OnlineCalculator360
The Compost Calculator works out exactly how much compost you need for any garden bed, raised bed, lawn area, border, or container — showing results in litres, cubic feet, cubic yards, and bag counts. Select your area shape and application mode, enter your dimensions, and the result updates instantly with a shopping-ready quantity and bag count.
Results are based on geometric volume calculations and a standard settling allowance, so they are purchasing estimates. Compost density and settling rate vary by product type and moisture content; for large landscaping projects, confirm quantities with your supplier before ordering in bulk. This calculator uses the standard depth × area volume method recommended by the Royal Horticultural Society (RHS) for home and garden use.
Spread compost on the surface as a layer — suits top-dressing lawns, beds, and borders each season.
Applying compost to multiple identical beds? Enter the count.
Advanced Options
Compost settles 10–20% after application and watering. This buffer ensures you buy enough.
Compost Needed
| Total area | -- | |
| Compost depth | -- | |
| Total (before settling) | -- | |
| Total to buy | -- | |
| Bags needed | -- | -- |
| Bulk equivalent | -- | |
How the Compost Calculator Works
Compost is decomposed organic matter — typically a mixture of garden waste, food scraps, leaves, and other organic materials broken down by microbes into a dark, crumbly soil amendment. Adding compost improves soil structure, water retention, and nutrient availability regardless of whether your starting point is heavy clay or fast-draining sandy soil.
The key variable in any compost calculation is how you intend to apply it. Spreading a thin surface layer requires far less material than filling a raised bed from scratch, and the two approaches involve different volume calculations. The calculator handles five distinct application modes so the result always matches your specific gardening task.
Top-Dressing Lawns and Beds
Top-dressing means applying compost as a thin surface layer — typically 1–5 cm (½–2 inches) — without incorporating it into the soil below. This is the most common home gardening approach: spread it in autumn or spring, and earthworms and rainfall work it in naturally over the following weeks. For lawns, a 1 cm (½ inch) layer applied twice a year is enough to measurably improve soil biology within a single growing season. For border beds, a 5 cm (2 inch) layer provides both weed suppression and the soil amendment benefit simultaneously.
Mixing Compost Into Existing Soil
Mixing compost into soil involves digging or tilling the compost into the upper layer of an existing bed before planting. The standard recommendation is a 25–33% compost amendment — roughly one part compost to three parts existing soil by volume. The calculator asks for your working depth (how deep you will dig) and your desired compost percentage, then computes only the compost fraction you need to purchase. A person amending a 3 m × 2 m bed to a 20 cm working depth at 30% will need approximately 360 litres — 14–15 × 25 L bags — compared to just 180 litres for a 5 cm top-dressing of the same bed.
Filling Raised Beds and Containers
A raised bed or container has no natural soil beneath it, so the full growing volume must be filled with a prepared growing medium. The most widely used recipe for raised beds is Mel's Mix, popularised by Mel Bartholomew in Square Foot Gardening: one-third compost, one-third peat moss or coir, one-third coarse vermiculite. The calculator defaults to 33% compost for raised beds and 40% for containers, both of which reflect standard growing media recommendations. Adjust the percentage to match your own blend. To calculate the full soil volume of a new raised bed across all three components, refer to our Raised Bed Soil Calculator.
Topping Up an Existing Compost Layer
Compost settles and breaks down over time, so established raised beds and borders need periodic refreshing. The Top-Up mode calculates only the additional compost needed to bring the current layer up to your target depth — not the full volume of the bed. Enter your existing (settled) depth and your target depth, and the calculator returns the difference volume as a buying quantity. This prevents over-ordering, which is the most common mistake when refreshing mature raised beds.
Compost Volume Formula
The calculator uses the standard depth-times-area volume formula, applied differently depending on the application mode selected.
Top-Dress and Top-Up volume:
Where:
- Area = length × width for rectangles and squares; π × r² for circles; ½ × base × height for triangles (all converted to m²)
- Depth = compost layer depth in centimetres
- 10 = conversion factor (1 m² × 1 cm of depth = 10 litres)
For example, if a rectangular bed measures 3 m × 1.2 m and the target top-dressing depth is 5 cm:
Mix Into Soil volume (compost fraction only):
For the same 3.6 m² bed, tilled to 20 cm at a 30% compost amendment:
Settling allowance:
For 180 litres with the default 10% settling allowance:
The calculator handles all unit conversions and the bag count ceiling automatically — the formulas are shown here for transparency.
What Your Result Means
Under 100 litres (up to 4 × 25 L bags) — A small weekend project. Most garden centres sell individual bags at this volume with no minimum purchase. Buy exactly what the calculator shows — there is no financial benefit to ordering bulk below this threshold.
100–400 litres (4–16 bags) — A standard garden project: one or two raised beds, a large border, or a full lawn top-dressing. At this volume, comparing the price per litre of bagged compost against a half-tonne bulk bag is worthwhile. Bulk becomes cheaper at roughly 350–400 litres in most markets.
400–800 litres (16–32 bags) — A bulk delivery is almost certainly more economical than buying individual bags. One standard dumpy bag (also called a builder's bag or tonne bag) holds approximately 700–800 litres of compost. Ask your supplier whether they can deliver in half-bag quantities if your total falls between 400 and 700 litres.
Over 800 litres (cubic yard range) — Plan a dedicated bulk delivery. At this volume, loose bulk compost delivered by the cubic yard is typically 40–60% less expensive per litre than bagged equivalents. The results table shows your total in cubic yards specifically to match the unit suppliers use for bulk orders.
Note that compost volumes quoted on bags are measured loose. Once spread and settled, the effective installed depth is 10–20% less than the raw volume suggests. The settling allowance in the calculator compensates for this so the final layer reaches your intended depth.
Compost Application Mode Reference
The table below summarises typical depth ranges, compost percentages, and appropriate scenarios for each application mode.
| Mode | Typical Depth | Compost % | Best For |
|---|---|---|---|
| Top-Dress | 1–5 cm (½–2 in) | 100% surface | Lawns, established borders, beds |
| Mix Into Soil | 15–25 cm working depth | 25–33% | New beds, soil preparation before planting |
| Raised Bed Blend | Full bed depth | 33% (Mel's Mix) | New raised beds with no existing soil |
| Container Blend | Full container depth | 40–50% | Pots, grow bags, window boxes |
| Top-Up Existing | Shortfall only | 100% of top-up | Refreshing settled raised beds |
Tips to Get the Most from Your Compost
- Keep the settling allowance on — Compost settles 10–20% after application and the first watering. The default 10% buffer in Advanced Options covers most bagged composts; raise it to 15% for fine-particle or peat-free products, which compact more than coarser mixes.
- Match compost type to application — Multi-purpose green-waste compost suits top-dressing and soil mixing. For raised beds and containers, use a sterilised growing compost or certified peat-free medium. Unsterilised bulk compost can introduce weed seeds at rates of 500–1,500 per litre, which quickly outweighs the cost saving.
- Time lawn top-dressing for autumn — Applying a 1 cm layer in early autumn gives earthworms the full winter to work it into the top 10 cm of soil before spring growth. Spring application also improves soil biology but competes with peak growing season activity.
- Fluff bags before measuring depth — Bagged compost that has dried or been compressed during transport will appear to have a larger installed volume than when moist. Lightly break up the bag contents before use so your measured depth reflects the true settled layer.
- Refresh containers annually — Container compost exhausts available nutrients faster than open ground and breaks down structurally within one growing season. A 40–50% replacement each year maintains water retention and root aeration. To plan your annual pot top-up by container shape and size, use our Potting Soil Calculator.
- Convert bulk quotes before ordering — Suppliers quote bulk compost by the tonne or cubic yard. Use the cubic yard figure in the results table when requesting delivery quotes, and ask whether the quoted weight is for dry or moist compost — moist compost weighs significantly more per cubic yard, which affects delivery feasibility.
- Plan multiple beds together — Use the Number of Areas field to calculate identically-sized beds in one step. For beds of different sizes, run the calculator separately for each and add the bag totals before buying — consolidating into a single trip reduces both cost and wasted partial bags.
Frequently Asked Questions
Multiply your bed area in square metres by the compost depth in centimetres, then multiply by 10 to get litres. For example, a 3 m × 2 m bed with a 5 cm top-dressing needs 3 × 2 × 5 × 10 = 300 litres. Divide by your bag size to find the bag count — 300 ÷ 25 = 12 bags. Add a 10% settling buffer so the final layer reaches the intended depth after watering and compaction.
Top-dressing places compost on the soil surface as a mulch layer, allowing rain and soil organisms to incorporate it slowly over weeks without disturbing existing roots. Mixing compost into soil means digging or tilling it to a specific depth, typically 15–25 cm, which is more disruptive but faster-acting. Top-dressing suits established beds and lawns; mixing suits new beds being prepared before first planting.
A 4 × 8 foot (approximately 1.2 m × 2.4 m) raised bed with a 30 cm depth at 33% compost — the Mel's Mix ratio — requires roughly 285 litres, or approximately 12 × 25 L bags. At 50% compost the total rises to about 432 litres, or 18 × 25 L bags. Use our Raised Bed Soil Calculator to plan the full soil blend across all three components if you are building a new bed from scratch.
A 10 × 10 foot (approximately 3 m × 3 m) garden at a standard 5 cm top-dressing depth needs roughly 450 litres — about 18 × 25 L bags. At a deeper 7.5 cm seasonal mulch the total rises to approximately 675 litres, or 27 × 25 L bags. At 675 litres, a bulk tonne bag is worth pricing against individual bags, as the per-litre cost difference is typically 40–60% in favour of bulk at this volume.
For general soil improvement, a 25–30% compost amendment — roughly one part compost to three or four parts existing soil by volume — is the standard recommendation for mineral soils. For raised beds, 33% compost reflects the classic Mel's Mix ratio. Containers and pots typically use 40–50% because they dry out faster than open ground and need higher organic content to retain moisture between waterings. Exceeding 50% compost in the soil reduces structural drainage and can promote anaerobic conditions at the root zone.
The volume calculation is geometrically exact — the result precisely reflects the depth and area entered. The main practical variables are compost settling (10–20% depending on product and moisture content) and the loose-fill vs. settled difference between the stated bag volume and the installed volume. The default 10% settling allowance covers most bagged composts; increase it to 15–20% for fine-particle or peat-free products, which compact more than coarser materials.
Yes. Compost settles because loose organic particles compress under their own weight and moisture. Settling is typically 10–15% for mature, fully decomposed compost and 15–20% for fresh or coarse material. This means a 5 cm layer may reduce to 4–4.5 cm after the first rainfall or watering. The calculator's settling allowance adds the corresponding extra volume to your purchase total so the final installed layer reaches the intended depth rather than falling short by 1–2 cm.
Bulk compost — delivered by the cubic yard or tonne bag — becomes cost-effective at approximately 400–500 litres, or roughly 16–20 × 25 L bags. At that volume, bulk compost typically costs 40–60% less per litre than individual bags at garden centre pricing. The calculator shows your result in cubic yards to make bulk ordering straightforward. For projects under 400 litres, buying bags is generally easier and avoids minimum-delivery charges. Also consider whether you have space to receive and store a tonne bag, as they require a flat, accessible area roughly 1 m × 1 m.