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 — litres, if that is how your bags are labelled — 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
Consider buying about ten per cent more than the number above as an optional purchasing allowance. 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 one adjustable example to start from — not an extension-verified formula — and the optional breakdown above splits your total that way. Adjust the percentages to suit your materials and crops.
Watch the ground underneath. A bed on open soil can be part-filled with rougher material. For a bed on a solid surface, provide drainage outlets so excess water can escape. Do not add a gravel or coarse layer solely to improve drainage; it can impede drainage rather than help.
Compare bulk vs bags past about eighteen bags. One cubic yard is 27 cubic feet — around 18 bags at 1.5 cubic feet. Get a delivered bulk quote and compare it against bag prices for your volume; delivery charges vary by supplier.
Bag labels do not all use the same unit. Bulk and raised bed soil is usually sold by the cubic foot, while potting mix is sold in US dry quarts, and the two are easy to confuse on a shelf. The soil volume converter turns one into the other in either direction, and garden soil vs potting soil vs topsoil covers which of them you actually want.
Getting it into the frame is its own job. How to fill a raised garden bed takes the volume this calculator gives you through levelling, layering and settling, in the order the work happens.
Sources
- Tools, tips, and terrible traditions for raised beds – Part 3 — The Garden Professors (Dr. Linda Chalker-Scott, WSU Extension). Against adding gravel or coarse material at the bottom of raised beds (perched water table).
- Soil Health in Raised Beds — Penn State Extension. 70% soil / 30% compost recommendation.