The recipe sets the settling allowance — see the results.
Bed 1
Advanced options
Only for beds over about 18 in deep.
0 = derive it from the recipe.
Vermiculite and perlite are the expensive part.
This raised bed soil calculator works out fill volume and splits it into topsoil, compost and aeration by recipe — with the settling allowance derived from how much compost your mix actually contains, rather than a single flat percentage.
The short answer
Fill volume is length × width × depth ÷ 12 for cubic feet. A standard 4 × 8 ft bed at 12 inches deep holds 32 ft³ — about 1.2 cubic yards.
Order more than that, because the mix settles. How much more depends on the recipe: a topsoil-heavy blend settles around 13%, pure compost around 30%.
On this page
How to use this raised bed soil calculator
Pick a recipe, enter the bed, and the calculator returns the volume of each component with settling already included. Four shapes:
- Rectangular — with a quantity field, so five identical beds are one entry
- Round — for circular planters and rings
- L-shaped — overall dimensions minus a cut-out
- Known volume — cubic feet, yards or litres
The header toggle switches between bags and bulk. Different beds in the same project? Use Add a different bed size and it totals them.
The volume formula
Cubic feet = Length (ft) × Width (ft) × Depth (in) ÷ 12
Cubic yards = Cubic feet ÷ 27
1 cubic foot ≈ 28 litres · 1 cubic yard = 27 cubic feet
Worked example — 4 × 8 ft bed, 12 in deep, 60/30/10 mix
26 bags of 1.5 ft³ in total, or 1.34 cubic yards in bulk.
The mix recipes
| Recipe | Composition | Best for |
|---|---|---|
| 60 / 30 / 10 | Topsoil, compost, perlite or grit | The default. Best cost-to-performance at any depth |
| 50 / 30 / 20 | Quality soil, compost, potting mix | Lighter and airier; good in heavy rainfall areas |
| 60 / 40 | Topsoil and compost only | Simple and cheap, but drains less well |
| Mel’s Mix | Equal thirds compost, coir or peat, vermiculite | Shallow intensive square-foot beds |
| Pure compost | Compost alone | Not recommended — see below |
The principle behind all of them is the same: roughly half structure, a generous share of compost, and something to keep it airy. The proportions vary; the logic doesn’t.
Two thirds of Mel’s Mix is coir and vermiculite, which cost several times what topsoil does. For a 4 × 8 bed at 12 inches, this calculator’s default prices put Mel’s Mix at 2.2 times the cost of 60/30/10 — which matches the “two to three times more” figure quoted independently elsewhere.
It’s an excellent mix, and its original context was 6 inch square-foot boxes where the volume is small. At 12 inches or deeper, 60/30/10 grows comparably for less than half the money.
Settling depends on the recipe, not the bed
Published settling allowances range from 10% to 25%, and it’s easy to read that as sources disagreeing. They’re not — they’re describing different mixes.
Settling is decomposition. Organic matter breaks down and its air pockets collapse; mineral soil and grit essentially don’t. So the settling rate follows directly from how much of your mix is organic:
| Recipe | Compost share | Settling |
|---|---|---|
| 60 / 30 / 10 | 30% | 13% |
| 50 / 30 / 20 | 30% | 14% |
| 60 / 40 | 40% | 15% |
| Mel’s Mix | 33% | 17% |
| Pure compost | 100% | 30% |
That reproduces the published spread precisely. Sources quoting “10–15%” are describing topsoil-heavy blends; sources quoting “15–25%” are describing compost-heavy ones. Applying a single flat figure to every recipe over-orders for one and under-orders for the other.
The calculator derives the settling percentage from whatever recipe you select, including a custom ratio. You can override it if you know your own materials behave differently.
Why not just fill it with topsoil?
In the ground, soil has structure — built over years by roots, worms, fungi and freeze-thaw cycles. Screened topsoil in a bag has none of that. Put it in a confined box and it settles into a dense mass that roots can’t penetrate and water can’t drain through.
Roots need oxygen as much as they need water. That is what the compost and the aeration component are doing in every recipe above — compost feeds and holds moisture, perlite or grit keeps air channels open.
The opposite extreme fails too. Pure compost settles about 30% in the first season, holds far too much water, and is nutritionally too rich for most vegetables. A bed filled with it will be visibly lower by autumn and can pack down into an airless layer.
How deep should a raised bed be?
| Depth | Suits |
|---|---|
| 6 in | Lettuce, herbs, radishes — square-foot style shallow beds over open soil |
| 8–10 in | Most leafy greens, bush beans, onions |
| 12 in | The general minimum for vegetables. Tomatoes, peppers, squash |
| 18 in | Carrots, parsnips, potatoes and other deep root crops |
| 24 in+ | Accessibility — waist height for gardening without bending |
Doubling a bed’s depth doubles the soil you need. Doubling its width doubles the soil and the lumber. A 4 × 8 bed at 12 inches takes 32 ft³; the same bed at 24 inches takes 64 ft³ — but so does an 8 × 8 bed at 12 inches, which needs four times the framing.
If the bed sits on open ground rather than a hard surface, roots will grow down past the frame into the soil beneath. In that case 12 inches of good mix is plenty even for deep-rooted crops.
Volume by bed size
At 12 inches deep with a 60/30/10 mix, including 13% settling.
| Bed | Holds | Order | Cubic yards | 1.5 ft³ bags |
|---|---|---|---|---|
| 2 × 8 ft | 16.0 ft³ | 18.1 ft³ | 0.67 | 13 |
| 4 × 4 ft | 16.0 ft³ | 18.1 ft³ | 0.67 | 13 |
| 3 × 6 ft | 18.0 ft³ | 20.3 ft³ | 0.75 | 14 |
| 4 × 8 ft | 32.0 ft³ | 36.2 ft³ | 1.34 | 25 |
| 4 × 12 ft | 48.0 ft³ | 54.2 ft³ | 2.01 | 37 |
And the same 4 × 8 bed at different depths:
| Depth | Holds | Order | Cubic yards | 1.5 ft³ bags |
|---|---|---|---|---|
| 6 in | 16.0 ft³ | 18.1 ft³ | 0.67 | 13 |
| 8 in | 21.3 ft³ | 24.1 ft³ | 0.89 | 17 |
| 12 in | 32.0 ft³ | 36.2 ft³ | 1.34 | 25 |
| 18 in | 48.0 ft³ | 54.2 ft³ | 2.01 | 37 |
| 24 in | 64.0 ft³ | 72.3 ft³ | 2.68 | 49 |
Bags, and why the components round up separately
A 4 × 8 bed needs 36.2 ft³ — which is 25 bags of 1.5 ft³ if you were buying a single material. Split across three components it comes to 26 bags, because each component rounds up on its own.
| Component | Needed | Bags | Supplies |
|---|---|---|---|
| Topsoil 60% | 21.7 ft³ | 15 | 22.5 ft³ |
| Compost 30% | 10.9 ft³ | 8 | 12.0 ft³ |
| Aeration 10% | 3.6 ft³ | 3 | 4.5 ft³ |
That surplus isn’t waste — you’ll want it for topping up after the first settling. But it’s worth knowing when comparing a bagged quote against a bulk one.
Above about a cubic yard, bulk delivery is usually 30–50% cheaper than bags, which a 4 × 8 bed at 12 inches already exceeds. Bulk suppliers often have minimum delivery charges, so the saving only materialises if you’re ordering enough — but two beds almost always justify it.
Filling the bottom with wood
On beds deeper than about 18 inches, most vegetable roots never reach the bottom third. Filling that space with logs, branches, twigs and leaves — the hugelkultur approach — has three advantages:
- It costs nothing and displaces a meaningful volume of purchased mix. On a 4 × 8 bed at 24 inches, the bottom third is over 21 ft³.
- Wood holds water like a sponge, releasing it slowly. Deep beds dry out from the top, and this buffers that.
- It feeds the bed for years as it slowly breaks down.
The trade-off is more settling — the wood decomposes and the bed drops. Expect to top up annually for the first few years. Use untreated wood, and avoid black walnut, which is allelopathic.
Topping up each year
Every raised bed loses volume. The organic fraction decomposes, which is exactly what you wanted it to do — it’s what releases nutrients — but the level drops as a result.
Add 1 to 2 inches of compost each spring and fork it lightly into the top few inches. For a 4 × 8 bed that’s about 3 to 5 cubic feet a year, which is two or three bags. Don’t dig it deeply in; raised bed soil structure improves when it’s left largely undisturbed.
A bed that has dropped by several inches after its first season hasn’t failed. That’s the settling allowance you calculated, made visible.
Raised bed soil mistakes to avoid
- Filling with topsoil alone. It compacts into a dense, airless block.
- Filling with pure compost. Too rich, too wet, and it settles 30%.
- Using one settling figure for every recipe. It scales with compost content.
- Mel’s Mix in a deep bed. Excellent mix, but 2.2× the cost for no gain at depth.
- Using garden soil dug from the yard. It brings weed seeds and compacts badly.
- Forgetting to top up. Add 1–2 inches of compost every spring.
- Buying bags for large beds. Above a cubic yard, bulk is 30–50% cheaper.
- Treated or walnut wood in a hugelkultur base. Both cause problems.
Frequently asked questions
How much soil for a 4×8 raised bed?
A 4 × 8 ft bed at 12 inches deep holds 32 cubic feet. Order about 36 cubic feet — 1.34 cubic yards — to allow for settling with a standard 60/30/10 mix. That’s roughly 26 bags at 1.5 ft³ each, split across the three components.
What is the best soil mix for a raised bed?
60% topsoil, 30% compost and 10% perlite or grit is the best value at any depth. The topsoil provides structure, the compost provides nutrition and moisture retention, and the aeration component keeps air channels open so roots can breathe. Never use any one of them alone.
How much does raised bed soil settle?
It depends on the recipe, because settling is decomposition of the organic fraction. A 60/30/10 mix settles about 13%, a 60/40 about 15%, Mel’s Mix about 17%, and pure compost about 30%. That’s why published figures range from 10% to 25% — they’re describing different mixes.
Can I fill a raised bed with just topsoil?
No. In-ground soil has structure built by roots, worms and freeze-thaw over years; bagged topsoil has none. In a confined bed it compacts into a dense mass that roots can’t penetrate and water can’t drain through. It needs compost and an aeration component.
How deep should a raised bed be?
12 inches is the general minimum for vegetables. Six to eight inches suits lettuce and herbs, 18 inches suits carrots and potatoes, and 24 inches or more is usually about accessibility rather than plants. If the bed sits on open soil, roots grow down past the frame and 12 inches is plenty.
What is Mel’s Mix?
Equal thirds of compost, peat or coir, and coarse vermiculite, from Mel Bartholomew’s Square Foot Gardening. It’s excellent in shallow intensive beds, which was its original context. At 12 inches or deeper it costs about 2.2 times what 60/30/10 does, because two thirds of it is expensive material.
How many bags of soil for a raised bed?
Divide the volume by the bag size and round up per component. A 4 × 8 bed at 12 inches needs 36 cubic feet, which is 26 bags at 1.5 ft³ split across topsoil, compost and aeration — one more than if you bought a single material, since each rounds up separately.
Is bulk soil cheaper than bags?
Usually 30–50% cheaper above about a cubic yard, which a single 4 × 8 bed at 12 inches already exceeds. The catch is minimum delivery charges, so the saving only materialises on a big enough order. Two or more beds almost always justify bulk.
Can I put logs in the bottom of a raised bed?
Yes, on beds over about 18 inches deep, where roots never reach the bottom third. It’s called hugelkultur — the wood displaces purchased mix, holds water like a sponge, and feeds the bed as it decomposes. Expect extra settling, and avoid treated lumber and black walnut.
Why has my raised bed soil dropped?
The organic fraction is decomposing, which is what it’s supposed to do — that’s how nutrients are released. A compost-rich mix can lose 15–30% of its volume in the first season. Top up with 1 to 2 inches of compost each spring rather than rebuilding.
Should I use garden soil from my yard?
Better not. It carries weed seeds and possible disease, and it compacts in a confined bed for the same reason bagged topsoil does. If you must use it, keep it well under half the mix and blend generously with compost and an aeration material.
Can I use this calculator in metric?
Dimensions are entered in feet and inches, but results include litres and cubic metres. The Known volume mode accepts litres directly. One cubic foot is about 28 litres, and one cubic yard is 0.765 cubic metres.
Sources and method
- Mix recipes cross-checked against seven published horticultural and calculator sources
- Settling derived from component decomposition rates; the model reproduces the full published 10–25% range as a function of compost share
- Cost model independently reproduces the published claim that Mel’s Mix costs two to three times a standard blend
- Square Foot Gardening method and Mel’s Mix from Mel Bartholomew’s published work
- Component prices are typical US retail and vary considerably by region and supplier
Related calculators
Compost CalculatorTopdressing and amendment rates
Cubic Yards CalculatorVolume and weight for any material
Mulch CalculatorBeds, tree rings and bags vs bulk
Square Footage CalculatorMeasure an irregular plot
Sand CalculatorCoarse sand as a drainage amendment
How this raised bed soil calculator works out its numbers
Fill volume is length times width times depth for each bed, multiplied by the quantity, and summed across every bed you enter. If you choose to fill part of the depth with wood, that fraction is deducted before anything else.
Settling is calculated from the recipe rather than applied as a flat figure. Each component is assigned a decomposition rate — compost highest, aeration material intermediate, mineral topsoil lowest — and the mix settling is their weighted average. A 60/30/10 blend comes out at 13% and pure compost at 30%, which brackets the range published elsewhere and explains why those published figures disagree.
The ordered volume is then split by your recipe percentages, and bag counts round up per component, since a partial bag of topsoil doesn’t help you buy compost. Custom ratios are normalised to 100% before splitting.
These are planning figures. Settling depends on the maturity of your compost and how wet the materials are when they go in. Bulk material also compacts in the pile, so a delivered cubic yard may not fill a cubic yard of bed. Order a little over rather than under — surplus mix is always useful for topping up.