You're standing in the garden with a tape measure and a vague sense of dread. The bags at the garden center say "2 cubic feet" but everything online talks about cubic yards. You don't want to make three trips — and you really don't want a pallet rotting in the driveway.
The math is three steps: measure your area, pick a depth, and do one quick calculation. Here's exactly how to work it out — and how to skip the math entirely.
Measure the Area First — This Is the Only Hard Part
Grab your tape measure and get the length and width of your bed in feet. For a rectangle, it's just length × width. A 10 × 20 ft bed is 200 sq ft; a 12 × 8 ft bed is 96 sq ft.
Irregular beds? Break them into sections. A curved bed is usually close to a rectangle — measure the longest and widest points and that's close enough. For circular beds, measure across the widest part to get the diameter, then halve it for the radius. You'll add a buffer at the end anyway, so a rough estimate works fine.
Depth Determines Everything: What Each Inch Actually Does
This number makes the biggest difference in total volume. A 2-inch layer refreshes an existing bed and retains moisture. Three inches is the standard for new beds — it blocks about 90% of weed seed germination and holds moisture through a hot summer.
At 3 inches deep, one cubic yard covers about 108 sq ft. At 2 inches it stretches to 162 sq ft. At 4 inches for heavy weed pressure, you only get 81 sq ft per cubic yard. Locking in your depth locks in the final number.
The Formula — and a Faster Way to Skip the Math
Here it is: Length (ft) × Width (ft) × Depth (ft) ÷ 27 = cubic yards. Convert your depth from inches to feet first — 3 inches becomes 0.25 ft, 2 inches becomes 0.17 ft.
Example: a 10 × 20 ft bed at 3 inches = 10 × 20 × 0.25 = 50 cu ft ÷ 27 = 1.85 cubic yards. That's 25 standard 2-cubic-foot bags.
Don't want to run the math yourself? Use our free mulch calculator — enter your dimensions, pick your bag size, and it gives you cubic yards, bag count, and estimated cost in one shot.
Add 10–15% Before You Order
Ground is never perfectly flat. Edges get sloppy. Some material compresses after spreading. Always round up before you buy.
For 1.85 cubic yards: add 10% → 2.04 cubic yards. Order 2 cubic yards or 28 bags (rounding up from 25). A little extra sitting in the garage is far better than running out halfway through the bed and having to make a second trip.
Frequently Asked Questions
How do I measure an oddly shaped garden bed?
Break it into simple shapes — rectangles, triangles, or circles. Measure each section separately, calculate the area, then add them together. You don't need perfect precision; adding a 10% buffer when you order covers irregular edges and gaps. Most oddly shaped beds are close enough to a rectangle.
Can I use the same formula for topsoil?
Yes. The formula is identical: length × width × depth (in feet) ÷ 27 = cubic yards. Topsoil, compost, and gravel all use the same volume calculation. The only difference is weight — bulk topsoil is heavier than mulch, so check your delivery truck's weight limit before ordering.
How long does it take to spread mulch?
One person with a wheelbarrow and pitchfork can spread about 1 cubic yard per hour on flat ground. Budget 2 hours for a typical 2-yard job. Getting bulk delivery dropped close to the bed saves significant hauling time compared to loading individual bags from your car.