CoverCalcPRO

Mulch calculator / Top-up calculator

Existing layer + target depth

How deep should mulch be? Calculate only the top-up

Measure the settled mulch already in the bed. This calculator subtracts it from your chosen finished depth, so the order covers only the additional layer.

Free · runs in browser
No accountNo saved measurementsImperial and metric

Planning a cardboard layer beneath a suitable sheet-mulch bed? The cardboard coverage calculator counts usable sheets with side and end overlaps. It does not choose the layer count or change the ground area used for top-layer mulch.

The measurement that prevents over-ordering

Added depth is different from finished depth

If a bed already has a usable settled layer, entering the full target depth as new material counts that existing layer twice. The top-up calculation uses added depth = maximum of target depth minus existing depth, or zero. It then multiplies the added depth by the measured area.

Example: a 100 ft² bed with 1 inch already present and a 3-inch target needs a 2-inch addition. That is 16.67 ft³, or about 0.617 yd³, before an allowance. A 10% allowance makes 18.33 ft³, or about 0.679 yd³ and 10 whole 2 ft³ bags.

Use a site-specific target

How deep should the finished mulch layer be?

There is no single depth for every bed. Iowa State University Extension says the effective range is generally 2 to 4 inches and that soil texture, climate, mulch type, plant age, and the purpose of the mulch all matter. Its guidance places coarse material toward the higher end and fine material toward the lower end.

Oregon State University Extension says to determine the existing depth before reapplying bark or wood chips. Its woody-landscape guidance says bark should not exceed 3 inches and wood chips should not exceed 4 inches. These are extension examples for particular materials and plantings, not automatic settings in this calculator.

For tree rings, several extension sources also warn against piling mulch against the trunk. Measure the area and layer while leaving the trunk flare exposed according to appropriate local guidance.

Cross section separating the existing mulch layer from the new top-up layer and showing clear space around a tree trunk
The tool calculates the new top-up layer. A finished layer should remain clear of trunks and plant crowns according to suitable local guidance.

Before entering a number

Measure the existing layer without hiding uncertainty

  1. Identify only the area that will receive new material. Exclude paths, edging, exposed trunk space, and structures.
  2. Gently level loose piles and take depth readings in several representative places.
  3. Record the settled depth you will use. If different sections vary materially, calculate them as separate areas.
  4. Choose a finished target using the material, plants, soil, drainage, and relevant local guidance.
  5. Keep the geometric result separate from any optional allowance and from the supplier's order increment.

This calculator does not decide whether old material is usable, compacted, contaminated, or due for removal. Inspect the bed before treating the measured layer as reusable depth.

Transparent calculation

Top-up volume and bag formulas

Added depth

Maximum of target finished depth − existing settled depth, or zero.

Exact volume

Area × added depth after converting inches to feet or centimeters to meters.

Planning volume

Exact volume × (1 + allowance ÷ 100). The allowance is shown separately.

Whole bags

Planning volume ÷ printed bag volume, rounded up to the next whole bag.

Common questions

Mulch top-up calculator FAQ

What if the existing layer is already deeper than my target?

The calculated addition is zero. The result does not tell you whether to remove or reuse material; inspect the condition, trunk clearance, drainage, and plant setting before deciding.

Should I enter the deepest measurement?

Use measurements that represent the area being calculated. If depths differ substantially, divide the bed into sections rather than hiding the variation in one input.

Does the allowance change the target depth?

No. It is applied after the geometric volume calculation and remains visible as a separate planning choice.

Can I calculate several beds?

Yes. The mulch refresh by bed calculator takes each bed's measured existing and target depth, then combines only the new material into one bag or bulk order. For one combined known area measured in m² and millimetres, the metric mulch calculator gives m³, litres and whole bags.