Planter Size and Compost Calculator
Ordering compost is easier once you know the volume you are filling. This page gives you the sums, a lookup table for every planter we make, and a rough guide to how many bags that means.
Working out the volume yourself
Measure the inside of the planter in centimetres, not the outside, and use the depth you actually intend to fill rather than the full height.
- Square or rectangular: length x width x depth, divided by 1000, gives litres.
- Round: 3.14 x radius x radius x depth, divided by 1000, gives litres. The radius is half the internal diameter.
- Hexagonal (measured across the flats): width x width, divided by 1.73, then multiplied by depth and divided by 1000.
- Octagonal (measured across the flats): width x width x 0.83, multiplied by depth and divided by 1000.
A worked example: a bed 116 cm long, 56 cm wide, filled to 30 cm deep is 116 x 56 x 30 divided by 1000, which is about 195 litres.
How much each of our planters holds
Bag counts assume 50 litre bags, which is the most common size sold in garden centres. Round up rather than down, because compost settles as soon as it is watered.
| Planter | Compost capacity | 50 litre bags |
|---|---|---|
| Round Barrel Planter 40cm | About 35 litres | 1 |
| Hexagonal Planter 60cm | About 55 litres | 2 |
| Herb Planter Table 80cm | About 60 litres | 2 |
| Planter with Trellis 45cm | About 65 litres | 2 |
| Square Planter 50cm | About 80 litres | 2 |
| Rectangular Trough 100cm | About 85 litres | 2 |
| Octagonal Planter 80cm | About 150 litres | 3 |
| Raised Garden Bed 120 x 60cm | About 200 litres | 4 |
What to fill it with
Pure multipurpose compost is light, dries out quickly and slumps by a surprising amount in the first season. For anything larger than about 50 litres a mix works far better:
- Ornamental planting: roughly two parts peat-free multipurpose to one part topsoil, with a handful of grit if you are growing anything that resents wet feet.
- Vegetables and fruit: roughly equal parts topsoil, garden compost or well-rotted manure, and peat-free multipurpose.
- Deep beds: the bottom third can be cheaper topsoil or turf laid grass-side down. Roots rarely reach it in the first year.
Buying loose soil by the bulk bag usually works out cheaper than bagged compost once you get past about 300 litres, but check the delivery vehicle can reach you.
Allow for settling
Expect the level to drop by around a tenth once everything is watered in. Fill to just below the top rail, water thoroughly, leave it a day, then top up. Adding 5 cm of fresh compost each spring keeps the depth where you want it.
Choosing a size
If you are still deciding, our comparison table lists internal dimensions, planting depth and weight for every planter side by side. If none of them fit, we build to measure, so send us your dimensions and we will quote.