Compost Calculator
Estimate compost volume, bags, bulk bags, weight and cost for borders, raised beds, pots, mulching and lawn top dressing.
Area, Depth And Pack Size
Compost To Buy
Compost settles after watering and handling. A small allowance is usually safer than ordering the exact mathematical volume.
What The Compost Estimate Means
Compost is usually bought by volume, not by weight. A bag labelled 50 litres contains 0.05 cubic metres. The calculator converts the area and depth into cubic metres, then into litres, bags and bulk bags. It also gives a broad weight estimate because moving a pallet, lifting bags into a car or carrying compost through a house can be harder than the volume alone suggests.
The result is designed for garden planning rather than laboratory measurement. Compost can be fluffy, wet, compacted, screened, bark-heavy or loam-based. A dry light multi-purpose compost bag can be much easier to move than a wet soil improver. For ordering, rely on the litre or cubic metre result first, then use the weight line as a handling check.
Formula And Method
Base cubic metres = area in m2 x depth in cm / 100Litres = cubic metres x 1,000Order litres = litres x (1 + allowance percentage)Bags to buy = order litres / bag litres, rounded upEstimated weight = order cubic metres x density tonnes per cubic metreThe calculation rounds bags up because shops do not sell part bags. Bulk bags are also rounded up so a project needing 0.82 m3 becomes two 0.75 m3 bags unless the supplier sells loose tipped loads. The allowance is included before rounding, so changing it can alter both the volume and the number of bags.
Typical Depths For Garden Jobs
| Job | Common depth | Practical note |
|---|---|---|
| Light soil improvement | 2 to 3 cm | Useful where soil already has decent structure. |
| Border top-up | 4 to 6 cm | Good for adding organic matter without burying crowns. |
| No-dig bed layer | 5 to 10 cm | Use more where weeds are suppressed by card or paper. |
| Mulch around shrubs | 5 to 8 cm | Keep compost away from woody stems. |
| Lawn top dressing | 0.3 to 1 cm | Brush in thinly; do not smother grass. |
| Raised bed fresh fill | 10 cm or more | Often needs a soil and compost blend, not compost alone. |
Worked Examples
Border Top-Up
A 12 m2 border with 5 cm of compost needs 0.6 m3 before allowance. With 10% extra, order about 660 L. In 50 L bags, that rounds to 14 bags.
Raised Bed
A 2.4 m by 1.2 m bed topped by 10 cm needs 288 L before allowance. A deeper new bed may need a mix of topsoil, compost and organic matter rather than compost only.
Container Batch
For pots, estimate each pot volume separately or use the total litres from labels. Keep a spare bag for topping up after watering because fresh compost often settles.
Buying Bags Or Bulk Compost
Small jobs are usually easier with bags because they can be carried, stored and opened as needed. Large borders, allotments and raised beds may be cheaper by bulk bag or loose load. Bulk deliveries need access, a place to tip, a wheelbarrow route and a plan for covering the pile in wet weather. If the calculator shows more than twenty small bags, compare a bulk price before ordering.
Peat-free composts can behave differently from old peat-based products. Some are wood-fibre rich and settle more after watering. Others hold water well but become heavy. For seed sowing, containers and ericaceous plants, choose a product made for the plant rather than buying the cheapest soil improver.
Before buying, check whether the product is a soil improver, a potting compost, a mulch or a specialist mix. Soil improver is often dug in or laid over borders, while potting compost is made for roots inside containers. A cheap mulch may look right on a shrub bed but lack the texture needed for sowing trays. Matching the product to the job keeps the volume estimate useful and prevents waste.
Measuring Tips
- Measure beds after edging, not from a rough memory of the garden.
- Split awkward shapes into rectangles and triangles, then add the areas.
- Use a thinner depth near plant crowns so stems are not buried.
- Add extra where the soil surface is uneven or compost will be walked in.
- Check car payload if collecting many wet bags.
FAQs
How many litres are in one cubic metre of compost?
One cubic metre is 1,000 litres. A 50 L bag is 0.05 m3, so twenty 50 L bags make one cubic metre before settling or spillage.
Should I add a waste allowance?
Yes for most garden jobs. Compost settles, spreads into hollows and can be lost during barrowing or tipping. Five to fifteen percent is a sensible range for many beds.
Can I fill a raised bed with compost only?
Usually not ideal for deep beds. Compost alone can settle and may hold too much or too little water. Many raised beds use a blend of topsoil, compost and organic matter.
Why does weight vary so much?
Moisture and product type drive weight. Wet loam-based compost can be much heavier than dry multi-purpose compost, even if the volume is the same.
How do I estimate compost for pots?
Use the pot volume in litres where available, or calculate pot volume separately. Add a small spare amount for topping up after watering.
Sources
- Royal Horticultural Society. (2026). Mulches and mulching. RHS. https://www.rhs.org.uk/soil-composts-mulches/mulching
- Royal Horticultural Society. (2026). Composts for containers. RHS. https://www.rhs.org.uk/soil-composts-mulches/composts-for-containers
- Garden Organic. (2026). Composting and soil improvement advice. Garden Organic. https://www.gardenorganic.org.uk/
- National Institute of Standards and Technology. (2008). Guide for the Use of the International System of Units (SI), Special Publication 811. NIST. https://www.nist.gov/pml/special-publication-811