About the Mulch Calculator
A layer of mulch does a lot of work for very little effort: it keeps weeds down, holds moisture in the soil, evens out soil temperature and makes beds look finished. Getting the quantity right is the only tricky part. Garden centres sell mulch in 2 or 3 cubic foot bags or in litres, landscape yards sell it by the cubic yard or cubic metre, and the depth that works is a fairly narrow band.
This mulch calculator turns the size of your beds and the depth you want into the volume to buy, in cubic yards or cubic metres, with a small settling allowance added. It gives the number of bags and bulk bags, the coverage of one cubic yard or metre at your depth, and an optional cost. It also warns you when the layer is too thin to stop weeds or so deep that it could harm your plants.
How to Use the Mulch Calculator
Choose feet and inches or metres and millimetres.
Pick the shape of the bed. Enter the length and width of a rectangle, or the diameter of a circle such as a ring around a tree. Split irregular beds into rectangles and add them up.
Enter the depth of mulch you want to spread.
Leave the settling allowance at 10% unless you know your mulch settles more or less.
Optionally, enter the price per cubic yard or cubic metre to see the cost.
How Much Mulch: the Formula
area = length × width (or π × diameter² ÷ 4)
volume = area × depth × (1 + settling %)
coverage = 324 ft² per yd³ ÷ depth in inchesThe volume is worked out in metres and converted back, so a 3-inch layer is read as a quarter of a foot, not 0.3 of one.
Step-by-Step Example
A border 20 by 10 feet, mulched 3 inches deep, at 40 per cubic yard.
Area: 20 × 10 = 200 ft²
Depth: 3 in = 0.25 ft
Volume: 200 × 0.25 = 50 ft³ = 1.85 yd³
Settling: 50 × 1.10 = 55 ft³ = 2.04 yd³
Bags: 55 ÷ 2 = 28 bags of 2 ft³ (or 19 of 3 ft³)
Cost: 2.04 × 40 = 81.48In metric, a bed 5 by 2 metres at 75 mm needs 0.75 m³, or 0.83 m³ with 10% for settling — fourteen 60 litre bags.
How Deep Should Mulch Be?
| Use | Typical depth |
|---|---|
| Flower beds and borders | 2–3 in / 50–75 mm |
| Around trees and shrubs | 2–3 in / 50–75 mm, clear of the trunk |
| Paths and play areas | 3–4 in / 75–100 mm |
| Topping up last year's mulch | about 1 in / 25 mm |
Under about 2 inches, enough light reaches the soil for weed seeds to germinate and the mulch dries out quickly. Over about 4 inches, the soil beneath can stay wet and short of air, and roots, stems and bark can start to rot. More is not better with mulch.
Coverage per Cubic Yard
| Depth | Area covered by 1 yd³ |
|---|---|
| 1 in | 324 ft² |
| 2 in | 162 ft² |
| 3 in | 108 ft² |
| 4 in | 81 ft² |
In metric, one cubic metre covers 20 m² at 50 mm, about 13.3 m² at 75 mm and 10 m² at 100 mm. The calculator shows the coverage for your exact depth.
Choosing a Mulch
Shredded bark and wood chips are the most common. They break down slowly, improving the soil as they go, and need topping up each year.
Compost and leaf mould feed the soil fastest but break down within a season and do less to suppress weeds.
Rubber mulch lasts for years and is popular in play areas, but it does nothing for the soil.
Gravel and slate are mulches too — for those, use the gravel calculator, because they are sold by weight rather than by volume.
Preparing a Bed for Mulch
Mulch works best on clean, moist soil, so a little preparation pays off.
Weed first. Pull or hoe out existing weeds, especially perennials with deep roots. Mulch smothers seedlings, but established weeds will push straight through it.
Water the soil. Mulch keeps moisture in as well as out. Spread it after rain or a good watering, not onto dry, dusty ground.
Edge the bed. A clean edge cut with a spade keeps the mulch in and the lawn out, and makes the bed look finished.
Consider a membrane only where needed. Under gravel a membrane helps, but under bark it stops the mulch feeding the soil and weeds soon root in the mulch on top. Most beds do better without one.
Understanding Your Result
Mulch needed is the headline volume, including settling.
Volume repeats it in cubic yards and cubic feet, or cubic metres and litres.
Area and depth confirms the measurements the calculation used.
Bags gives 2 ft³ and 3 ft³ bags, or 60 litre bags and 1 m³ bulk bags.
Coverage is how much ground one cubic yard or metre covers at your depth.
Cost multiplies the volume by your price, if entered.
Worth knowing gives the usual depths, or warns when your depth is too thin or too deep.
When Should You Use This Calculator?
Mulching new beds after planting.
Topping up existing beds each spring or autumn.
Laying a bark path or play area, which needs a deeper layer.
Comparing bags with a bulk delivery before you buy.
Checking a landscaper's quote for the quantity of mulch.
Common Mistakes
Reading 3 inches as 0.3 feet. It is 0.25 feet, and the error adds 20%.
Piling mulch against stems and trunks. Keep it a few inches clear to avoid rot.
Spreading too deep. Beyond about 4 inches, the soil stays waterlogged.
Forgetting what is already there. Measure the old layer and top up only the difference.
Buying dozens of bags for a big area. Above two or three cubic yards, bulk delivery is usually far cheaper.