How the raised bed soil calculator works
A raised bed is an open box, so the soil it holds is the inside length times the inside width times the depth you fill. Measure inside the boards: 5 cm timbers on a 2.4 m frame leave only 2.3 m of growing space, and that difference adds up across several beds. The result is multiplied by the number of beds, and 15% is added because a fresh, fluffy mix of topsoil and compost sinks noticeably after the first watering and over the first season.
Mix = length × width × depth × beds × 1.15, then split into topsoil and compostThe split follows the ratio you choose. A 60/40 blend of screened topsoil and compost is a good all-round filling: the soil gives structure and holds water, the compost brings nutrients and life. Bags are counted separately for each part, so you know exactly what to put in the car.
Worked example
Two beds of 2.4 m × 1.2 m filled 30 cm deep hold 2.4 × 1.2 × 0.30 = 0.864 m³ each, 1.728 m³ together. With 15% for settling you need 1.99 m³ of mix. At 60/40 that is 1,192 litres of topsoil and 795 litres of compost, or 24 and 16 bags of 50 litres: forty bags in all. The same job in imperial units, two 8 × 4 ft beds 12 in deep, needs 73.6 ft³ (2.73 yd³): 30 bags of 1.5 cu ft topsoil and 20 of compost. At that size, a bulk delivery of mixed soil is usually cheaper than bags.
Soil volume for common bed sizes
| Bed (inside) | Depth | Settled volume | Bags before settling allowance |
|---|---|---|---|
| 1.2 × 1.2 m | 30 cm | 432 L | 9 × 50 L |
| 2.4 × 1.2 m | 30 cm | 864 L | 18 × 50 L |
| 2.4 × 1.2 m | 45 cm | 1,296 L | 26 × 50 L |
| 4 × 4 ft | 12 in | 16 ft³ (0.59 yd³) | 11 × 1.5 cu ft |
| 8 × 4 ft | 12 in | 32 ft³ (1.19 yd³) | 22 × 1.5 cu ft |
| 8 × 4 ft | 18 in | 48 ft³ (1.78 yd³) | 32 × 1.5 cu ft |
How deep does a raised bed need to be?
Most vegetables root in the top 30 cm (12 in). Salad leaves, radishes and herbs manage with 15–20 cm (6–8 in), tomatoes, peppers and squash prefer 30–45 cm (12–18 in), and long carrots or parsnips like 30 cm or more of loose soil. If the bed sits on garden soil, roots carry on downward, so 30 cm is plenty; on paving or a roof the bed itself must give the full depth.
Tips for filling raised beds
- Fill tall beds with bulk first. Beds deeper than about 45 cm (18 in) can have the bottom third filled with logs, branches and turf turned upside down. Reduce the depth in the calculator by that layer, and expect it to settle further as it rots.
- Do not use bagged potting mix alone. It is light, dries out fast and shrinks badly in a bed. Blend it with real topsoil.
- Mix before you fill. Tip the topsoil and compost onto a tarp and turn them together, or fill in alternate layers, so the bed is even from top to bottom.
- Top up every year. Beds lose 3–5 cm (1–2 in) a season as organic matter breaks down; a layer of compost each spring keeps the level up.
- Weigh up treated timber. For food beds, many gardeners prefer untreated durable wood or a liner between soil and boards.
Frequently asked questions
How much soil do I need for a 4x8 raised bed?
A 4 × 8 ft bed filled 12 in deep holds 32 ft³, about 1.2 cubic yards. Allow 15% for settling: roughly 37 ft³, or 25 bags of 1.5 cu ft.
How many bags of soil fill a raised bed?
Divide the bed volume by the bag size. A 2.4 × 1.2 m bed 30 cm deep holds 864 litres, so about 20 bags of 50 litres once you add 15% for settling.
What is the best soil mix for raised beds?
A blend of about 60% screened topsoil and 40% compost suits most vegetables. Heavy feeders do well at 50/50; 70/30 holds water better in hot, dry places.
Can I fill the bottom of a raised bed with something else?
In beds deeper than about 45 cm (18 in), the bottom layer can be logs, branches or upturned turf. Keep at least 30 cm (12 in) of real soil mix on top.
How much does it cost in bags versus bulk?
Above roughly 1 m³ or 1.5 yd³, bulk delivery of blended soil is usually cheaper than bags. Compare the price per cubic metre or yard, not per bag.
Last reviewed: 2026-09-27 · How we calculate