Free tool
Compost & mulch calculator
Enter your bed size and we'll tell you how much compost, topsoil or mulch you need — in litres, 50-litre bags and bulk bags.
Filling a bed is usually 20–30 cm deep.
Standard bags of compost are about 50 litres.
Compost to fill this bed
720 litres
0.72 m³
50-litre bags
15
rounded up
Bulk bags
1.0
~0.7 m³ each
Estimates are guidance for UK gardens — compost settles as it breaks down, so it is worth buying a little extra. Topping a no-dig bed each year needs only a 3–5 cm layer.
How to use it
Choose whether you are filling a new bed or topping up an existing one with a mulch layer, then enter the bed's length and width in metres and the depth in centimetres. The results update as you type — no button to press. If you know the size of the bags you'll buy, set it (most bagged compost is 50 litres) and the calculator shows how many bags, plus the equivalent in bulk bags for bigger jobs.
How the calculation works
The maths is simple volume: length × width × depth, all converted to metres, gives the volume in cubic metres. Multiplying by 1,000 converts that to litres, which is how compost is sold. Bags are then rounded up so you don't come up short. Compost settles and breaks down over time, so it's always worth buying a little more than the exact figure — and if you make your own, you can fill a bed for almost nothing. See our guide to making your own compost and the no-dig method, which needs only a thin annual top-up.
Frequently asked questions
- How much compost do I need for a raised bed?
- Multiply the bed's length × width × depth (all in metres) to get the volume in cubic metres, then multiply by 1,000 for litres. A standard 1.2m × 2.4m bed filled to 25cm needs about 720 litres — roughly fourteen 50-litre bags. This calculator does the maths for you.
- How many litres are in a bulk bag of compost?
- A 'bulk bag' (also called a dumpy or jumbo bag) holds roughly 0.7 cubic metres, or about 700–800 litres. For anything more than a few bags, bulk is usually far cheaper per litre.
- How deep should a mulch layer be?
- For mulching, a 5–8cm layer is plenty — enough to suppress weeds and hold moisture without smothering plants. For a no-dig bed you only need to top up with a 3–5cm layer of compost each year.