Measure your space and convert to square feet
Start by measuring the length and width of the area where you want to lay mulch, in feet. Multiply those two numbers together to get the total square footage. If your space is irregular — an L-shape, a curved bed, or multiple sections — break it into rectangles, calculate each one separately, and add them together.
For example, a garden bed that is 10 feet long and 4 feet wide is 40 square feet. A bed that is 15 feet long and 6 feet wide is 90 square feet. If you have two separate beds of 40 and 90 square feet, your total is 130 square feet.
Write down your total square footage. You will need it for the next step.
Key Takeaways
- Multiply the length and width of your space in feet to find square footage; break irregular shapes into rectangles and add them together.
- Decide on your mulch depth — 2 to 3 inches is standard for most garden beds, while 3 to 4 inches works for areas with heavy foot traffic.
- Use the formula: (square feet × depth in inches) ÷ 324 = cubic yards of mulch needed.
- Round up to the nearest half yard when you order, because mulch settles and compacts over time.
- One cubic yard of mulch covers roughly 108 to 162 square feet, depending on how deep you spread it.
Choose your mulch depth
Mulch depth matters because it affects how much material you need and how well it does its job. A depth of 2 to 3 inches is standard for most garden beds and flower borders. This depth suppresses weeds, holds moisture, and looks finished without looking excessive.
Use 3 to 4 inches if your bed gets heavy foot traffic, if you live in a dry climate, or if you are laying mulch over an area that currently has weeds. Use 1 to 2 inches only if you are topping off mulch you laid last year — fresh mulch that thin will not do much work.
Do not go deeper than 4 inches in most cases. Mulch that is too thick can trap moisture against the soil, rot plant stems, and create a home for pests.
Do the calculation: square feet to cubic yards
The formula is straightforward: (square feet × depth in inches) ÷ 324 = cubic yards.
Here is a worked example. You have a bed that is 130 square feet and you want mulch 3 inches deep.
130 × 3 = 390 390 ÷ 324 = 1.2 cubic yards
Another example: a bed of 200 square feet at 2 inches deep. 200 × 2 = 400 400 ÷ 324 = 1.23 cubic yards
The number 324 is the constant that converts square feet and inches into cubic yards. You do not need to understand why — just use it every time.
Round up and account for settling
When your calculation gives you 1.2 cubic yards, order 1.5 cubic yards. When it gives you 1.23, order 1.5. Mulch settles and compacts as it sits on the soil and as rain and foot traffic work it down. Ordering a half yard more than your calculation prevents you from running short after a few weeks.
If your calculation comes out to 2.6 cubic yards, order 3 cubic yards. If it comes out to 0.8, order 1 cubic yard — most suppliers have a one-yard minimum anyway.
This buffer also accounts for the fact that mulch is sold by loose volume, not compressed volume. The mulch in your truck bed is fluffier than the mulch in the bag at the garden center, so the same cubic yard takes up different space depending on how it was stored.
Use a quick reference table for common sizes
| Square Feet | 2 Inches Deep | 3 Inches Deep | 4 Inches Deep |
|---|---|---|---|
| 100 | 0.6 yards | 0.9 yards | 1.2 yards |
| 200 | 1.2 yards | 1.9 yards | 2.5 yards |
| 300 | 1.9 yards | 2.8 yards | 3.7 yards |
| 400 | 2.5 yards | 3.7 yards | 4.9 yards |
| 500 | 3.1 yards | 4.6 yards | 6.2 yards |
Find your square footage in the left column, then read across to the depth you chose. Round up to the nearest half yard when you order.
Check the math with coverage rates
One cubic yard of mulch covers roughly 108 square feet at 3 inches deep, or 162 square feet at 2 inches deep. You can work backward to double-check your calculation.
If you need to cover 300 square feet at 3 inches deep: 300 ÷ 108 = 2.78 cubic yards. Round up to 3 cubic yards. This matches the table above.
If you need to cover 300 square feet at 2 inches deep: 300 ÷ 162 = 1.85 cubic yards. Round up to 2 cubic yards. Again, this matches the table.
Use this method if you prefer to think in coverage rates rather than formulas. Both approaches give the same answer.
Account for obstacles and edges
Subtract the square footage of any trees, shrubs, or structures that sit inside your mulch bed. If your bed is 200 square feet but contains a tree with a 3-foot-diameter base, subtract roughly 7 square feet (the area of a circle 3 feet across). Your new total is 193 square feet.
You do not need to subtract small plants or perennials — mulch around them is part of the job. But large trees, deck posts, or raised beds that sit inside the mulch area should be subtracted.
Do not subtract the width of the bed edge itself. Mulch goes right up to the border, and the small amount that overhangs is normal.
Frequently Asked Questions
What if my garden bed is circular or curved?
Measure the radius (the distance from the center to the edge) and use the formula: 3.14 × radius × radius = square feet. For a circular bed with a 5-foot radius: 3.14 × 5 × 5 = 78.5 square feet. For curved beds, break them into sections that approximate rectangles or circles, calculate each, and add them together.
Do I need to remove old mulch before laying new mulch?
No. You can lay new mulch over old mulch if the old layer is still in decent shape. If the old mulch is matted, moldy, or has sunk into the soil, rake it out first. When topping off, use 1 to 2 inches of new mulch rather than a full 3-inch layer.
How much does a cubic yard of mulch weigh?
Mulch typically weighs 400 to 600 pounds per cubic yard when wet or freshly delivered, and 300 to 400 pounds when dry. Weight varies by mulch type — shredded hardwood is heavier than pine bark. Ask your supplier for the weight of their specific product if you are renting a truck or need to know the load.
Can I calculate mulch in bags instead of cubic yards?
Yes. A standard mulch bag is 2 cubic feet. One cubic yard equals 13.5 bags. So if you need 2 cubic yards, multiply 2 × 13.5 = 27 bags. Bags are more expensive per cubic foot than bulk mulch, but they work for small beds.
What if I want mulch deeper than 4 inches?
Avoid it. Mulch deeper than 4 inches can suffocate plant roots, trap moisture, and create conditions for rot and pest damage. If you want more color or coverage, use 3 to 4 inches and refresh it yearly rather than piling it deeper.