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Raised bed soil calculator
Enter your bed's inside dimensions and get the volume in cubic feet, cubic yards and litres, the number of bags to buy, and the Mel's Mix three-way split. Works in feet, inches, centimetres or metres.
Mel's Mix — equal thirds by volume
Volume in cubic feet = width × length × depth, all in feet. Divide by 27 for cubic yards, or multiply by 28.317 for litres. A 4 × 8 bed at 12 inches deep is 4 × 8 × 1 = 32 cubic feet, which is 1.19 cubic yards or 906 litres.
Measure the inside, not the outside
A bed built from 8-foot boards of 2-inch stock holds 7 feet 8 inches of soil, not 8 feet. Over four sides that discrepancy is a bag or two — annoying if you are one bag short on a Sunday afternoon. Measure the soil cavity.
Common bed sizes at three depths
| Bed size | 6" deep | 12" deep | 18" deep |
|---|---|---|---|
| 2 × 4 ft | 4 cu ft · 113 L | 8 cu ft · 227 L | 12 cu ft · 340 L |
| 3 × 6 ft | 9 cu ft · 255 L | 18 cu ft · 510 L | 27 cu ft · 765 L |
| 4 × 4 ft | 8 cu ft · 227 L | 16 cu ft · 453 L | 24 cu ft · 680 L |
| 4 × 8 ft | 16 cu ft · 453 L | 32 cu ft · 906 L | 48 cu ft · 1,359 L |
| 4 × 12 ft | 24 cu ft · 680 L | 48 cu ft · 1,359 L | 72 cu ft · 2,039 L |
| 2 × 8 ft | 8 cu ft · 227 L | 16 cu ft · 453 L | 24 cu ft · 680 L |
How deep does a raised bed need to be?
| Depth | Suits |
|---|---|
| 6 inches | Salad, spinach, radish, herbs — only if the bed sits on decent open soil roots can reach |
| 10–12 inches | The practical standard. Tomatoes, brassicas, beans, beetroot, most of what you'd grow |
| 18 inches | Carrots, parsnips, daikon, potatoes; also beds on concrete, decking or heavy clay |
| 24 inches + | Waist-height beds for accessibility. Consider filling the lower half with coarse material |
Beds on a hard surface need more depth. If your bed sits on concrete, paving or decking, roots cannot escape downward — add about 4 inches to whatever the crop would normally need.
Mel's Mix, and why not just garden soil
The square foot gardening recipe is one third compost, one third peat moss or coconut coir, one third coarse vermiculite, by volume. Each part does a distinct job: compost feeds, peat or coir holds moisture, vermiculite keeps the mix open so it drains and stays aerated.
Ordinary garden soil is the usual mistake. Out of the ground and confined in a box it compacts, drains poorly and sets like a brick in a dry spell. If you want to use some, keep it below a third of the total and add plenty of compost and grit.
Coconut coir is the more sustainable substitute for peat and behaves nearly identically — worth choosing given how peat extraction damages bog habitats.
Bags or bulk?
Bagged compost is convenient and expensive. Bulk delivery is cheaper per unit but arrives as a heap on your drive. The crossover is usually around one cubic yard — roughly 27 cubic feet, or a single 4×8 bed at 12 inches.
- Under 1 cubic yard: bags are usually easier and not much dearer.
- 1–3 cubic yards: bulk bags (dumpy bags) tend to win.
- Over 3 cubic yards: loose bulk delivery is far cheaper.
Beds settle — buy about 10% extra
Fresh mix settles by roughly a tenth over the first few weeks as it is watered in, and organic matter continues to break down over the season. Most gardeners top up an inch or two each spring. Ordering a little over is cheaper than a second delivery.
Hügelkultur: filling the bottom cheaply
For a deep bed, filling the whole thing with bought mix is expensive. The hügelkultur approach fills the bottom third with logs, branches, cardboard and leaves, then puts good mix on top. The woody layer rots down, feeds the bed for years and holds water. The catch: expect the bed to sink noticeably in year one, so plan to top it up.
Related
- Plants per square foot calculator — what to put in the filled bed
- Raised bed planner app — lay the bed out on a grid
- Square foot gardening guide — the full method
Bed filled? Now plan what goes in it.
Niwa grids the bed, spaces the plants and dates every crop. Free on iPhone.
Questions gardeners ask
How much soil do I need for a 4x8 raised bed?
A 4x8 foot bed filled to 12 inches deep needs 32 cubic feet of soil, which is about 1.19 cubic yards or 906 litres. In 2-cubic-foot bags that is 16 bags; in 40-litre bags it is about 23. At 6 inches deep you need half that, and at 18 inches you need 48 cubic feet.
Plan a 4x8 bedWhat is Mel's Mix and what are the proportions?
Mel's Mix is the soil recipe from Mel Bartholomew's square foot gardening method: one third compost, one third peat moss or coconut coir, and one third coarse vermiculite, measured by volume. It is light, drains freely and holds nutrients. Using ordinary garden soil in a raised bed is the most common mistake — it compacts and drains badly once it is out of the ground.
The full square foot methodIs Niwa free to use?
Yes. Niwa is free to download from the App Store, and the core planner — the garden grid, the plant library and the planting calendar — works without paying. Optional in-app purchases unlock additional features.
See it on the App StoreHow many plants fit in one square foot?
It depends on the crop's spacing requirement. A plant needing 3 inches fits 16 per square foot, 4 inches fits 9, 6 inches fits 4, and 12 inches fits 1. Large plants such as vining tomatoes or zucchini need more than one square each. Niwa works this out for you and prints the count straight onto the grid cell.
Plants per square foot calculatorCan Niwa plan a raised bed of any size?
Yes. You set the bed dimensions — 4×4, 4×8, 2×8 or a custom size — and Niwa draws the square foot grid to match. You can keep several beds and several gardens in the app at once and switch between them.
Raised bed plannerHow does Niwa know when I should plant?
Niwa uses your location to find your average last spring frost and first autumn frost, then works backwards from those dates for every crop in your bed. Each plant gets its own sow-indoors, transplant and harvest window on a month-by-month timeline, so the dates are yours rather than a generic zone table.
How the planting calendar worksDoes Niwa warn me about bad companion plants?
Yes. As you place plants on the grid, Niwa puts a green dot between neighbouring squares that benefit each other and a red dot between squares that are better kept apart, so you see the conflict while you are still planning.
Companion planting in NiwaHow much soil do I need to fill a raised bed?
Multiply length × width × depth in feet to get cubic feet, then divide by 27 for cubic yards. A 4×8 bed at 12 inches deep needs 32 cubic feet, roughly 1.2 cubic yards or sixteen 2-cubic-foot bags.
Raised bed soil calculatorDoes Niwa work offline, and do I need an account?
The whole plant library and all your layouts are stored on the device, so the app works with no signal — useful on an allotment. There is no account to create and your garden data is never uploaded to a server.
See what's includedWhich growing zones and countries does Niwa support?
Niwa works worldwide. It recognises USDA hardiness zones in the United States and derives frost dates from your location elsewhere, including the UK, Canada, Australia and Europe.
Find your frost dates and zoneHow is Niwa different from other garden planner apps?
Three things: it is a native iPhone app rather than a desktop planner viewed in a mobile browser, it shows companion-planting conflicts on the grid as you place plants instead of in a separate chart, and it works fully offline with no account.
Compare Niwa with other plannersIs Niwa available on Android?
Not yet. Niwa is a native iPhone app and needs iOS 18.0 or later. An Android version is not currently available.
Read the gardening guides in the meantime