🪴 Garden
Potting Mix Volume
Work out how much potting mix a set of containers needs in litres, quarts and bags. Enter the container dimensions, then press Calculate.
About the Potting Mix Volume
Potting mix is sold by volume, usually in litres or dry quarts. A rectangular container's volume is length × width × height; a cylindrical pot uses π × radius² × height. Leave 2–3 cm below the rim for watering, and expect the mix to settle by 5–10% after the first few waterings.
A 40 cm × 30 cm × 30 cm planter is 36 litres, or about 38 dry quarts. Bag sizes vary: 20 L, 40 L, 1.5 cu ft (about 42 L) and 2 cu ft (about 57 L). Buy a little extra rather than running short mid-pot.
How the calculation works
- Rectangular volume (L) = L × W × H ÷ 1,000 (cm).
- Cylindrical volume (L) = π × (diameter ÷ 2)² × H ÷ 1,000.
- 1 L = 0.908 dry quarts; 1 US gallon = 3.785 L.
- Bags = total litres ÷ bag size.
Reference values
| Container | Dimensions | Volume | Dry quarts |
|---|---|---|---|
| Small pot | 15 cm diameter × 15 cm | 2.7 L | 2.4 qt |
| 1-gallon pot | 20 cm diameter × 20 cm | 6.3 L | 5.7 qt |
| 3-gallon pot | 28 cm diameter × 25 cm | 15.4 L | 14.0 qt |
| Planter | 40 × 30 × 30 cm | 36 L | 32.7 qt |
| Raised bed | 120 × 60 × 30 cm | 216 L | 196 qt |
Worked example: filling a planter
A 60 × 30 × 30 cm planter is 54 L gross. Leaving 3 cm of rim gives 60 × 30 × 27 = 48.6 L. A 40 L bag fills it with 8.6 L to spare; two 20 L bags are exactly 40 L and fall short. A 30 cm diameter pot, 30 cm deep, holds π × 15² × 30 ÷ 1,000 = 21.2 L.
Fill loosely and water thoroughly; compacting removes air and can make the mix waterlogged. Expect 5–10% settling after the first few waterings, so buy a little extra rather than running short mid-pot. For a large planter, a mix of 60–70% compost and 30–40% drainage material is a common starting point.
Raised beds and larger containers
A raised bed is usually the largest volume most gardeners calculate, and the answer is often bigger than expected. A bed 2.4 m long, 1.2 m wide and 30 cm deep holds 0.864 m³, or 864 litres — about 22 bags of 40 L. Filling the same bed to 45 cm would need 1,296 L, half as much again, so decide the depth before ordering.
Bulk suppliers sell by the cubic metre or cubic yard, which is far cheaper than bags at that scale. Check access before delivery: a bulk bag is often 0.7–1 m³ and needs a hard, level surface close to the bed. If the bed sits on soil rather than a hard surface, mixing in some native soil at the base reduces the volume required and improves drainage continuity.