.
The first-year layout that does not overwhelm you
Build one bed. Not four. One 4 by 8 bed is 32 square feet, which is genuinely enough food to matter and few enough plants that you will keep up with the watering in August. The single most common first-year failure is building more garden than you will tend.
Divide the bed into 32 one-foot squares in your head. Put tall crops on the north end so they do not shade everything else, and give them a trellis.
| Position | Crop | Squares | Plants |
|---|---|---|---|
| North row, trellised | Cucumbers | 4 | 4 |
| North row, trellised | Pole beans | 4 | ~24 |
| Second row | Tomatoes, staked | 6 | 3 |
| Second row | Basil | 2 | 8 |
| Third row | Peppers | 4 | 4 |
| Third row | Lettuce | 4 | 16 |
| Front row | Bush beans | 3 | ~27 |
| Front row | Carrots | 3 | ~48 |
| Front row | Radish | 2 | ~32 |
The spacing behind those plant counts, useful for any crop you swap in:
| Plants per square foot | Crops |
|---|---|
| 16 | Radish, carrots |
| 9 | Bush beans, spinach, beets |
| 4 | Lettuce, chard, basil |
| 1 | Peppers, eggplant, broccoli, cabbage |
| 1 per 2 squares | Tomatoes (staked or caged) |
| 1 per 4 to 9 squares, or trellised | Squash, melons |
Want it simpler? Cut it to five crops. Tomatoes, lettuce, bush beans, carrots and cucumbers on a trellis will feed you and forgive mistakes. Radish and lettuce come out fast, which frees squares for a second sowing.
Growing timing is the other half of this plan: our seed starting schedule covers counting backwards from your last frost, and the companion planting chart covers which neighbors are worth bothering with.
Plan on paper, then plan on last year
Sketch the bed on graph paper before you buy a single seed. It takes twenty minutes and stops you from ordering six squash plants for a bed that fits one.
Then keep that sketch. Year two is where raised beds get good, because you know which corner drains badly, which crop you never ate, and which family sat in which bed so you can rotate them. GardenOS holds your bed layouts, crops and sowing dates together with a garden journal that carries across seasons, in a single offline HTML file on your own device. The plan and the notes belong in the same place, because last season’s notes are the thing that makes next season’s plan actually good. A layout drawn from memory is just optimism.
Do this next: measure your sunniest 4 by 8 foot patch and time the sun across one day. If it clears 6 hours, run the soil math (32 cubic feet at 12 inches deep), price a cubic yard and a half from a local bulk supplier, and compare it to 22 bags. That one phone call usually decides both the budget and the build date.
Frequently asked questions
How much soil do I need for a 4x8 raised bed?
Multiply length by width by depth in feet, then divide by 27 to get cubic yards. A 4 by 8 foot bed filled 12 inches deep is 32 cubic feet, which is about 1.2 cubic yards, or roughly 22 bags at 1.5 cubic feet each. Order about 10 to 20 percent extra, because the mix settles noticeably in the first season.
How wide should a raised bed be?
Four feet is the standard maximum when you can walk around both sides, because most adults can comfortably reach about two feet in. If the bed sits against a wall or fence with access from one side only, keep it to three feet. The rule behind both numbers is simple: you should never have to step into the bed and compact the soil.
How deep should a raised bed be?
Ten to twelve inches suits almost every vegetable if the bed sits on existing soil, since roots grow down past the frame. Six inches works for lettuce and other shallow greens. Go to eighteen inches or more if the bed is on concrete, on a patio, or if you want long carrots and parsnips.
What is the best soil mix for a raised bed?
A common bulk recipe is roughly half to sixty percent screened topsoil, thirty to forty percent compost, and ten to twenty percent something for aeration like coarse bark or pumice. Do not fill a bed with pure compost, which slumps badly as it breaks down, and do not fill a large bed with bagged potting mix, which is expensive and dries out fast.
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