Gardeners Toolkit
Free UK beds & borders calculator

Compost Calculator

Work out how much compost or soil improver to spread across beds and borders — from measured area and application depth to litres and whole bags.

Multiple bedsEditable depthBag sizes you choose
Diagram showing a garden bed area, compost layer depth and bag conversionbed areadepthbaglitres

For compost spread over soil — not for filling pots

This tool is for beds, borders and other measured garden areas. It calculates the volume created by area × compost depth, then converts that volume into the bag size you actually plan to buy.

Filling a pot or planter?

Container geometry is a different problem. Use the Pot & Container Compost Calculator for round, tapered and rectangular containers.

Calculate compost for beds and borders

Choose how you know the main area, add any extra beds, then set the layer depth and bag size.

Depth help
Step 1

Measure the area

Extra beds or border sectionsAdd any sections you already know in square metres.

Awkward border? Use the Garden Area Calculator first and enter its total here.

Step 2

Choose application depth

Examples only. The right amount depends on whether you are lightly improving soil, preparing a border or using compost as a mulch.
L
Enter the litres printed on the bag you plan to buy.
%

Your compost estimate

Calculated volume—

— litres across — m² at the depth you selected.

Whole bags — base—
With allowance—
Whole bags — order—
Base requirement vs buying quantity

The base figure is the mathematical volume. The order figure adds only the allowance you chose, then rounds bags up to a whole pack.

You know the compost quantity — what next?

Use the next tool that matches the job rather than forcing one volume formula onto every project.

Filling pots or planters?Calculate round, tapered and rectangular container volumes.Filling a raised bed?Use internal bed dimensions and subtract any existing fill.Using compost as mulch?Compare the job with the dedicated mulch quantity workflow.Need topsoil as well?Calculate soil volume separately so the two materials are not confused.

How much compost should you spread?

The maths is simple; choosing the purpose of the compost layer is the important part. Compost used as a light soil improver is not the same job as filling a new bed, and compost used as an organic mulch may be applied more deeply. RHS guidance for creating a garden border says to add at least 5cm of organic matter and aim for 10cm on sandy or heavy clay soil, while its organic-mulch guidance uses at least 5cm and ideally 7.5cm. Treat these as horticultural examples, not mandatory calculator defaults.

Source note

Current RHS guidance distinguishes soil improvement, mulching and no-dig bed creation rather than giving one universal compost depth. Check the purpose of your project before choosing the number you enter.

20 m²× 50 mm1,000 LArea × depth = volume20 m² covered to 50 mm is 1 m³, or 1,000 litres.
50 Lper bag1,000 L= 20 bagsUse the bag labelBag sizes vary, so the calculator lets you enter the litres printed on the product.

Why this is not the pot calculator

A border is measured as a surface area with a layer spread over it. A pot has three-dimensional container geometry, often with tapered sides. Keeping those jobs separate avoids the common mistake of treating a 40cm tapered pot like a 40cm straight cylinder.

Worked example

A 6m × 3m border has an area of 18m². At a chosen 50mm application depth, the volume is 0.9m³ or 900 litres. If the product is supplied in 40-litre bags, the mathematical requirement is 22.5 bags, so you would need 23 whole bags before any optional allowance.

Common ordering mistakes

  1. Using the outside size of an edged bed when the compost only covers the inside planting area.
  2. Mixing millimetres and metres without converting the depth first.
  3. Assuming every bag contains the same litres.
  4. Adding an allowance without showing it separately from the true calculated volume.
  5. Using this page for containers where taper, rootballs and a watering gap change the real fill volume.

Calculation method

Volume in cubic metres = area in m² × depth in metres. One cubic metre is 1,000 litres. Whole bags are rounded upward because a fraction of a retail bag is not normally purchasable. See How our calculations work for unit conversions and rounding principles.

Check the depth & volume referenceVerify litres per square metre and area × depth conversions.