Mulch calculator: bags and cubic yards

Quick answer

One cubic yard of mulch covers 324 sq ft 1 in deep, or 108 sq ft at 3 in, as the Clemson Cooperative Extension also states. A 2 cu ft bag covers 8 sq ft at 3 in, so a cubic yard is 13.5 bags of 2 cu ft. A bed of 12 × 5 ft at 3 in takes 9 bags of 2 cu ft with 10 percent extra.

What this calculator does

It calculates the mulch for one bed or several identical beds of three shapes: a rectangle, a circle and a ring around a tree, where the circle around the trunk stays bare. It gives the number of bags, the volume in cubic yards or cubic metres for a loose delivery, and the area covered.

Bark, wood chip and other mulches are sold by volume, so the kind of mulch does not change the result. The depth does, and so does the size of the bag.

How to use it

1. Choose metric or US units. In US units you can type feet and inches, such as 12' 6", and fractions.
2. Choose the shape and enter its size: length and width, the diameter of a circle, or the outer diameter of a ring and the bare circle around the trunk.
3. Enter the depth of mulch and, for several beds of the same size, how many there are.
4. Choose the bag size printed on the bag. The volume is filled in, and you can change it.
5. Set the allowance and select Calculate.

How the result is calculated

1. Rectangle: area = length × width. Circle: area = π × (diameter ÷ 2)². Ring: area = π × (outer diameter ÷ 2)² − π × (inner diameter ÷ 2)².
2. Volume = area × number of beds × depth × (1 + allowance ÷ 100).
3. Bags = volume ÷ volume of one bag, rounded up to a whole bag.

The depth is entered in millimetres or inches and converted. One cubic yard is 27 cu ft; one cubic metre is 1000 L. The volume for a loose delivery is rounded up to 0.01.

Worked example

A bed 4 m long and 1.5 m wide gets 75 mm of bark. Area: 4 × 1.5 = 6 m². Volume: 6 × 0.075 = 0.45 m³, with 10 percent allowance 0.495 m³, shown as 0.5 m³. In 60 L bags: 495 ÷ 60 = 8.25, so 9 bags.

In US units: a bed of 12 ft by 5 ft with 3 in of mulch is 60 sq ft. Volume: 60 × (3 ÷ 12) = 15 cu ft, with 10 percent allowance 16.5 cu ft, which is 0.62 cubic yards rounded up. In 2 cu ft bags: 16.5 ÷ 2 = 8.25, so 9 bags.

A ring around a tree with an outer diameter of 6 ft and a bare circle of 1 ft: 3.14159 × (3² − 0.5²) = 27.5 sq ft. At 3 in with 10 percent allowance: 7.56 cu ft, or 4 bags of 2 cu ft.

Coverage by depth

The area that a given volume covers is the volume divided by the depth. The table uses the same geometry as the calculator, without allowance.

Depth and bag sizes

Advice on depth differs by source and by mulch. The Clemson Cooperative Extension advises no more than 2 to 3 in for fine wood or bark mulch and up to 4 in for coarse pine bark nuggets. The Royal Horticultural Society recommends at least 5 cm, ideally 7.5 cm. Melcourt, a UK bark supplier, advises 50 to 100 mm and 5 to 10 percent extra for settling. The examples use 75 mm and 3 in.

Bag sizes on the presets: 1.5 and 2 cu ft (Scotts Nature Scapes) and 3 cu ft (Kurtz Bros) in the United States; 60 L (Melcourt, UK) and 100 L (Westland, UK). The 50 L and 70 L presets are sizes seen at Australian retailers. Check the volume printed on your bag.

Common mistakes

• Entering the depth in feet or metres. The calculator expects inches or millimetres: 3 in is a quarter of a foot, and 75 mm is 0.075 m.
• Using the diameter of a round bed as its length and width. A circle covers π ÷ 4, about 79 percent, of the square around it.
• Forgetting to leave out the bare circle around a tree trunk, or counting the trunk area twice.
• Counting bags by weight. Mulch bags are sold by volume, which is printed on the bag.
• Ordering no extra. Fresh mulch settles after it is spread.