The basic formula: length × width × depth ÷ 27
To find how many cubic feet of mulch you need, measure the area you want to cover in feet, then multiply length by width by the depth you want (usually 2 to 4 inches). Divide that number by 27 to convert cubic inches to cubic feet. Most mulch is sold by the cubic yard, and one cubic yard equals 27 cubic feet, so this final step tells you how many yards to order.
The math works the same whether you are filling a flower bed, a playground, or the base of trees. The only variables are the size of your space and how thick you want the mulch layer.
Key Takeaways
- Measure your bed length and width in feet, multiply them together, then multiply by your desired depth in feet (2 to 4 inches is standard).
- Divide the result by 27 to convert to cubic yards, which is how mulch is priced and delivered.
- A 2-inch layer works for most flower beds; use 3 to 4 inches around trees or in high-traffic areas where mulch settles faster.
- Irregular shapes are easier to calculate if you break them into rectangles, calculate each one, then add the totals together.
- Mulch settles over time, so ordering 10 to 15 percent extra accounts for settling and gives you material for touch-ups later in the season.
Converting inches to feet for the depth measurement
Depth is the trickiest part because mulch depth is usually given in inches, but the formula works in feet. Convert your desired depth by dividing inches by 12. A 2-inch layer is 0.17 feet. A 3-inch layer is 0.25 feet. A 4-inch layer is 0.33 feet.
If you are unsure what depth to use, 2 inches is standard for flower beds and shrub plantings. Use 3 to 4 inches around the base of trees, in areas where foot traffic is heavy, or where mulch tends to compact quickly. Deeper layers suppress weeds better but cost more and can trap moisture against plant stems if piled too high.
Working with irregular bed shapes
Most garden beds are not perfect rectangles. Break an irregular shape into smaller rectangles, calculate the cubic feet for each one, then add them together. For example, an L-shaped bed can be split into two rectangles: one for the long part and one for the short part.
For circular beds, measure the radius (the distance from the center to the edge), then use the formula: radius × radius × 3.14 × depth ÷ 27. If you do not want to do the math, sketch the bed on graph paper, count the squares it covers, and use that as a rough estimate of square footage.
A worked example: a 10-by-6-foot flower bed
Say you have a flower bed that is 10 feet long and 6 feet wide, and you want a 2-inch layer of mulch. Here is the calculation:
- Length × width: 10 × 6 = 60 square feet
- Convert depth to feet: 2 inches ÷ 12 = 0.17 feet
- Multiply by depth: 60 × 0.17 = 10.2 cubic feet
- Convert to cubic yards: 10.2 ÷ 27 = 0.38 cubic yards
You would order 0.5 cubic yards (most suppliers sell in half-yard increments). If you want a 3-inch layer instead, the math changes at step 2: 3 ÷ 12 = 0.25 feet, so 60 × 0.25 = 15 cubic feet, or about 0.56 cubic yards. Round up to 1 cubic yard to account for settling.
Why mulch settles and how much extra to order
Mulch compacts over the first few weeks as it absorbs water and as foot traffic and weather break down the material. A layer that looks full in June may look thin by August. Ordering 10 to 15 percent extra gives you material to top up bare spots and accounts for this natural settling.
If your calculation comes to 2.5 cubic yards, order 3 cubic yards instead. The extra mulch is not wasted—you will use it for maintenance throughout the season, and any leftover can be stored in a dry spot for next year or spread thinly in other beds.
Mulch depth by use and plant type
Different areas of your yard have different needs. Around established shrubs and in perennial beds, 2 inches is sufficient. Around newly planted trees, use 3 to 4 inches, but keep it a few inches away from the trunk to prevent rot and pest problems. In vegetable gardens, 2 to 3 inches works well and helps retain soil moisture during hot months.
High-traffic pathways and play areas benefit from 3 to 4 inches because mulch compacts faster in those spots. Shade beds under trees can use a thinner layer (1.5 to 2 inches) because less evaporation happens there, and deeper mulch can stay too wet. Adjust based on your climate—drier regions benefit from thicker layers, while humid areas should use less to prevent fungal problems.
Frequently Asked Questions
How much does one cubic yard of mulch weigh?
Mulch weight varies by type and moisture content. Hardwood mulch typically weighs 400 to 600 pounds per cubic yard when dry, and more when wet. Pine mulch is lighter, around 300 to 400 pounds per yard. If you are ordering delivery, the supplier can tell you the weight of their specific product.
Can I use the same calculation for wood chips or bark?
Yes. The volume calculation is the same regardless of mulch type. The only difference is settling rate—finely shredded mulch compacts faster than large wood chips, so you may want to order slightly more of the finer product to account for settling.
What if my bed is on a slope?
Measure the average depth you want across the slope, or break the bed into sections and calculate each one separately. Mulch on slopes tends to wash downhill during heavy rain, so you may need to refresh it more often than a flat bed, but the initial calculation stays the same.
Do I need to remove old mulch before adding new mulch?
If the old mulch is still in good condition and not compacted into a hard mat, you can add new mulch on top. If it is matted or decomposed into soil, remove the top 1 to 2 inches first. Either way, your calculation should be based on the depth you want the final layer to be, not how much you are adding.
What if I want to buy mulch by the bag instead of by the yard?
Standard mulch bags are usually 2 or 3 cubic feet. Divide your total cubic feet by the bag size to find how many bags you need. A 10-cubic-foot bed would need five 2-cubic-foot bags or four 3-cubic-foot bags (round up). Bagged mulch costs more per cubic foot than bulk delivery but works well for small areas.