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Today's Gardens Project Planner

Compost calculator

https://todaysgardens.org/compost-calculator/ Planning estimate — generated in your browser and not stored.

Compost is sold two different ways — by volume in litres and by weight in kilograms — and the two are only related if you know the density of the material in front of you. This tool keeps the modes separate rather than converting between them behind your back.

How is your compost measured?

The two modes are separate tools. Switching between them clears the other one's result, because there is no honest conversion between volume and weight without a density.

Measurements

Add the area you are covering or improving. Set a section to subtract for planting you are working around. The measurements stay put when you switch mode.

Depth and packaging

Required. The presets below are cited figures you can change.

Cited depth presets

Surface mulch / no-dig — 5cm: At least 5cm is required for weed control and no-dig surface use, which the source puts at about 50 litres per square metre. Source: RHS — Organic Matter: How to Use in the Garden (checked 6 August 2026).

Surface mulch — 8cm: The source describes 8cm as easier and more effective in practice. Source: RHS — Organic Matter: How to Use in the Garden (checked 6 August 2026).

Bagged compost is sold in litres.

Rate and pack size

Required in this mode. The presets below are the band the cited source gives for improving soil.

Cited rate presets

5 kg per m²: The lower end of the 5–10 kg/m² band the source gives for improving soil. Source: RHS — Organic Matter: How to Use in the Garden (checked 6 August 2026).

10 kg per m²: The upper end of the 5–10 kg/m² band the source gives for improving soil. Source: RHS — Organic Matter: How to Use in the Garden (checked 6 August 2026).

Read this off the bag. Weight-based products are usually sold in 25 kg or 50 kg packs, but check.

Calculations run in your browser. Enable JavaScript to use this calculator.

Your estimate

Worked example — not your result

A 100 m² bed at 50 mm depth, with 50 L bags.

5,000 litres (L)

Volume
5.00 m³
Measured area
100.00 m²
Bags at 50 L
100
Assumptions in this example
Depth 50 mm
Overage not entered — no allowance added
Bag size 50 L

This is an example, not your result. It matches golden vector C2 in the calculation methodology. Enter your own measurements and select Calculate.

Compost is sold two different ways

Bagged compost is sold by volume, in litres. Bulk deliveries and improvement rates are usually talked about by weight, in kilograms. Those two are only related through the density of the material in front of you, and the density of compost varies enormously with how wet it is. That is why this tool keeps the modes apart instead of quietly converting between them: volume mode never shows a kilogram figure, and rate mode never shows a litre figure.

By depth, for surface use and no-dig

A 100 m² bed at 5 cm takes 5 m³, which is 5,000 litres. There is a useful cross-check here: 5 cm over exactly 1 m² is 50 litres, which matches the figure the cited source gives per square metre. This is the one place on the site where a source figure is used to check our arithmetic rather than to supply it.

One honest note about that 8 cm figure. Our arithmetic says 8 cm over the same area needs 60% more material than 5 cm, not 50%. The source's "about 50% more" is a practical rule of thumb rather than a calculation, so this site reports its own arithmetic and tells you where the two differ, rather than bending one to match the other.

By rate, for improving soil

Rate mode multiplies your area by kilograms per square metre and divides by the pack weight you enter. Ten square metres at 7 kg/m² is 70 kg, which is three 25 kg packs once the count is rounded up. No litre figure appears anywhere in that result, because converting it would need a density this tool refuses to assume.

Timing, and what to use

The guidance this site cites puts mulching in mid-to-late spring and autumn, and is firm that only well-rotted material should be dug in — undecomposed matter draws nitrogen out of the soil as it breaks down and shrinks, leaving an uneven surface.

If you want a bark or wood chip mulch rather than compost, the bark and mulch calculator carries the cited depth guidance for biodegradable mulches. If you are blending improver into bed soil rather than spreading it on top, the soil calculator splits a fill volume into soil and improver.

How this figure is worked out

Every step is arithmetic you can repeat by hand. Full precision is kept through the calculation and rounded once for display.

Formulae used on this page
Step Formula
Depth to metres depth m = mm ÷ 1000, or cm ÷ 100, or m × 1
Cubic metres to litres 1 m³ = 1000 L
Net volume net volume m³ = net area m² × depth m
Gross volume after overage gross volume = net volume × (1 + overage % ÷ 100)
Bag count bags = ceiling(gross litres ÷ bag litres)
Compost by weight rate total kg = net area m² × rate kg/m²
Pack count by weight packs = ceiling(total kg ÷ pack kg)

The calculation methodology sets out every formula, bound and rounding rule this site uses.

Sources

SourceRHS — How to Lay Turf

https://www.rhs.org.uk/lawns/lawns-from-turf

checked 6 August 2026. Source key rhs-lay-turf.

  • Complete the site preparation well before buying or having the turf delivered.
  • Get rid of perennial weeds such as couch grass and bindweed first, forking out the roots.
  • Dig or rotovate the soil to a depth of 20–25cm so there is no compaction.
  • Incorporate plenty of well-rotted manure or other organic matter, especially on sandy soil — and only well-rotted material, because undecomposed matter shrinks and makes the surface sink unevenly.
  • Measure the area and check the dimensions of the turf pieces carefully before buying.
  • Lay turf within 24 hours of delivery if possible; if laying is delayed more than a few days, unroll it flat and water it if dry.
  • Stand on boards to avoid making footprints, stagger the joints like bricks in a wall, and push the pieces together tightly.
  • Turf is best laid in mid-autumn and can be laid from mid-autumn to early spring whenever the soil is not too wet or frosty; laying from mid-spring to early autumn is best avoided.

SourceRHS — Mulches and Mulching

https://www.rhs.org.uk/soil-composts-mulches/mulch

checked 6 August 2026. Source key rhs-mulch.

  • To be effective, biodegradable mulches need to be between at least 5cm and ideally 7.5cm thick.
  • Take care not to pile mulch up against the stems of woody plants.
  • Lay mulches over moist soil, after removing weeds, when the soil is not frozen.
  • Single trees and specimen shrubs are best mulched to the radius of the canopy.
  • Mulches are best applied from mid- to late spring and in autumn, and around new plantings at any time of year.

SourceRHS — Organic Matter: How to Use in the Garden

https://www.rhs.org.uk/soil-composts-mulches/organic-matter-how-to-use-in-garden

checked 6 August 2026. Source key rhs-organic-matter.

  • To improve soil it is usual to apply about 5–10kg per square metre.
  • For weed control as a mulch, or as a surface application in no-dig gardening, at least 5cm is required, which amounts to about 50 litres per square metre.
  • In practice it is easier and more effective to apply 8cm, which needs about 50% more organic matter.
  • Check quantities by weight where you can, because wet manures and composts are much heavier than drier materials.
  • Only use well-rotted material when digging it in, to avoid depleting nitrogen from the soil.
  • The coarser the material, the longer it takes to break down.