Mulch Calculator: How Much Mulch Do I Need?
Mulch is usually bought by eye, and that is why so many people come home with too many bags — or too few. The right amount depends on just three things: the area of the bed, the depth you want, and the size of the bag or load you are buying. This page does that maths for you in imperial or metric units, shows the formula it uses, and helps you compare the cost of bags against a bulk load before you spend anything.
Quick answer
One cubic yard of mulch covers 324 square feet at 1 inch deep. Every extra inch of depth divides that coverage: 162 sq ft at 2 inches, 108 sq ft at 3 inches, and 81 sq ft at 4 inches. So for a standard 3-inch layer, divide your bed area by 108 to get cubic yards. A 200 sq ft bed at 3 inches needs 200 ÷ 108 = 1.85 cubic yards, which is 25 bags of 2 cu ft. Use the calculator below for your own measurements, bag sizes and prices.
| Depth | Coverage per cubic yard |
|---|---|
| 1 inch | 324 sq ft |
| 2 inches | 162 sq ft |
| 3 inches | 108 sq ft |
| 4 inches | 81 sq ft |
A note for UK readers: in the UK, mulch and bark are normally sold by the litre, in bags and in bulk bags of roughly 600–900 litres, rather than by the cubic yard. The metric mode of the calculator covers this: enter your area in square metres, the depth in centimetres and a bag size of 50 L or 60 L, and it works out cubic metres and bags. One cubic metre is 1,000 litres, so a 750-litre bulk bag holds about 0.75 cubic metres.
Mulch Calculator
Choose your units, pick how you want to enter the area, then add a depth. Results update as you type, or press Calculate. Prices are optional and work in any currency — enter amounts in your own currency and read the costs in that same currency.
Enter your bed size and depth, then press Calculate — the results update as you type.
The formula
Everything on this page rests on one idea: mulch is a volume, so you multiply an area by a depth. For a rectangle, the area is length × width. For a circle, the area is π × radius², where the radius is half the diameter. The only awkward part is the units, because area is measured in feet while depth is measured in inches. Converting the depth to feet (inches ÷ 12) and remembering that one cubic yard holds 27 cubic feet gives the imperial shortcut used by the calculator: cubic yards = (square feet × depth in inches) ÷ 324. The number 324 is simply 27 × 12, the two conversions combined into one step.
Bags are just smaller volumes. A cubic yard is 27 cubic feet, so it contains 27 ÷ 2 = 13.5 bags of 2 cu ft, or 27 ÷ 3 = 9 bags of 3 cu ft. The calculator divides your total cubic feet by the bag size and always rounds up, because you cannot buy part of a bag. In metric, the same logic runs in litres: cubic metres × 1,000 gives litres, and litres ÷ bag litres gives bags. The settling and waste percentage is applied last, on top of the exact figure, so a 10% allowance turns 1.85 cubic yards into about 2.04.
Worked example
Take a flower bed that measures 10 ft by 20 ft, to be mulched 3 inches deep with 2 cu ft bags, with no waste allowance:
- Step 1 — area: 10 ft × 20 ft = 200 sq ft.
- Step 2 — volume: 200 sq ft × 3 inches ÷ 324 = 1.85 cubic yards.
- Step 3 — bags: 1.85 cubic yards × 13.5 bags per cubic yard = 25 bags of 2 cu ft.
- Step 4 — with a 10% allowance: 1.85 × 1.10 = about 2.04 cubic yards, which rounds up to 28 bags.
Those are exactly the numbers the calculator gives for the same inputs, so you can check any result on this page by hand in under a minute.
How deep should mulch be?
Depth is the decision people most often get wrong, usually by adding too much. As a guide: 2 inches is enough for a refresh or top-up over mulch that is still mostly there; 3 inches is the standard depth for flower beds and borders, deep enough to suppress most weeds and hold moisture without smothering the soil; and 3–4 inches suits the ground around trees and shrubs, where roots are deeper and competition from grass is stronger. Never go beyond 4 inches in a garden bed. A thicker layer keeps water and air from reaching the soil, and excess depth starves roots of oxygen — roots then grow up into the mulch itself, where they dry out and suffer in hot and cold weather. If last year’s mulch is still 2 inches deep, top it up rather than starting again: you only need to restore the total to about 3 inches.
Bags vs bulk: where is the break-even?
Bagged mulch wins on convenience. It is clean to carry, easy to spread a little at a time, simple to store, and there is no minimum order — ideal for one or two small beds, a few tree rings, or a top-up. Bulk mulch wins on price per unit of volume once the job gets bigger, because you are not paying for plastic, packing and handling on every two cubic feet. In practice, bulk usually becomes the better buy beyond roughly 2–3 cubic yards, but the honest answer depends on your local prices, which is why the calculator lets you enter both.
Two costs catch people out. The first is delivery: a bulk load that looks much cheaper per yard can lose its advantage entirely once a delivery fee is added to a small order, so add that fee to the bulk total before you decide. The second is handling — a bulk load is tipped in one heap, usually on a driveway, and has to be barrowed to the beds, while bags can be carried straight to where they are needed. For a small job, bags can honestly be cheaper overall once delivery is counted; for a whole garden, bulk normally wins by a wide margin.
Irregular beds, circles and tree rings
Few beds are perfect rectangles, and that is fine — the maths does not need them to be. Break an odd-shaped bed into rough rectangles and circles, work out each piece, and add the areas together before you enter the total under “I know my area”. An L-shaped bed is just two rectangles. A curved border can be measured as a long rectangle using its average width; mulch is forgiving, and being within a few percent is close enough, especially with a small waste allowance added.
Circles use the diameter in the calculator, which halves it to get the radius and works out π × radius² for you. A tree ring is the classic example: a ring 6 ft across has a radius of 3 ft, so its area is about 28.3 sq ft. At 3 inches deep that is 28.3 × 3 ÷ 324 = about 0.26 cubic yards, or roughly 4 bags of 2 cu ft. Measure a few rings, add their areas together, and buy for the total rather than rounding every ring up separately.
Common mistakes
- Mulch volcanoes. Piling mulch in a cone against a tree trunk holds moisture against the bark, invites rot and pests, and encourages roots to circle. Spread it flat and wide instead, like a doughnut, not a volcano.
- Going deeper than 4 inches. Extra depth does not mean extra weed control — it starves roots of oxygen and sheds rain before it reaches the soil.
- Mulching dry soil. Mulch locks in whatever moisture is already there, so water the bed well first and let the mulch keep it there.
- Fabric under mulch where you want soil improvement. Landscape fabric blocks the cycle by which decomposing mulch feeds the soil, and weeds eventually root on top of it anyway. On beds you want to enrich, put organic mulch straight onto the soil.
Whatever the shape or depth, keep mulch a few inches away from trunks and stems — mulch should cover soil, never touch bark. One exception worth knowing about: the 6–12 inch depths sometimes quoted for playgrounds are a fall-protection safety standard for play surfaces, not a garden recommendation, and should never be copied onto beds or around trees.
Mulch guides
Prefer to work it out on paper first, or want the reasoning behind the numbers? These two guides walk through the maths step by step:
Frequently asked questions
How much mulch do I need for 100 sq ft?
At the standard 3 inches deep, 100 sq ft needs 100 × 3 ÷ 324 = about 0.93 cubic yards, which is roughly 13 bags of 2 cu ft. At 2 inches deep it is about 0.62 cubic yards, or around 9 bags.
How many bags are in a cubic yard?
A cubic yard is 27 cubic feet, so it contains 13.5 bags of 2 cu ft or 9 bags of 3 cu ft. In practice you would buy 14 of the smaller bags to make up a full cubic yard.
How deep should mulch be?
Three inches is the standard depth for beds and borders. Use 2 inches for a top-up, 3 to 4 inches around trees and shrubs, and never more than 4 inches, because greater depth can starve roots of oxygen.
Is bulk or bagged mulch cheaper?
Per cubic yard, bulk is usually cheaper, and it normally wins overall beyond about 2 to 3 cubic yards. For small jobs, bags can be cheaper once a bulk delivery fee is added, so compare both prices for your actual quantity.
How much does a 2 cu ft bag cover?
A 2 cu ft bag covers about 8 sq ft at 3 inches deep. It covers roughly 12 sq ft at 2 inches and about 6 sq ft at 4 inches.
How often should mulch be refreshed?
Check the depth once a year, usually in spring. Organic mulch decomposes and feeds the soil, so most beds need a light top-up each year to bring the layer back to about 3 inches rather than a full new layer.