Compost Calculator – How Much Compost Do I Need?
Borders, raised beds or pots — measure up and we'll work out the litres, bags and bulk bags of compost, with a topsoil mix option for deep beds.
Compost Calculator
What are you doing?
50mm of mulch suppresses weeds and holds moisture. Spread it over damp soil, keeping it off plant stems.
Measure rectangles length by width. Split an L-shape into two rectangles, or measure a circle straight across its widest point.
Measure your border
15 m²
Odd-shaped garden? Split it into rectangles, triangles and circles — we add them up for you.
You need
825litres of compost
Including 10% settling allowance
- Total volume
- 0.83 m³
- 60L bags
- 14
- 1000L bulk bags
- 1
- Before settling
- 750 L
How we worked this out
Your shopping list
14 × 60L bags of multi-purpose compost
Or 1 × 1000L bulk bag if you would rather buy loose.
£98.00
Compost needed at a glance
| Area | 50mm mulch | 75mm dug in | 100mm new border |
|---|---|---|---|
| 5 m² | 250 L | 375 L | 500 L |
| 10 m² | 500 L | 750 L | 1,000 L |
| 20 m² | 1,000 L | 1,500 L | 2,000 L |
| 50 m² | 2,500 L | 3,750 L | 5,000 L |
| 1.8 × 1.2m raised bed, 450mm deep | — | — | 972 L |
How this calculator works
Compost is sold by volume in the UK, so everything here works in litres. One cubic metre is 1,000 litres, which gives a handy shortcut: the depth in millimetres is the same number as the litres per square metre. Spread 50mm of mulch and you need 50 litres per square metre; 100mm on a new border needs 100 litres per square metre.
For borders we take area × depth. For raised beds we take length × width × internal height × number of beds. For pots we add up the litres of each container — a 30cm pot holds roughly 13 litres, a 50cm pot about 60.
Every result gets a 10% settling allowance because compost slumps once it has been watered in and trodden down. Bags are then rounded up to whole numbers, because no merchant will sell you four fifths of a bag.
Worked example: mulching a 10m² border
A border 5m long by 2m wide is 10m². A 50mm mulch layer is 10 × 50 = 500 litres. Add the 10% settling allowance and you are at 550 litres, which is ten 60-litre bags — about £70 at £7 a bag.
Compare that with a 1,000-litre bulk bag at around £95. You would have 450 litres left over, but the price per litre is roughly half. If you have more borders to do, or a raised bed to fill, the bulk bag is the better buy — just make sure the lorry can get a crane or forklift close enough to drop it where you want it.
Filling raised beds without spending a fortune
A standard 1.8m × 1.2m raised bed at 450mm deep swallows nearly 1,000 litres. Filling that with bagged compost alone costs well over £100. Two better options: fill the bottom third with rough material — turf laid upside down, small branches, cardboard and old compost — then top with a 60/40 topsoil and compost mix, or simply buy a bulk bag of topsoil and a bulk bag of compost and blend them with a shovel on a tarpaulin.
The 60/40 default in this calculator gives structure and water retention while still being rich enough for vegetables. For carrots and parsnips go lighter on the compost; for courgettes and beans go heavier.
Tips from the trade counter
- Mulch onto damp soil. Mulching bone-dry ground locks the dryness in. Wait for rain, or soak the border first.
- Keep mulch off stems. Leave a 50mm collar around trunks and woody stems so they do not rot.
- Check litres, not weight. Compost bags are sold by volume. A "heavy" bag usually just means wet compost.
- Buy peat-free. Retail peat sales are being phased out in the UK and modern peat-free mixes perform well — just water pots a little more often.
- Order slightly over for beds. Fill proud; you will lose 10–20% in the first month and topping up later means another delivery charge.
Bulk bag deliveries
Bulk bags weigh several hundred kilos even when they are sold by the litre. Make sure there is firm, level ground for the delivery and that you are happy for it to be craned over a fence — most drivers will not carry bags through a house or down a side path.
Frequently asked questions
How many litres of compost do I need per square metre?
At 50mm deep you need 50 litres per m², at 75mm it is 75 litres and at 100mm it is 100 litres. The rule is simple: depth in millimetres equals litres per square metre.
How much compost is in a bulk bag?
A garden bulk bag of compost is usually quoted as around 1,000 litres (1m³), though some suppliers sell 600–800 litre bags. Check the litres, not the weight — compost weight varies hugely with moisture.
Can I fill a raised bed with compost alone?
It is not ideal. Pure compost slumps, dries out fast and can be too rich. A mix of roughly 60% topsoil to 40% compost gives structure, holds water and still feeds plants.
How deep should mulch be?
50mm is the usual mulch depth on borders — enough to suppress weeds and hold moisture. Keep it clear of plant stems and tree trunks to avoid rot.
Why does the calculator add 10%?
Compost settles once it is watered and walked on, typically by 10–20%. The settling allowance means you are not two bags short at the end, and you can always top up in spring.
Is peat-free compost as good?
For beds, borders and most containers, yes — and peat is being phased out for UK retail sale. Peat-free dries out faster at the surface, so water a little more often in pots.
Should I buy bags or a bulk bag?
Above roughly 15–20 standard bags, a bulk bag is usually cheaper per litre once delivery is counted. Below that, bags are easier to handle and store.