Getting this sum wrong is expensive in both directions. Guess low and you finish the weekend with a half-filled bed and another trip to the garden centre. Guess high and you have six unopened bags in the garage until next spring. This calculator gives you the volume in cubic feet, cubic yards and liters, and the number of bags at whatever size your shop sells.
How to use it
- Pick the shape. Rectangle covers the usual timber-framed bed. Circle covers round planters, galvanised rings and stock tanks.
- Measure inside the frame, not outside. This is where most people lose a bag. A bed built from two-inch lumber is four inches narrower inside than its outside dimensions suggest — on a four by eight foot bed that is nearly two cubic feet.
- Enter the depth you will actually fill to, not the height of the walls. Most people stop an inch or two below the rim so watering does not wash soil over the edge.
- Set the number of beds if you are filling several the same size.
- Choose the bag size your shop stocks. One, one and a half, two and three cubic feet are the common US sizes; 20 to 70 liters is usual elsewhere. Bag size is what decides how many you carry home, so it is worth checking before you drive over.
- Switch to metric at the top if you would rather work in metres and centimetres. The numbers already in the form convert with it.
How it is calculated
For a rectangular bed:
cubic feet = length (ft) x width (ft) x depth (in) / 12
The division by twelve is the whole trick. Depth is in inches while length and width are in feet, so the depth has to be converted to feet before the three multiply together. Skip it and the answer comes out twelve times too large.
For a round bed, the area is a circle rather than a rectangle:
cubic feet = pi x (diameter / 2)^2 x depth (in) / 12
Then the conversions, which are fixed:
- Cubic yards = cubic feet / 27, because a yard is three feet and 3 x 3 x 3 = 27.
- Liters = cubic feet x 28.317.
- Bags = total volume / bag size, always rounded up, because a shop will not sell you two thirds of a bag.
A worked example
A bed eight feet long, four feet wide, filled twelve inches deep:
- Area: 8 x 4 = 32 square feet
- Volume: 32 x 12 / 12 = 32 cubic feet
- In cubic yards: 32 / 27 = 1.19 cubic yards
- In liters: 32 x 28.317 = 906 liters
- At 1.5 cubic feet a bag: 32 / 1.5 = 21.3, so 22 bags
Halve the depth to six inches and every figure halves with it.
Common bed sizes
| Bed size | 6 in deep | 8 in deep | 12 in deep | Bags at 1.5 cu ft, 12 in |
|---|---|---|---|---|
| 2 x 4 ft | 4 cu ft | 5.3 cu ft | 8 cu ft | 6 |
| 3 x 6 ft | 9 cu ft | 12 cu ft | 18 cu ft | 12 |
| 4 x 4 ft | 8 cu ft | 10.7 cu ft | 16 cu ft | 11 |
| 4 x 8 ft | 16 cu ft | 21.3 cu ft | 32 cu ft | 22 |
| 4 x 12 ft | 24 cu ft | 32 cu ft | 48 cu ft | 32 |
| 5 ft circle | 9.8 cu ft | 13.1 cu ft | 19.6 cu ft | 14 |
Tips and common mistakes
Buy about ten per cent more than the number above. Fresh mixes settle as the organic matter in them breaks down, and a bed filled level in April usually wants topping up by the following spring.
Bagged volumes are measured loose. Compressed compost swells when you break it up, and a very wet bag weighs more but does not contain more. The cubic foot figure on the bag is what to trust.
Do not fill a deep bed entirely with pure compost. It holds water like a sponge at first and then collapses as it rots down. A mix of roughly 60% topsoil, 30% compost and 10% of something coarse for drainage is a reasonable starting point, and the optional breakdown above splits your total that way.
Watch the ground underneath. A bed on open soil can be part-filled with rougher material. A bed on a patio, a balcony or a liner needs the whole volume in bought mix, and something coarse at the base to stop it going sour.
Order bulk soil in cubic yards, not bags, past about eighteen bags. One cubic yard is 27 cubic feet — around 18 bags at 1.5 cubic feet — and that is usually the point where bulk delivery starts to win on price.