How Much Mulch Do I Need? The Formula, With Worked Examples
The mulch math is one formula and thirty seconds of arithmetic. Measure the bed in feet, pick your depth in inches, and you get cubic yards for bulk orders and bag counts for store runs. Below is the formula, four worked examples, and a quick table you can use without touching a calculator.
To find how much mulch you need: multiply bed length by width in feet, multiply by depth in inches, divide by 12 for cubic feet, then divide by 27 for cubic yards. A 10 by 10 foot bed at 3 inches deep needs 25 cubic feet, about 1 cubic yard or 13 two-cubic-foot bags. Round beds use pi times radius squared for the area. Add 10 percent for irregular shapes.
The one formula
Every mulch quantity comes from the same calculation:
Cubic feet = length (ft) x width (ft) x depth (in) / 12
Cubic yards = cubic feet / 27
Bags (2 cu ft) = cubic feet / 2
A cubic yard is 27 cubic feet, and most bagged mulch comes in 2 cubic foot bags. That is all you need to convert between bulk orders and store bags. The only judgment call is the depth, and the depth guide below covers that.
Worked example 1: a 10 by 10 foot bed
Take a square 10 by 10 foot flower bed at 3 inches deep:
10 x 10 x 3 / 12 = 25 cubic feet.
25 / 27 = 0.93, so about 1 cubic yard.
25 / 2 = 12.5, so 13 two-cubic-foot bags.
One yard is the answer most people expect here, and the math confirms it. If you are buying bags, grab 13 and you will have a touch to spare.
Worked example 2: a 20 by 6 foot border
A long foundation border, 20 feet by 6 feet, at 3 inches:
20 x 6 x 3 / 12 = 30 cubic feet.
30 / 27 = 1.11, so 1.1 cubic yards, and you would order 1.5 yards to be safe.
30 / 2 = 15 two-cubic-foot bags.
Border beds eat more mulch than people guess because the length adds up fast. This is the bed where bag buyers underestimate: 15 bags looks like a lot at the store, but it is exactly right.
Round beds and tree rings
For a circle, the area is pi times the radius squared. A 10 foot diameter bed has a 5 foot radius:
3.1416 x 5 x 5 = 78.5 square feet.
At 3 inches: 78.5 x 3 / 12 = 19.6 cubic feet, or 0.73 yards, or 10 bags.
For a tree ring, measure the diameter of the ring, not the trunk. A 6 foot diameter ring at 3 inches is 28.3 square feet, which is 7.1 cubic feet, about 0.26 yards or 4 bags. Small rings are the classic bag-buying job.
Odd shapes, slopes, and the 10 percent rule
Real beds are rarely perfect rectangles. Break a curved bed into rectangles and circles, add the areas, and run the formula. For anything with curves, jogs, or a slope, add 10 percent to the result before ordering. Slopes need the extra because mulch washes and creeps downhill, and curves always hide more square footage than the eye estimates.
If your depth is not uniform, use the average. A bed that is 2 inches at the edges and 4 inches in the middle is a 3 inch bed for ordering purposes.
Quick reference: yards and bags by bed size (3 inches deep)
100 sq ft (10x10): 0.9 yards, 13 bags.
120 sq ft (20x6): 1.1 yards, 15 bags.
200 sq ft: 1.9 yards, 25 bags.
300 sq ft: 2.8 yards, 38 bags.
500 sq ft: 4.6 yards, 63 bags.
1,000 sq ft: 9.3 yards, 125 bags.
These are at the standard 3 inch depth. For 2 inches, multiply by two thirds; for 4 inches, multiply by four thirds.
Rounding rules
Always round up, never down. Mulch is cheap and a second trip is not. Bulk yards round up to the nearest half yard (a supplier will sell you 1.5 yards). Bags round up to the whole bag. And if your total lands within 10 percent of the next whole yard, just order the whole yard; the price difference is small and the leftover tops up the thin spots.
Skip the arithmetic
Have your measurements but not the patience for the math? Enter the length, width, and depth and get cubic yards, bag counts, and cost in seconds.
How much mulch do I need: questions
How many bags of mulch do I need?
Divide your cubic feet by 2 for standard 2 cubic foot bags. A 10 by 10 foot bed at 3 inches is 25 cubic feet, so 13 bags. A 20 by 6 foot border at the same depth is 30 cubic feet, so 15 bags. Always round up to the whole bag.
How many cubic feet are in a yard of mulch?
One cubic yard is 27 cubic feet. That is 13.5 two-cubic-foot bags, so buy 14 bags to equal a yard. It is also 9 three-cubic-foot bags. The 27 figure is the bridge between bulk pricing and bag pricing.
How much mulch is in a bag?
Most bagged mulch is sold in 2 cubic foot bags; some brands sell 3 cubic foot bags. Check the label, because the bag count math changes. One 2 cubic foot bag covers 12 square feet at 2 inches deep, or 8 square feet at 3 inches deep.
How many yards of mulch do I need?
A quick estimate: divide your bed square footage by 108 for a 3 inch depth. A 300 square foot bed needs 2.8 yards, so order 3. For 2 inches, divide by 162; for 4 inches, divide by 81.
Is there a free mulch calculator?
Yes, this site has one. Enter the bed length, width, and depth and it returns cubic yards, bag counts for 2 and 3 cubic foot bags, and an optional cost estimate. It also handles round beds by diameter.