The basic formula: square footage times depth
To find how much mulch you need, multiply the area you're covering (in square feet) by the depth you want (in inches), then divide by 324. That number is how many cubic yards of mulch to order. The math works because one cubic yard covers 324 square feet at a depth of one inch.
For example: a garden bed that is 10 feet long and 4 feet wide is 40 square feet. If you want mulch 3 inches deep, multiply 40 × 3 = 120, then divide by 324. You need about 0.37 cubic yards, which rounds to roughly one-third of a cubic yard. Most suppliers sell in whole or half cubic yards, so you'd order a half yard to be safe.
The depth matters more than most people think. Mulch settles and breaks down over time, so what looks like 3 inches in spring may be 2 inches by midsummer. Ordering slightly more than your calculation suggests gives you a buffer.
Key Takeaways
- Multiply your garden bed's square footage by your desired mulch depth in inches, then divide by 324 to get cubic yards needed.
- Most garden beds need mulch 2 to 4 inches deep; 3 inches is the standard for weed suppression and moisture retention.
- Mulch settles and compacts over the season, so ordering 10 to 15 percent extra prevents thin spots by late summer.
- Measure irregular beds by breaking them into rectangles, finding the square footage of each, then adding them together.
- Bulk mulch is usually cheaper per cubic yard than bagged mulch, but bagged works better for small areas or steep slopes.
Measuring irregular and curved beds
Not every garden bed is a perfect rectangle. For L-shaped or curved beds, break the area into simpler shapes you can measure separately. An L-shaped bed becomes two rectangles: measure the length and width of each section, multiply to get square footage, then add them together.
For a curved bed, estimate by measuring the longest length and the widest width, multiply them, then reduce the result by 20 to 30 percent to account for the curve. This gives you a close approximation without needing to calculate the exact arc. If the bed is very irregular, lay a garden hose or rope along the edge and measure that perimeter, then estimate the average width from the center to the edge.
When in doubt, round up slightly. Leftover mulch can be stored in a dry place or spread thinly in other areas. Running short and having to order a second delivery costs more in time and money than buying a little extra.
Depth recommendations for different purposes
The depth you choose depends on what you want the mulch to do. For weed suppression, 3 inches is the standard—thick enough to block most weed seeds from germinating, but not so thick that it traps moisture against plant stems. For moisture retention in hot or dry climates, 4 inches works better, though you'll need to keep it a few inches away from tree trunks and shrub bases to prevent rot.
In cooler, wetter climates, 2 inches may be enough and reduces the risk of fungal issues. Around newly planted trees and shrubs, start with 2 to 3 inches and avoid piling mulch against the trunk—this creates a "mulch volcano" that can girdle the plant and invite pests. As mulch breaks down (usually within 12 to 18 months), you'll need to top it up to maintain the depth.
Bulk mulch versus bagged mulch
Bulk mulch from a landscape supplier or garden center is usually $25 to $60 per cubic yard, depending on type and location. A half cubic yard costs roughly half that. Bagged mulch from a hardware store typically runs $3 to $8 per bag, and most bags are 2 to 3 cubic feet—so you'd need 10 to 15 bags to equal one cubic yard, bringing the total to $30 to $120 for the same amount.
Bulk makes sense if you're covering more than 100 square feet. For smaller beds, bagged mulch is easier to handle and store. Bulk also comes in a truck, which means delivery fees ($50 to $150) may explore if you're not near a supplier. Check whether your supplier offers free delivery on orders over a certain size, or whether you can pick it up yourself.
Both bulk and bagged mulch come in different types—wood chips, shredded bark, colored mulch, and others. The type doesn't change how much you need, only the cost and appearance.
Accounting for settling and annual top-ups
Mulch compacts under its own weight and breaks down as it decomposes. A 3-inch layer applied in spring often settles to 2 inches or less by late summer. This is normal and actually beneficial—as mulch breaks down, it adds organic matter to the soil. However, it also means your weed suppression weakens over time.
Plan to add a 1 to 2 inch top-up layer each year, usually in spring or early fall. You don't need to remove the old mulch first; fresh mulch on top works fine. This annual refresh is cheaper than replacing everything and keeps your beds looking full. If you're calculating mulch for the first time and want it to last through the season without looking thin, add 10 to 15 percent extra to your initial order.
Common mistakes that lead to ordering too much or too little
The most common error is forgetting to subtract the area taken up by plants. If your bed is 100 square feet but contains shrubs and perennials that occupy 20 square feet, you only need mulch for 80 square feet. Measure around the base of each plant or estimate the space they occupy, then subtract from your total.
Another mistake is overestimating depth. Many people think they need 4 to 6 inches because mulch looks thin when first spread, but that's before it settles. Start with 3 inches and add more next year if needed. Ordering too much mulch upfront wastes money and creates storage problems.
Finally, don't forget to account for slopes. Mulch on a steep slope will shift and wash downhill, especially in heavy rain. You may need to order 20 to 30 percent extra for sloped areas, or consider using landscape fabric underneath to hold it in place.
Quick reference table for common bed sizes
| Bed Size (feet) | Square Footage | Cubic Yards at 2 inches | Cubic Yards at 3 inches | Cubic Yards at 4 inches |
|---|---|---|---|---|
| 4 × 8 | 32 | 0.2 | 0.3 | 0.4 |
| 6 × 10 | 60 | 0.37 | 0.55 | 0.74 |
| 10 × 10 | 100 | 0.62 | 0.92 | 1.23 |
| 10 × 20 | 200 | 1.23 | 1.85 | 2.47 |
| 15 × 20 | 300 | 1.85 | 2.78 | 3.7 |
Frequently Asked Questions
Do I need landscape fabric under the mulch?
Landscape fabric helps block weeds but isn't required. It works best on new beds or areas with heavy weed pressure. On established beds with healthy soil, mulch alone usually does the job. Fabric can trap moisture and prevent organic matter from reaching the soil, so some gardeners skip it. If you use fabric, you'll still need the same amount of mulch on top.
What if I order too much mulch?
Store it in a dry location—a tarp over a pile works fine. Mulch stored dry keeps for months. You can spread extra mulch thinly in other garden areas, use it as a base layer under new beds, or offer it to neighbors. Most suppliers won't take back opened orders, so it's better to have extra than to run short.
How do I know if my mulch is settling or if I ordered too little?
Measure the depth with a ruler a few weeks after spreading. If it's dropped from 3 inches to 2 inches, that's normal settling. If it's dropped to 1 inch or less, you either ordered too little or the mulch was very loose when delivered. Either way, a top-up layer in a few months will fix it.
Can I use the same calculation for wood chips as for shredded bark?
Yes. The formula works for any mulch type because it's based on volume, not material. Wood chips, shredded bark, and colored mulch all compress similarly over time. The only difference is cost and appearance, not the amount you need.
Should I mulch right up to the base of trees and shrubs?
No. Keep mulch 3 to 6 inches away from tree trunks and shrub stems. Mulch piled against the base traps moisture, invites pests and disease, and can eventually girdle the plant. This means you'll need slightly less mulch than your calculation suggests—subtract a small circle around each plant base if you want to be precise.