Planning a new raised bed or refilling an existing one is much easier when you know the exact volume of soil, compost or growing medium you need before you order. This calculator covers rectangular beds, circular beds, triangular borders and L-shaped beds, supports both metric and imperial dimensions, and converts the result to litres, cubic metres and US gallons in one step. It also tells you how many bags to buy and, optionally, what the material will cost.
How it works
Soil volume is a straightforward geometry problem:
Volume = Base area × Fill depth
The base area depends on your bed shape:
- Rectangle or square:
length × width - Circle:
π × radius²(enter the diameter; the calculator halves it) - Triangle:
½ × base × height— useful for corner beds or diagonal borders - L-shape:
total length × total width − cutout length × cutout width
Once the area is known it is multiplied by the fill depth. The raw volume in cm³ is divided by 1,000 to give litres, then converted to m³ and US gallons. A user-adjustable overfill percentage (default 10 %) is applied before the final figure so that soil settlement in the first season is already budgeted for. The bag count is the total litre volume divided by the bag size, rounded up to the nearest whole bag, because partial bags cannot be returned.
Worked example
A standard raised bed is 200 cm × 100 cm (2 m × 1 m), filled to a depth of 20 cm — a good depth for most vegetables.
| Step | Calculation | Result |
|---|---|---|
| Base area | 200 cm × 100 cm | 20,000 cm² |
| Volume | 20,000 cm² × 20 cm | 400,000 cm³ |
| Convert to litres | 400,000 ÷ 1,000 | 400 L |
| +10 % overfill | 400 × 1.10 | 440 L |
| Bags (50 L each) | 440 ÷ 50 → round up | 9 bags |
Now imagine the same bed but deeper — 30 cm for root vegetables:
- Volume: 600,000 cm³ = 600 L
- +10 %: 660 L
- Bags (50 L): 14 bags
That extra 10 cm adds five bags, which is easy to underestimate when ordering by eye.
Depth reference
Different crops have different root-depth requirements. Use the preset dropdown or these guidelines:
| Crop type | Recommended fill depth |
|---|---|
| Salad leaves, herbs, radishes | 15 cm (6 in) |
| Tomatoes, courgettes, beans | 20 cm (8 in) |
| Onions, leeks, chard | 25 cm (10 in) |
| Carrots, parsnips, beetroot | 30–45 cm (12–18 in) |
| Fruit bushes, small shrubs | 45 cm (18 in) or more |
Everything is calculated in your browser — no figures are ever sent to a server or stored.