Raised Bed Soil Calculator: Volume, Mix Ratio and What to Buy
Enter the bed size and how deep you will actually fill it. The total is split into the components of your chosen mix, because the part that catches people out is not the volume — it is how much of each ingredient to buy.
Why the volume is the easy half
Working out how much a raised bed holds is arithmetic: length times width times the depth you actually fill. Every calculator on the first page of results will give you that number, and they will all agree.
What none of them tell you is how to split it. A 4 x 8 ft bed filled to 11 inches takes about 29 cubic feet — but 29 cubic feet of what? Bagged "raised bed soil" at that volume is the most expensive way there is to fill a bed, and a yard of screened topsoil and nothing else gives you a bed that sets like a path by August. The split below is the part worth getting right.
| Example | One bed | All beds | Same total in cubic yards | Same total in litres | Compost | Bulk base — topsoil, coir or peat | Aeration — vermiculite or perlite | Bags if you buy it all bagged |
|---|---|---|---|---|---|---|---|---|
| 4 x 8 ft bed, 11 in deep — half and half | 29.3 cu ft | 29.3 cu ft | 1.09 cu yd | 831 L | 14.7 cu ft | 14.7 cu ft | 0.0 cu ft | 20 |
| 4 x 8 ft bed, 6 in deep — shallow, over existing soil | 16.0 cu ft | 16.0 cu ft | 0.59 cu yd | 453 L | 8.0 cu ft | 8.0 cu ft | 0.0 cu ft | 11 |
| 4 x 4 ft bed, 11 in deep — thirds mix | 14.7 cu ft | 14.7 cu ft | 0.54 cu yd | 415 L | 5.0 cu ft | 4.8 cu ft | 4.8 cu ft | 10 |
| 2 x 8 ft narrow bed, 17 in deep — loam-based | 22.7 cu ft | 22.7 cu ft | 0.84 cu yd | 642 L | 6.8 cu ft | 13.6 cu ft | 2.3 cu ft | 16 |
| Three 4 x 8 ft beds, 11 in deep | 29.3 cu ft | 88.0 cu ft | 3.26 cu yd | 2,492 L | 44.0 cu ft | 44.0 cu ft | 0.0 cu ft | 59 |
| Mix | Compost | Bulk base — topsoil, coir or peat | Aeration — vermiculite or perlite |
|---|---|---|---|
| half | 0.50 | 0.50 | 0 |
| mels | 0.34 | 0.33 | 0.33 |
| loam | 0.30 | 0.60 | 0.10 |
These are the three mixes that extension services and long-running raised-bed guides actually recommend, expressed as fractions of finished volume. The thirds mix is the Square Foot Gardening formula; it is the most expensive of the three by a wide margin, because vermiculite costs several times what topsoil does. Sources disagree on how much compost a bed can take — the published range runs from roughly 25% to 50% — and this table uses the upper end only for the half-and-half mix, which is meant to be topped up with compost annually rather than rebuilt. More compost is not better: beds built at well over half compost slump heavily in the first season and can run high enough in nutrients and salts to hurt germination.
Assumptions and limits
- Measurements are the INSIDE of the frame. Sleepers and thick boards can take two to four inches off each dimension, which on a 4 ft bed is most of a bag.
- Fill depth is not the same as bed height. Beds are filled an inch or two below the rim so water and mulch stay in, and a bed sitting on open soil does not need filling to the bottom — roots carry on down into the ground beneath.
- Bulk and bagged material both settle. Expect to lose roughly 10-20% of the depth over the first season, more if the mix is compost-heavy. Budget a top-up rather than over-filling on day one.
- The component split is by volume, not by weight or by bag count. Compost, coir and vermiculite have very different densities, so a cubic foot of each does not weigh the same.
- This calculates volume only. A filled 4 x 8 ft bed at 11 in holds well over a ton once wet, which matters if it sits on a roof, a deck or a balcony.
Fill depth versus bed height
The most common way to over-buy is to enter the height of the frame. Two things reduce it. You fill to an inch or two below the rim, or the first heavy watering washes mix over the edge. And if the bed sits on open ground rather than a hard surface, roots grow straight down through the base into the soil underneath — so a 17-inch frame on soil rarely needs 17 inches of bought mix.
A bed on a deck, a roof or a concrete yard is the exception: there is nothing underneath, so the fill depth is the whole rooting depth and the figures here are the real requirement.
Choosing between the three mixes
Half and half is the default for good reason — cheap, forgiving, and the topsoil brings mineral content that pure organic mixes lack. Its weakness is structure: it slumps as the compost fraction breaks down, so it wants topping up every spring.
Thirds holds its structure far longer because vermiculite does not decompose, which is why it is the standard for shallow beds where every inch counts. It is also, by a wide margin, the most expensive of the three, and the cost scales with volume — reasonable for a 4 x 4 ft bed, painful across six long ones.
Loam-based is the compromise for deep beds and hungry crops: mostly mineral soil, enough compost to feed, a little aeration to stop it setting. It is the heaviest of the three, which matters only if the bed is not on the ground.
Bags or bulk
The cubic-yard figure is the one to check before ordering. Bulk delivery usually carries a minimum and a flat delivery charge, so below roughly half a yard bags win on total cost even though they cost several times as much per cubic foot. Above about a yard bags stop making sense — a single 4 x 8 ft bed is already 20 to 30 bags at the common 1.5 cu ft size.
If the number lands just over a round yard, check whether an inch off the fill depth brings it under. On a 4 x 8 ft bed an inch is about 2.7 cubic feet.