Trying to figure out how much soil you need for your new planter? This tool should help you determine roughly how much compost you need based on the dimensions of your planter. If you’re looking for a bespoke steel planter, we can supply a custom-built one specced exactly how you need.
Compost Calculator
How Our Calculator Works
To find out how much compost a planter holds, you need the area of its base and the depth you’ll fill it to. The depth is the inside height, minus the gap at the top and any drainage or filler at the bottom.
Planter Area Formulas by Shape
- Square: width × width
- Trough or rectangle: length × width
- Round: 3.14 × radius × radius
- Hexagon: 0.866 × flat to flat × flat to flat
- L-shaped: (long side + short side − arm width) × arm width
Measure in metres, multiply the area by the depth, and you have the volume in cubic metres. Multiply that by 1,000 to get litres, which is how compost is usually sold.
Example: Filling a 120cm Trough
If we use a trough 120cm long, 40cm wide and 50cm tall, filled to 3cm below the rim, we can use these calculations:
- Base area: 1.2m × 0.4m = 0.48m²
- Compost depth: 50cm − 3cm = 47cm, or 0.47m
- Volume: 0.48 × 0.47 = 0.226m³, which is 226 litres
- Add 10% for settling: 248 litres, or 4 × 70 litre bags
So 4 70 Litre bags of soil will fill this planter, with some left over. Our calculator just provides a simpler, visual way of working this out
Using Less Compost in a Deep Planter
You don’t have to fill a tall planter to the top with compost. Most plants only need around 30 to 45cm, so you can fill the bottom of a deep planter with upturned plastic pots or bottles and buy fewer bags. Our calculator has an option to select a drainage layer, so it will work out the savings for you. It keeps the compost at the right depth for what you’re growing and never fills more than the bottom half.
For more on liners, drainage and planting up, see our guide on how to fill a large planter and manage drainage.
Bespoke Steel Planters
If you need a planter in a specific size or shape for your garden, the team at Luxunique would be happy to manufacture a bespoke planter in corten or mild steel to suit your needs. Just fill in the form on or contact page and we’ll get back to you
Filling a Planter: Common Questions
Bedding plants and herbs manage in about 20cm. Most vegetables want 30cm, perennials and grasses 40cm, and shrubs around 45cm. Small trees and climbers need 60cm or more.
Fresh compost is full of air. Once it’s watered and firmed in, it can drop by 10% or more over the first few weeks, so it’s worth having a bag spare to top up.
Damp multipurpose compost weighs around 500kg per cubic metre. John Innes and other loam-based composts are much heavier, and topsoil can be 1.4 to 1.5 tonnes. If weight matters, on a balcony or roof terrace for example, fill the bottom with upturned pots or bottles.
For bedding, vegetables and other seasonal planting, a peat-free multipurpose compost is fine. Shrubs, trees and anything else staying put for years do better in a loam-based compost such as John Innes No.2 or No.3, which holds its structure for longer. Camellias, rhododendrons and blueberries need ericaceous compost.