What this calculator works out
This calculator multiplies length, width and depth to give the volume of compost needed in litres, then divides by your bag size to give a bag count.
Bulk garden materials are sold by volume, and volume is where the ordering goes wrong. A bed that looks like it needs a couple of bags routinely needs six.
Enter your details
Before you order
- Depth is in centimetres. A mulch layer is 5 to 10 cm; filling a new bed is 20 to 30 cm or more.
- Standard bag sizes are commonly 50 or 60 litres. Check the bag rather than assuming.
- Bagged compost settles once watered and trodden — allow roughly 10 to 20% more than the calculated volume.
- For anything above about ten bags, compare the price against a bulk bag, which typically holds 700 to 900 litres.
That single relationship covers most bulk garden calculations. A 10 m² bed mulched to 5 cm needs 500 litres; a 2 m² bed filled to 30 cm needs 600 litres. Once the relationship is memorable, the arithmetic stops needing a calculator.
How this calculator works
A volume converted to litres, then to bags:
Litres = length × width × (depth ÷ 100) × 1,000
Bags = litres ÷ bag size, rounded upDividing depth by 100 converts centimetres to metres so all three dimensions share a unit; multiplying by 1,000 converts the resulting cubic metres to litres. The bag count rounds up, since part bags cannot be bought.
Worked example: a 2 m by 1 m bed
Using the default figures — 2 m by 1 m at 10 cm deep, in 50 litre bags:
- Volume: 200 litres
- Bags: 4
Four bags for a small bed mulched to 10 cm. Extend that to a 10 m² border at the same depth and it becomes 1,000 litres — twenty bags, or a bulk bag with some left over. The point at which bags stop making sense arrives sooner than most people expect, and bulk is usually less than half the price per litre.
Common mistakes
- Entering depth in metres. The field expects centimetres.
- Not allowing for settling. Compost loses 10 to 20% of its volume once watered in.
- Buying bags for a bulk job. Above roughly ten bags, a bulk bag is usually cheaper.
- Confusing compost with topsoil. They serve different purposes and are not interchangeable by volume alone.
- Mulching too deeply against stems. Keep mulch clear of trunks and crowns.
Frequently asked questions
What is the difference between compost, topsoil and manure?
Compost is decomposed organic matter, high in nutrients and structure but low in mineral content. Topsoil is mineral soil with some organic matter and is what gives a bed body. Well-rotted manure is a soil improver with more nutrients than compost. A new raised bed usually wants a mix — commonly around 60 to 70% topsoil to 30 to 40% compost — rather than compost alone.
How much does a bulk bag hold?
Typically 700 to 900 litres, though suppliers vary and some sell smaller bags under the same name. Ask for the volume in litres or cubic metres rather than accepting 'bulk bag' as a measure. At the larger end, one bulk bag replaces around fifteen 60 litre bags.
Can I make enough compost myself?
For mulching a whole garden, rarely — a typical household bin produces a few hundred litres a year. Home compost is best used where its quality matters most, such as sowing and potting mixes or feeding vegetable beds, with bought material making up the bulk for mulching.
Is peat-free compost as good?
Modern peat-free composts perform well for most purposes, though they behave differently — many dry out faster at the surface while staying wet lower down, so watering habits need adjusting. Sales of peat compost to amateur gardeners have been subject to phase-out commitments in the UK, and peat extraction damages carbon-storing habitats.
Is what I enter stored?
No. Measurements are processed in your browser and never transmitted or retained.
Related tools
References
- GOV.UK — guidance on peat use, garden waste and soil
- Royal Horticultural Society — practical advice on composts, soil improvers and mulching
Sources are checked at publication and can change — how I choose and check references.