Measure your garden beds to find the square footage
The first step is to measure the area you want to cover. Use a measuring tape to find the length and width of each bed, then multiply length times width to get square footage. If your bed is L-shaped or irregular, break it into rectangles, calculate each one separately, and add them together.
For example, a bed that is 10 feet long and 4 feet wide is 40 square feet. A bed that is 8 feet long and 3 feet wide is 24 square feet. If you have both, your total is 64 square feet.
Write down each measurement as you go. You will use this number in the next step, and it is straightforward to forget if you are measuring multiple beds.
Key Takeaways
- Multiply the length and width of each bed to find square footage, then add all beds together to get your total.
- Mulch depth matters: most gardens need 2 to 3 inches, but vegetable beds and newly planted areas often need 3 to 4 inches.
- Divide your total 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 to the unit your supplier uses.
- Order 10 to 15 percent extra to account for settling and uneven spreading.
Decide how deep you want the mulch layer
Depth affects both how much mulch you need and how well it works. A shallow layer breaks down faster and does less to suppress weeds. A thick layer lasts longer but costs more and can trap moisture against the soil if you overdo it.
Most ornamental beds do well with 2 to 3 inches. Vegetable gardens and newly planted areas benefit from 3 to 4 inches. If you are using mulch mainly for appearance and already have good weed control, 2 inches is enough. If weeds are a problem, go to 3 or 4 inches.
Do not go deeper than 4 inches unless you have a specific reason — thick mulch can prevent water from reaching the soil and can cause rot in tree bark and plant crowns if it piles against them.
Convert square footage to cubic feet using a depth chart
Once you know your square footage and desired depth, use this conversion: multiply your square footage by the depth in inches, then divide by 12. The result is cubic feet.
For example, 64 square feet at 3 inches deep: 64 × 3 = 192, then 192 ÷ 12 = 16 cubic feet. If you wanted 2 inches instead: 64 × 2 = 128, then 128 ÷ 12 = 10.67 cubic feet.
Here is a quick reference for common sizes:
| Square Footage | 2 inches deep | 3 inches deep | 4 inches deep |
|---|---|---|---|
| 100 sq ft | 16.7 cubic feet | 25 cubic feet | 33.3 cubic feet |
| 200 sq ft | 33.3 cubic feet | 50 cubic feet | 66.7 cubic feet |
| 300 sq ft | 50 cubic feet | 75 cubic feet | 100 cubic feet |
| 500 sq ft | 83.3 cubic feet | 125 cubic feet | 166.7 cubic feet |
Account for mulch coverage rates on the bag
Mulch bags list how many square feet one cubic foot covers. This number varies by mulch type and how densely it is packed. Shredded bark typically covers 8 to 10 square feet per cubic foot. Wood chips cover about 10 to 12 square feet per cubic foot. Compost-based mulch covers 6 to 8 square feet per cubic foot.
Check the bag label for the exact coverage rate, then divide your total square footage by that number. If your bed is 100 square feet and your mulch covers 10 square feet per cubic foot, you need 10 cubic feet.
This method works if you are buying by the bag and want to know how many bags to get. If you already calculated cubic feet using depth, you can skip this step — the two methods give the same answer.
Convert cubic feet to cubic yards if ordering in bulk
Mulch suppliers often sell by the cubic yard instead of cubic feet. One cubic yard equals 27 cubic feet. Divide your cubic feet total by 27 to get cubic yards.
For example, if you calculated 50 cubic feet, divide by 27 to get 1.85 cubic yards. Most suppliers will round up, so you would order 2 cubic yards. If you calculated 16 cubic feet, that is 0.59 cubic yards, which rounds to 1 cubic yard.
Bulk mulch is usually cheaper per cubic foot than bagged mulch, but you need enough to make delivery worthwhile. Check with your supplier about their minimum order and delivery cost before deciding whether bulk or bags make sense for your project.
Add extra mulch to account for settling and spreading
Mulch settles and compacts over time, especially in the first few weeks. It also spreads unevenly — some areas end up thicker than others depending on how you rake it out. Order 10 to 15 percent extra to account for this loss.
If you calculated 50 cubic feet, add 5 to 7.5 cubic feet (10 to 15 percent). Order 55 to 57.5 cubic feet, which rounds to 2 cubic yards. If you calculated 16 cubic feet, add 1.6 to 2.4 cubic feet, so order 18 to 19 cubic feet or 1 cubic yard.
Having a little left over is better than running short partway through the job. Extra mulch can go in a storage bin or be spread thinner in less visible areas.
Frequently Asked Questions
Do I need to remove old mulch before adding new mulch?
Not always. If the old mulch is still in good condition and not matted down, you can add new mulch on top. If it is decomposed into a thin layer or packed solid, rake it out first. Mixing old and new is fine — just break up any clumps so water can reach the soil.
What if my garden bed is circular or has curved edges?
Measure the radius (distance from center to edge) and use the formula for circle area: radius squared times 3.14. For a circle with a 5-foot radius: 5 × 5 × 3.14 = 78.5 square feet. For curved edges, approximate by breaking the shape into rectangles and triangles, measure each, and add them together.
Should I put landscape fabric under the mulch?
Landscape fabric can reduce weeds but can also trap moisture and prevent mulch from breaking down into the soil. Many gardeners skip it and rely on mulch depth alone. If you use fabric, make sure water can still reach the soil and do not let mulch pile against plant stems.
How often do I need to add more mulch?
Mulch breaks down over time — usually 6 to 12 months depending on the type and climate. Add a fresh layer of 1 to 2 inches once a year in spring. You do not need to remove the old mulch unless it has become compacted or moldy.
Can I use the same calculation for wood chips as for shredded bark?
The depth calculation is the same, but coverage rates differ. Wood chips cover more square footage per cubic foot than shredded bark because they are larger and less dense. Always check the bag label for the specific coverage rate of the product you are buying.