Mulch Calculator
Estimate top-up mulch for multiple beds and compare whole bags with bulk volume.
This tool runs entirely in your browser. Your data is never uploaded, never stored, and never leaves your device.
Estimate additional mulch across several beds using target minus existing depth, then compare bulk volume with whole bags using label volume or coverage at a stated depth.
How to use it
- 1Enter non-overlapping bed rectangles, exclusions, target depth and existing usable depth.
- 2Choose bulk, bag volume or bag coverage with its label depth; density is not needed.
- 3Review added volume, your allowance and any whole-bag surplus.
- 4Download a local JSON estimate; changes remove old output.
Example
- Input
- 8 m² topped up by 50 mm plus 2 m² at 50 mm; no allowance; 50 litre bags.
- Output
- 0.5 m³ added volume; 10 bags; zero bag-rounding surplus.
A quantity example, not a recommended mulch depth.
What happens to your data
All measurements are processed locally without upload or storage. The optional JSON includes the active bed inputs, bag basis and assumptions.
Last updated October 2026
Buying mulch by area alone misses an essential measurement: the depth being added. This mulch calculator handles several rectangular beds, subtracts excluded areas and calculates the additional layer after accounting for existing usable mulch. Choose bulk volume, a labelled bag volume or a bag's coverage at a stated depth.
The tool never asks for density to answer a volume question. It separates measured volume, any chosen allowance, whole bags and surplus. It also avoids ordering more for a bed already at its entered target depth, while clearly explaining that an excessive existing layer may still need attention rather than more mulch.
How it works
For each bed, net area is length × width − excluded area. Added depth is max(target depth − existing usable depth, 0). Multiplying those two quantities gives the added volume; different bed depths are handled individually before the volumes are combined. Apply your chosen allowance to that combined quantity. No default settling, expansion or decomposition factor is imposed.
Metric mode accepts metres and millimetres, while US mode accepts feet and inches. The result includes m³, ft³ and yd³. In bulk mode these are volume estimates, not an automatically rounded whole-yard order. Ask the supplier about practical delivery increments, minimum quantities and the meaning of its stated loose or settled volume.
For bag-volume mode, enter litres in metric mode or cubic feet in US mode from the actual package. Required volume divided by one bag's stated volume is rounded up once across all beds. If a label instead says how much area a bag covers at a particular depth, enter both values. The implied volume is coverage area × stated depth. This is a uniform-layer approximation, not evidence that a compressed or expanding product can be modelled without its own instructions.
Target depth is a user measurement, not a horticultural recommendation. Extension guidance cautions against mulch piled against trunks and excessive layers. Species, material, existing conditions and the site matter. Existing material should count only when it is usable toward the intended layer. A computed zero addition means the entered target is already met; it does not approve an overly deep layer or diagnose plant health. Exact decimal calculations avoid accidental extra bags at mathematical boundaries, but physical coverage can still differ.
Worked example: Top up existing mulch and cover another bed
- Input
- 4 × 2 m bed: 75 mm target, 25 mm existing; 2 × 1 m bed: 50 mm target, none existing; no allowance; 50 litre bags.
- Result
- 0.4 + 0.1 = 0.5 m³ added volume; 10 bags; zero whole-bag rounding surplus.
These depths demonstrate the arithmetic, not a recommendation for particular plants or materials.
Common use cases
- Plan a top-up across beds with different target and existing depths instead of buying a full new layer everywhere.
- Compare a volume-labelled bag with a bag labelled by coverage and depth.
- Obtain an unrounded bulk volume in cubic yards or cubic metres for a supplier quote.
- Keep an auditable local JSON breakdown of the bed measurements, bag basis and purchasing assumptions.
Frequently asked questions
How do I calculate mulch volume?＋
Multiply net bed area by added depth in matching length units. With feet and inches, divide area × inches by 12 for cubic feet, then by 27 for cubic yards. The tool performs those conversions and combines the separate bed quantities.
Do I need mulch density?＋
No, not when buying a stated volume or calculating coverage at a depth. Density would be needed for a separate weight estimate and varies with the material and moisture. This calculator does not invent a pounds-per-yard conversion.
How is existing mulch handled?＋
It subtracts existing usable depth from target depth, never below zero. For a 75 mm target and 25 mm existing layer, only 50 mm is added. If existing depth meets or exceeds target, that bed adds zero volume. Check whether excess or unsuitable mulch needs removal instead.
What if the bag only gives square-foot coverage?＋
Coverage must include a depth. In coverage mode a bag covering 12 ft² at 2 inches corresponds to 2 ft³ for this uniform-layer calculation. Area alone cannot establish volume. Use the manufacturer's actual volume or expansion instructions when those apply.
How deep should mulch be?＋
This calculator does not choose a depth. Follow guidance for your material, plants and site, and keep mulch away from trunks and stems. Excessive existing material is not a reason to add another full layer; extension resources below explain the risks.
Does bulk mode round up to a whole cubic yard?＋
No. It reports the calculated volume with your stated allowance. Supplier minimum orders and increments differ. Bag modes separately round to whole bags and show the extra volume from that rounding.
Are the bed measurements private?＋
The computation runs locally without uploading or saving your entries. The optional JSON download contains active measurements and assumptions on your device. Changing an input or buying method invalidates the previous result and download.
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