What this calculator does
Filling a raised bed, garden bed or large pot with soil or potting mix comes down to one calculation: length times width times depth gives the volume needed. The part that usually goes wrong is depth, which is normally worked with in centimetres while length and width are in metres, so the units need converting before they are multiplied together.
This soil calculator takes the bed or pot dimensions and a fill depth, works out the volume in litres and cubic metres, and converts that to a number of bags using a bag size you can set yourself, since bag sizes vary between brands and between garden soil, potting mix and compost.
The formula
Multiply length by width to get the area in square metres, then multiply by the fill depth (converted from centimetres to metres) to get the volume in cubic metres. Multiply by 1,000 to convert to litres, then divide by the bag size to get the number of bags, rounded up since a partial bag still needs to be bought.
| Term | Meaning |
|---|---|
| Fill depth | How deep the layer of soil or potting mix needs to be, typically 15 to 30 cm for a vegetable bed or planter. |
| Bag size | The volume of soil in a single bag, in litres, as printed on the product; common sizes include 25 L and 30 L but this varies by brand. |
| Volume | The total amount of soil needed to fill the bed or pot at the chosen depth, in litres and cubic metres. |
The inputs explained
| Field | What to enter |
|---|---|
| Bed/pot length (m) | The length of the bed or pot, in metres. |
| Bed/pot width (m) | The width of the bed or pot, in metres. For a round pot, use the diameter as an approximation. |
| Fill depth (cm) | How deep the soil layer needs to be, in centimetres. |
| Bag size (L) | The volume of a single bag of soil or potting mix, in litres, from the product packaging. |
When to use it
Filling a new raised garden bed
A raised bed built from sleepers or boards has a fixed length, width and height; filling it to a sensible depth for root growth is the most common use of this calculator.
Topping up an existing bed
Beds settle over a season as soil compacts and organic matter breaks down, so working out the volume for a shallower top-up depth avoids buying far more than needed.
Filling several large pots or planters
Treating a pot as a rectangular approximation using its diameter for both length and width gives a workable estimate of the bags needed, even though a round pot holds slightly less than a square one of the same width.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How much soil does a 1.2 m × 0.9 m bed need at different depths?
A fixed 1.2 m by 0.9 m bed footprint, filled to a range of depths.
| Fill depth | Soil needed | Bags needed |
|---|---|---|
| 10 cm | 108.00 L | 5 × 25 L bags |
| 15 cm | 162.00 L | 7 × 25 L bags |
| 20 cm | 216.00 L | 9 × 25 L bags |
| 25 cm | 270.00 L | 11 × 25 L bags |
| 30 cm | 324.00 L | 13 × 25 L bags |
| 40 cm | 432.00 L | 18 × 25 L bags |
How does the bag count change with bag size, for the same bed?
The same 216 L of soil needed, split across a range of bag sizes.
| Bag size | Bags needed |
|---|---|
| 10 L | 22 × 10 L bags |
| 20 L | 11 × 20 L bags |
| 25 L | 9 × 25 L bags |
| 30 L | 8 × 30 L bags |
| 40 L | 6 × 40 L bags |
| 50 L | 5 × 50 L bags |
Questions
How deep should soil be in a raised bed?
Most vegetables and flowers do well with 20 to 30 cm of soil depth, though shallow-rooted crops like lettuce can manage with less and deep-rooted crops like carrots benefit from more. Check the depth guidance for whatever is being planted.
Does this account for soil settling over time?
No, it calculates the volume to fill the bed to the depth entered today. Soil typically settles and compacts over a season, so beds are often topped up later rather than overfilled at the start.
How do I calculate soil for a round pot?
Use the pot's diameter as both the length and width. This treats a round pot as if it were square, which slightly overestimates the true volume, since a circle covers about 79% of the area of the square that contains it, so treat the bag count as an upper estimate for round containers.
Why does the bag count round up instead of showing a decimal?
Bags are bought whole, so a calculated requirement of 8.6 bags still means buying 9. The calculator rounds up to the nearest whole bag for that reason.
For bulk garden materials priced by the cubic metre rather than by the bag, see the mulch, soil and gravel calculator.