The Basic Formula for Mulch Coverage

To figure out how much mulch you need, measure the length and width of each garden bed in feet, multiply those numbers together to get the square footage, then divide by the coverage rate of your mulch. Most mulch covers 100 to 150 square feet per cubic yard when spread 2 to 3 inches deep — the standard depth for weed suppression and moisture retention. A cubic yard is 27 cubic feet, which is the unit mulch is usually sold in.

Here's the math in order: length × width = square footage. Then take your total square footage and divide it by the coverage rate. For example, a bed that is 10 feet long and 8 feet wide is 80 square feet. If your mulch covers 100 square feet per cubic yard at 3 inches deep, you need 0.8 cubic yards — which rounds up to 1 cubic yard in practice, since you can't buy 0.8 of a yard.

The coverage rate changes based on depth. If you spread mulch only 2 inches deep instead of 3, one cubic yard covers roughly 150 square feet. At 4 inches deep, it covers about 75 square feet. Deeper mulch lasts longer and suppresses weeds better, but uses more material. Most homeowners find 3 inches is the practical middle ground.

Key Takeaways

  • Measure the length and width of your bed in feet, multiply them together, then divide by the coverage rate (usually 100 to 150 square feet per cubic yard at 3 inches deep).
  • One cubic yard of mulch covers roughly 100 square feet at 3 inches deep, 150 square feet at 2 inches deep, and 75 square feet at 4 inches deep.
  • Always round up when your calculation gives you a fraction — you cannot buy half a cubic yard, and it's better to have extra than to run short.
  • Irregular or curved beds need to be broken into rectangles or triangles, measured separately, and added together for total square footage.

Measuring Beds with Straight Edges

For rectangular or square beds, the measurement is straightforward. Use a measuring tape to find the length and width in feet. If your bed is 12 feet long and 4 feet wide, that's 48 square feet. Write down every measurement — it's straightforward to forget a bed once you've measured five or six.

If you have multiple beds, add all the square footages together before dividing by the coverage rate. Say you have three beds: one that's 48 square feet, one that's 30 square feet, and one that's 20 square feet. Your total is 98 square feet. At 100 square feet per cubic yard, you need just under 1 cubic yard — so you'd order 1 cubic yard and have a small amount left over.

Measuring Irregular and Curved Beds

Curved or oddly shaped beds are harder to measure, but you can break them into simpler shapes. Divide the bed mentally into rectangles and triangles, measure each section, then add them together.

For a triangular section, measure the base and the height (the perpendicular distance from the base to the opposite point), multiply them together, then divide by 2. For example, a triangle with a 6-foot base and a 4-foot height is (6 × 4) ÷ 2 = 12 square feet. If your curved bed is roughly circular, measure across the widest point (the diameter), divide that number by 2 to get the radius, then use the formula: radius × radius × 3.14. A circular bed with a 10-foot diameter has a radius of 5 feet, so 5 × 5 × 3.14 = 78.5 square feet.

For very irregular shapes, sketch the bed on graph paper, count the squares, and use that as your estimate. This method is less precise but works when the shape doesn't fit standard formulas.

Accounting for Depth and Settling

The depth you choose affects both how much mulch you buy and how long it lasts. At 2 inches deep, mulch suppresses weeds but breaks down faster and may need topping up in a year. At 3 inches, it's the standard for most gardens — good weed control and lasts 12 to 18 months before you need to add more. At 4 inches or deeper, it lasts longer but uses significantly more material and can trap moisture against plant stems, which some plants don't tolerate.

Mulch also settles and compacts over time, especially after rain. Plan to add a 1-inch top-up layer once a year if you want to maintain consistent depth. This means if you spread 3 inches initially, you might add 1 inch the following spring. That top-up layer requires about one-third of the original amount of mulch.

Choosing Between Mulch Types and Coverage Rates

Different mulch materials have slightly different coverage rates because of their density and particle size. Shredded hardwood mulch, the most common type, covers about 100 to 120 square feet per cubic yard at 3 inches. Pine bark nuggets are lighter and fluffier, so they cover closer to 120 to 150 square feet per cubic yard at the same depth. Dyed mulch is usually denser and covers about 100 square feet per cubic yard.

When you buy mulch, the supplier should tell you the coverage rate for that specific product. If they don't, ask — it's a standard piece of information. Using the wrong coverage rate is the most common reason people buy too little or too much. A bag of mulch sold at a garden center usually covers 8 to 10 square feet at 3 inches deep, so you can calculate how many bags you need if you prefer bagged mulch to bulk delivery.

Calculating for Slopes and Uneven Ground

Sloped beds need more mulch than flat ones because the material spreads thinner when it's on an angle. If your bed slopes noticeably, add 10 to 20 percent to your total. For example, if a sloped bed calculates to 2 cubic yards on flat ground, order 2.2 to 2.4 cubic yards instead.

Uneven ground with dips and high spots is trickier. Measure the bed as if it were flat, then visually estimate how much of the bed is significantly lower or higher than average. If about a quarter of the bed is notably sunken, add 25 percent to your order. This is less precise than the other methods, but it accounts for the reality that mulch settles into low spots and doesn't cover high spots as well.

Ordering and Delivery Considerations

Most landscape suppliers sell mulch by the cubic yard and deliver in bulk. A standard delivery is usually 5 to 10 cubic yards, though some suppliers have minimums. If you need only 2 cubic yards, you might pay for a 5-yard minimum, so it's worth asking about smaller quantities or whether you can split an order with a neighbor.

Mulch is heavy — one cubic yard weighs 400 to 500 pounds depending on moisture content and type. Make sure the delivery truck can access your garden beds without driving across your lawn or damaging hardscape. Ask the supplier where they'll dump the pile, and plan to spread it within a few days so it doesn't kill the grass underneath.

Bagged mulch from a garden center is more expensive per cubic yard but works well if you have a small area or prefer to spread it gradually. Calculate how many bags you need using the coverage rate on the bag, then add one or two extra bags for settling and uneven spreading.

Frequently Asked Questions

What if my calculation gives me a decimal, like 2.3 cubic yards?

Always round up. You cannot buy 0.3 of a cubic yard, and it's better to have leftover mulch than to run short and have bare patches. Leftover mulch keeps in a dry place for next year's top-up layer.

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

Not always. If the old mulch is still in decent condition and not compacted hard, you can spread new mulch on top. However, if it's matted down, moldy, or more than 4 inches deep already, rake it out first. Spreading new mulch over old can create a layer that sheds water instead of absorbing it.

How much mulch do I need if I'm filling a raised bed?

Measure the length, width, and depth of the bed in feet. Multiply length × width × depth to get cubic feet, then divide by 27 to convert to cubic yards. For example, a raised bed that's 4 feet long, 2 feet wide, and 1 foot deep is 8 cubic feet, or about 0.3 cubic yards — so you'd order 1 cubic yard and have plenty left over.

Can I use the same coverage rate for all mulch types?

No. Different mulches have different densities and particle sizes, so coverage rates vary. Always check the coverage rate for the specific product you're buying, or ask the supplier. Using the wrong rate is a common reason for ordering too much or too little.

Should I measure in inches or feet?

Always measure in feet for the length and width of your bed. If you measure in inches by accident, convert to feet by dividing by 12 before multiplying. The coverage rate is always given in square feet per cubic yard, so working in feet keeps the math consistent.