Gardeners Toolkit
Free UK garden calculator

Mulch Calculator

Calculate the volume of mulch needed for beds and borders, convert it into litres and see how many bags to buy at the depth you choose.

Multiple bedsDepth by purposeEditable bag size
Garden bed cross section showing a layer of organic mulch kept away from a shrub stemkeep clear of stemdepth

Choose the mulch depth before you choose the bag count

Mulch quantity is a simple volume calculation, but the useful decision is the layer depth. For organic mulches, RHS guidance uses at least 5cm and ideally 7.5cm in many beds and borders. Your material and purpose can change that, so the depth stays editable.

Mulch is bought by volume

Unlike gravel, converting to weight usually adds confusion. Enter the litres printed on the bag or bulk product and let the calculator stay volume-first.

Calculate your mulch quantity

Measure the bed area, choose the layer depth and enter the bag size you are actually buying.

Depth help
Step 1

Measure the beds and borders

Extra beds or bordersAdd areas you have already measured in square metres.

For curved beds, split the space into simple zones or use the Garden Area Calculator first.

Step 2

Choose the mulch depth

Choose the depth for your material and purpose; presets are not universal product instructions.
L
Replace the example with the litres printed on your bag.
%

Your mulch requirement

Base volume required—

Use this as the geometry result, then round packaging up to whole bags.

Total area—
Litres—
Whole bags—
Selected depth—
Volume with allowance—
Bags with allowance—
—

Check whether you are topping up an existing layer before ordering the full calculated depth again.

Use the number in the garden

Choose the depth deliberately, prepare the bed, then use the calculator result as a purchasing estimate.

Choose the mulch depthUse horticultural guidance before accepting a default layer.Adding compost instead?Calculate a soil-improver layer separately from surface mulch.Need to build soil level?Topsoil is a different job from surface mulching.

How the mulch calculation works

Volume = area × depth. Convert depth to metres, multiply by square metres, then multiply cubic metres by 1,000 for litres.

A 20m² border mulched to 50mm (0.05m) needs 1m³, which is 1,000 litres. If your bags contain 50 litres, that is 20 whole bags before any optional allowance.

Mulch depth around a planted garden bedmulch depthDepth is the key decisionAt the same bed area, a 75mm layer needs 50% more mulch than a 50mm layer.
Diagram showing cubic metres converted to litres and mulch bagsm³volumeLlitresbagsUse the volume printed on the bagBag weight varies with material and moisture, so litres are the cleaner comparison for mulch.

Why the 5cm and 7.5cm examples appear here

RHS guidance for organic mulches says the layer should be at least 5cm (2in) and ideally 7.5cm (3in) in many beds and borders. The RHS also advises applying mulch over moist, weeded soil and keeping it away from the stems of woody plants. Those are horticultural starting points, not a rule for every mulch material or every garden.

If you are topping up an existing mulch layer, measure what remains and calculate only the additional depth you actually want to add.

Keep mulch away from woody stems

Piling damp mulch against trunks or woody stems can create problems. RHS advice recommends a mulch-free circle around woody stems. The calculator measures the bed volume; it does not add extra material for piling against plants.

Common mulch-calculator mistakes

  • Using bag weight instead of bag volume. Enter the litres stated on the product.
  • Calculating the full target depth when an old mulch layer remains. Estimate the top-up depth instead.
  • Measuring the outer rectangle of a curved border. Split it into smaller areas or use the Garden Area Calculator.
  • Assuming deeper is always better. Mulch type, plants and purpose matter.
  • Burying plant crowns or woody stems. Spread around plants rather than banking mulch against them.
Horticultural source note

Depth, timing and stem-clearance guidance was checked against current RHS organic mulching advice and the RHS overview of mulches and mulching.

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