Measure your garden beds to find the square footage

Start by measuring the length and width of each garden bed in feet. Multiply length by width to get the square footage of that bed. If your bed is an irregular shape, break it into rectangles, measure each one separately, and add the totals together.

For circular beds, measure the distance across the center (the diameter), divide it by two to get the radius, then use the formula: radius × radius × 3.14. This gives you the square footage. Write down each measurement — you'll need the total square footage of all beds combined to calculate how much mulch to order.

Key Takeaways

  • Multiply the length and width of each bed in feet to find square footage, then add all beds together for your total.
  • Most gardens need 2 to 4 inches of mulch depth; 3 inches is the standard for weed suppression and moisture retention.
  • Divide your total square footage by 100 to find how many cubic yards of mulch you need at 3 inches deep.
  • Mulch settles and breaks down over time, so plan to add a 1-inch top-up layer each year.
  • Bulk mulch is usually cheaper per cubic yard than bagged mulch, but requires delivery or a truck to transport.

Decide on the depth you want

The depth of mulch you spread affects both how much you need and how well it works. A 2-inch layer is the minimum for basic weed suppression. A 3-inch layer is the standard — it suppresses weeds effectively, retains soil moisture, and moderates soil temperature without piling mulch against tree trunks or plant stems.

A 4-inch layer offers stronger weed control and moisture retention but costs more and can trap moisture against woody stems if you're not careful about placement. Most homeowners choose 3 inches as the balance between effectiveness and cost. If you're mulching over existing weeds or bare soil, you may want to go to 4 inches on the first process.

Use the square footage-to-cubic-yard conversion

Once you know your total square footage and your desired depth, the math is straightforward. At 3 inches deep, divide your square footage by 100 to get the number of cubic yards you need. At 2 inches deep, divide by 150. At 4 inches deep, divide by 75.

Example: A 500-square-foot bed at 3 inches deep needs 500 ÷ 100 = 5 cubic yards. A 1,000-square-foot bed at 2 inches deep needs 1,000 ÷ 150 = about 6.7 cubic yards (round up to 7). These conversions account for the fact that one cubic yard of mulch covers 100 square feet at 3 inches of depth.

Account for settling and annual top-ups

Mulch compacts and breaks down as it ages. After the first year, you'll lose roughly 1 inch of depth. Rather than re-mulching the entire bed, you can add a 1-inch top-up layer each year to maintain your original depth. A 1-inch layer across 100 square feet requires about 0.3 cubic yards.

If you're calculating for the first time and want to account for settling in year one, add 10 to 15 percent to your initial order. This means if your calculation says you need 5 cubic yards, order 5.5 to 5.75 cubic yards instead. After that, plan to add a small top-up layer each spring or fall.

Compare bulk mulch versus bagged mulch costs

Bulk mulch is sold by the cubic yard and is usually the cheaper option per unit, especially for larger quantities. A cubic yard of bulk mulch typically costs between $25 and $50 depending on your region and mulch type, but you need a truck or trailer to haul it, or you pay for delivery. Delivery fees often start at $50 to $100 and can be higher for rural areas.

Bagged mulch comes in 2-cubic-foot bags, which means you need 13.5 bags to equal one cubic yard. A single bag usually costs $3 to $8, making a cubic yard of bagged mulch cost $40 to $100 when you buy it this way. Bagged mulch is convenient for small beds or if you don't have truck access, but it becomes expensive fast for anything larger than 200 square feet. For a 500-square-foot bed needing 5 cubic yards, bulk mulch with delivery is almost always cheaper than buying 67 individual bags.

Order slightly more than your calculation suggests

Mulch settles during transport and spreads less evenly than you might expect. Ordering 10 to 15 percent more than your calculation gives you a buffer for uneven spreading, settling in the truck, and the inevitable thin spots you'll notice after the first watering. It's easier to use leftover mulch in a nearby bed or save it for next year's top-up than to run short and have to place a second order.

If you're buying bagged mulch, round up to the nearest whole bag. If you're ordering bulk, round up to the nearest half cubic yard. Most suppliers won't charge you significantly more for the small overage, and you'll avoid the frustration of finishing your beds and running out.

Frequently Asked Questions

How do I measure an oddly shaped garden bed?

Break the bed into straightforward shapes — rectangles, triangles, or circles — measure each one separately, and add the square footage together. For a triangle, multiply base by height and divide by two. For a circle, use radius × radius × 3.14. Sketch the bed on paper and label each section so you don't lose track of your measurements.

Can I use the same mulch calculation for different types of mulch?

Yes. The square-footage-to-cubic-yard conversion works the same whether you're using wood chips, shredded bark, or straw. The depth you choose may vary slightly — some mulches compact more than others — but 3 inches is a safe standard across all common types. Check with your supplier if you're using an unusual mulch type.

What if I want to mulch around trees and shrubs instead of in beds?

Measure the diameter of the area you want to cover around each plant, calculate the square footage using the circle formula (radius × radius × 3.14), and add all the circles together. Keep mulch 3 to 4 inches away from tree trunks and plant stems to prevent rot and pest problems. The calculation stays the same; only the layout changes.

Do I need to remove old mulch before adding new mulch?

If your old mulch is still in decent condition and less than 1 inch thick, you can add new mulch on top. If it's matted, moldy, or thicker than 2 inches, rake it out first. Removing old mulch also lets you check for pest damage or disease before you re-mulch. Plan on removing old mulch every 2 to 3 years depending on how quickly it breaks down in your climate.

What's the difference between cubic yards and square feet?

Square feet measures the area of your garden bed (length × width). Cubic yards measures volume — how much mulch you need to fill that area to a certain depth. You need both numbers: square footage tells you how wide your coverage is, and depth tells you how deep. Together, they convert to cubic yards, which is how mulch is sold.