Working out soil volume
Running out of soil halfway through filling a bed means a second trip and a second delivery fee. Over-ordering leaves a heap on the drive. Both come from the same two mistakes: mixing up units, and measuring the outside of the bed instead of the inside.
Keep every dimension in the same unit
This is where most calculations go wrong. Beds are measured in feet, but depth is almost always quoted in inches. Multiply 4 × 8 × 10 without converting and you get 320 cubic feet instead of 26.7 — twelve times too much. Divide inches by 12 first, so 10 inches becomes 0.83 feet. In metric, divide centimetres by 100.
Measure the inside of the bed
A raised bed built from 2-inch scaffold boards has an interior four inches narrower than its outside dimensions. On a 4 ft × 8 ft frame that is nearly 8% of the volume. Measure from inner face to inner face.
How deep to fill
Depth drives cost more than footprint does — doubling the depth doubles the soil bill. These are working minimums rather than ideals:
| Crop | Minimum | Comfortable |
|---|---|---|
| Salad leaves, herbs | 6 in / 15 cm | 8 in / 20 cm |
| Peppers, beans, brassicas | 8 in / 20 cm | 10 in / 25 cm |
| Tomatoes, courgettes | 12 in / 30 cm | 18 in / 45 cm |
| Carrots, parsnips | 12 in / 30 cm | 18 in / 45 cm |
| Potatoes | 12 in / 30 cm | 24 in / 60 cm |
A bed sitting on open ground can be shallower than the table suggests, because roots carry on down into the soil beneath. A bed on paving or a membrane cannot, so treat the minimum as firm.
Choosing between the four common mixes
- Topsoil
- Cheapest by volume, heavy, quality varies a lot by supplier. Good for levelling ground or bulking out the bottom of a deep bed. Not for containers.
- Garden soil
- Screened topsoil with organic matter added. Fine in the ground or as one component of a raised bed blend, still too dense on its own in a pot.
- Raised bed mix
- Topsoil, compost and a drainage material such as perlite or coarse sand. Holds moisture without waterlogging. The default choice for a vegetable bed.
- Potting mix
- Peat or coco coir with perlite and vermiculite. Light, drains fast, resists compaction. The only one of the four that belongs in a container.
Bringing the cost down
- Switch to bulk past ~1.5 cubic yards. Below that the delivery charge cancels the saving.
- Fill the bottom third of a deep bed with woody material. Logs and prunings rot down slowly and hold water. On a 24-inch bed this cuts bought soil by roughly a third.
- Blend your own. Equal parts topsoil, compost and coir usually beats bagged raised bed mix on price.
- Check for municipal compost. Many councils sell green-waste compost far below retail. Coarser than bagged, fine mixed in.
Irregular and L-shaped beds
Split the shape into rectangles, run each through the calculator, and add the results. For a curved border, fit the largest rectangle you can inside it, then add roughly a third again for the curved edges. Erring high is cheap — spare soil tops up other beds; running short costs another delivery.
Three worked examples
Seeing the formula applied to a few different shapes makes it easier to trust the number your own bed produces.
A 24-inch round pot, filled 14 inches deep
Radius is 12 inches, or 1 foot. Area = π × 1² = 3.14 sq ft. Depth is 14 ÷ 12 = 1.17 ft, so volume = 3.14 × 1.17 = 3.66 cu ft. Add 10% for settling and it's 4.03 cu ft — three 2-cubic-foot bags of potting mix, with a little left over.
A triangular corner bed, 6 ft base, 4 ft height, 8 inches deep
Area = ½ × 6 × 4 = 12 sq ft. Depth is 8 ÷ 12 = 0.67 ft, so volume = 12 × 0.67 = 8.0 cu ft. With settling that's 8.8 cu ft — five 2-cubic-foot bags.
A 4 × 8 bed filled 18 inches deep, for deep-rooted crops
32 sq ft × 1.5 ft = 48 cu ft, plus settling = 52.8 cu ft, or 1.96 cubic yards. That crosses the roughly-1.5-cubic-yard line where bulk delivery usually beats bags — eighteen 3-cubic-foot bags versus about two yards from a bulk supplier.
Mistakes that throw off a soil estimate
- Confusing bed height with fill depth. A 12-inch-tall raised bed that already has 4 inches of old soil in it only needs 8 inches of new soil, not 12. Measure what you're actually adding.
- Treating a tapered pot as a straight cylinder. Most pots are narrower at the base than the rim. Averaging the top and bottom diameter before calculating gets much closer than using the rim measurement alone, which overstates volume.
- Assuming a labeled "2 cu ft" bag delivers 2 cu ft in the bed. Bagged mixes are compressed for shipping and expand somewhat when opened, but they also settle fast once watered — treat the label figure as a rough guide, not an exact one.
- Buying "topsoil" and expecting planting-ready soil. Screened topsoil is a volume product, not a fertility product — it's often low in organic matter and needs compost worked in before anything grows well in it.
Planning what goes into the bed next? The Square Foot Planner works out how many plants it will hold, and the Garden Cost Estimator totals up soil, timber and plants for a whole plot.