Measure your garden beds to find the square footage
The amount of mulch you need depends on the area you are covering and how deep you want the layer to be. Start by measuring the length and width of each garden bed in feet. If a bed is irregularly shaped, break it into rectangles, measure each section separately, then add the totals together.
For a rectangular bed that is 10 feet long and 4 feet wide, multiply those numbers: 10 × 4 = 40 square feet. If you have a bed that is 8 feet long and 3 feet wide, that is 24 square feet. Add all your beds together to get your total square footage. Write this number down—you will need it for the next step.
Key Takeaways
- Measure the length and width of each bed in feet, multiply them together, and add all the beds to get your total square footage.
- Most garden beds need mulch 2 to 4 inches deep; 3 inches is the standard depth that suppresses weeds while letting water through.
- Divide your square footage by the coverage rate on the mulch bag (usually 8 to 10 square feet per cubic foot) to find how many cubic feet you need.
- Mulch is sold by the cubic yard (27 cubic feet) or cubic foot, so convert your total and round up to account for settling and uneven ground.
- Bulk mulch from a landscape supplier is usually cheaper per cubic yard than bagged mulch, especially for areas larger than 200 square feet.
Decide on your mulch depth
The depth of mulch you explore affects both how much you need and how well it works. A layer that is too thin will not suppress weeds or retain moisture. A layer that is too thick can trap moisture against plant stems and cause rot.
For most garden beds, 3 inches is the standard depth. This thickness blocks sunlight from reaching weed seeds, holds moisture in the soil, and breaks down at a rate that lets you refresh it every year or two. If you are mulching around trees or shrubs, 3 to 4 inches is appropriate. For vegetable gardens or annual flower beds where you turn the soil each season, 2 inches is often enough. Never pile mulch directly against the trunk or stem of a plant—keep it 2 to 3 inches away.
Convert square footage to cubic feet
Once you know your square footage and desired depth, you can calculate how many cubic feet of mulch you need. The formula is: square footage × depth in inches ÷ 12 = cubic feet.
Here is a worked example: You have 100 square feet of bed space and want mulch 3 inches deep. The math is: 100 × 3 ÷ 12 = 25 cubic feet. If you wanted 4 inches deep instead, it would be: 100 × 4 ÷ 12 = 33.3 cubic feet. Write down your cubic feet total—this is what you will order or buy.
Account for mulch settling and uneven ground
Mulch compacts and settles over time, especially in the first few weeks after you spread it. It also fills in low spots and dips in the ground that you may not notice when you measure. To account for this, add 10 percent to your cubic feet total before you order.
Using the earlier example of 25 cubic feet: 25 × 1.10 = 27.5 cubic feet. Round up to 28 cubic feet to be safe. This small buffer means you will not end up short and have to make a second trip. If your ground is very uneven or you are filling in a new bed, adding 15 percent instead of 10 percent is reasonable.
Choose between bagged and bulk mulch
Mulch is sold two ways: in bags at garden centers and home improvement stores, or in bulk by the cubic yard from landscape suppliers. Bagged mulch is convenient for small areas (under 100 square feet) because you can carry it home and spread it yourself. Each bag typically covers 8 to 10 square feet at 3 inches deep, though this varies by brand and mulch type.
Bulk mulch is more economical for larger areas. One cubic yard equals 27 cubic feet and usually costs less per cubic foot than bagged mulch. If you need 28 cubic feet, that is just over 1 cubic yard. Many suppliers deliver for a fee, or you can rent a truck and pick it up yourself. For areas larger than 200 square feet, bulk mulch almost always saves money even after delivery charges.
Calculate the number of bags you need
If you are buying bagged mulch, divide your cubic feet total by the coverage per bag. Most bags cover 8 to 10 square feet at 3 inches deep, which equals about 0.75 to 1 cubic foot per bag. Check the label on the specific brand you choose—manufacturers list coverage differently.
Suppose you need 25 cubic feet and each bag covers 1 cubic foot. You would need 25 bags. If the bags cover 0.75 cubic feet each, you would need 25 ÷ 0.75 = 33.3 bags, so round up to 34 bags. Always round up, never down, so you have enough to reach your target depth.
Use a straightforward reference table for common bed sizes
| Bed Size (feet) | Square Footage | Cubic Feet at 3 inches | Cubic Feet at 4 inches | Bags Needed (1 cu ft per bag) |
|---|---|---|---|---|
| 4 × 8 | 32 | 8 | 10.7 | 8–9 |
| 4 × 12 | 48 | 12 | 16 | 12–13 |
| 6 × 10 | 60 | 15 | 20 | 15–16 |
| 8 × 10 | 80 | 20 | 26.7 | 20–21 |
| 10 × 20 | 200 | 50 | 66.7 | 50–51 |
This table shows common raised bed and garden bed dimensions. Find your bed size in the left column, then read across to see how many cubic feet you need at 3 or 4 inches deep. The rightmost column shows how many 1-cubic-foot bags to order. If your beds do not match these sizes exactly, use the formula from the earlier section instead.
You can also use this table to compare costs: calculate the price per bag at your local garden center, multiply by the number of bags needed, then compare that total to the price of bulk mulch from a landscape supplier. The table makes it straightforward to see at a glance whether bagged or bulk is the better choice for your project size.
Frequently Asked Questions
Do I need to remove old mulch before adding new mulch?
You do not have to remove old mulch completely, but you should rake it back and break up any matted layer on top. If the old mulch is more than 4 inches deep, remove some so the total does not exceed 4 inches. Fresh mulch mixed with decomposed mulch works fine and actually improves soil as it breaks down further.
What if my garden bed is circular or has curved edges?
Measure the radius (distance from the center to the edge) and use the formula for the area of a circle: 3.14 × radius squared. For a circular bed with a 5-foot radius, the area is 3.14 × 5 × 5 = 78.5 square feet. For curved beds, break them into simpler shapes (rectangles and triangles) and add the areas together, or estimate by measuring the longest and widest points and treating it as a rectangle.
How often do I need to add more mulch?
Mulch breaks down over time as it decomposes and becomes part of the soil. Most gardeners refresh their mulch once a year, usually in spring. You may only need to add 1 to 2 inches rather than replacing all of it. If mulch is still 2 to 3 inches deep and looks intact, you can wait another year.
Can I use the same calculation for wood chips and bark mulch?
Yes, the calculation is the same regardless of mulch type. Wood chips, shredded bark, and compost-based mulch all use the same square footage and depth formula. The main difference is how quickly they break down—bark mulch lasts longer than shredded hardwood, so you may refresh it less often.
What if I order too much mulch?
Extra mulch can be stored in a dry place for next season, spread thinner in existing beds, or used to start a new planting area. Mulch does not spoil, though it will continue to decompose slowly. If you have significantly more than you need, you can offer it to neighbors or a community garden.