How to measure a bed for mulch
Measure the bed area in square feet, the same way you would for sod: length times width for a rectangle, and for an irregular bed, break it into smaller rectangles and circles you can measure separately and add together. A kidney-shaped bed around a tree is usually close enough to a rectangle plus a circle, or a circle with a smaller circle cut out for the trunk. This calculator's preview panel scales to match what you enter, but the math underneath is exactly what you would do by hand: length times width for a rectangle, pi times radius squared for a circle.
Depth is the part sod does not need and mulch does. Mulch is sold and measured by volume, not just area, because how thick you spread it changes how much you need for the same footprint.
How depth drives quantity
Volume in cubic feet is area in square feet times depth in inches, divided by 12 (since there are 12 inches in a foot and area is already in square feet). That means depth and quantity scale together directly: doubling the depth from 2 inches to 4 inches doubles the amount of mulch you need for the same bed, even though the footprint on the ground has not changed at all. A 200 square foot bed needs about 33 cubic feet at 2 inches, but about 67 cubic feet at 4 inches. This is the single most common source of a mulch order coming up short: people measure the bed correctly but guess at the depth, or forget to account for it at all.
Why 2 to 4 inches, and what too thick does
The University of California Statewide IPM Program recommends coarse-textured mulches up to about 4 inches deep for weed control, since larger particles need more depth to block light from reaching the soil. Fine-textured mulches pack more tightly and should stay closer to 2 inches, since a thick layer of fine material holds water and stays wet, which can actually help weeds germinate inside the mulch instead of stopping them. Michigan State University Extension horticulturist Bert Cregg recommends about 3 inches for a new bed, then a light yearly top-dress to keep it looking fresh rather than building up depth every season.
Both sources warn against going past about 4 inches. Mulch piled too deep holds excess moisture against the soil and plant roots, which can starve roots of oxygen and encourage rot, essentially the opposite of what mulch is supposed to do. The same problem shows up in miniature with a "mulch volcano," mulch piled directly against a tree trunk or shrub stem. Keep mulch a few inches back from any trunk or stem so the base stays dry, even if the surrounding bed is at a full 3 to 4 inches.
Bags vs. bulk cubic yards
A cubic yard is 27 cubic feet, and a standard 2 cubic foot bag is the most common size sold, which works out to 13.5 bags per cubic yard (round up to 14 bags to actually cover a full yard, since you cannot buy half a bag). For a small bed, bags are usually more convenient: no minimum order, no truck, no leftover pile in the driveway. For anything past a few cubic yards, bulk delivery is almost always cheaper per cubic foot and means far less packaging to deal with, most suppliers price bulk mulch by the cubic yard and it undercuts the equivalent number of bags once you are ordering more than roughly 3 to 5 yards, though the exact break-even point depends on local pricing. If you are unsure which way to go, price both: multiply your bag count by the per-bag price, then compare that to a bulk quote for the same cubic yardage.
Why order about 10 percent extra
Ground is rarely perfectly flat, edges settle, and mulch compacts somewhat after the first watering or rain, all of which eat into your measured volume. A simple rectangular bed with square edges needs only about 5 percent extra. A typical bed with a curved edge or a tree well needs closer to 10 percent. A bed with lots of curves, mulch rings around several trees, or noticeably uneven ground should plan for 15 percent. It is cheaper to have a few leftover bags on hand for next year's top-dress than to run short mid-project and have to make a second trip.
If part of the project is a gravel path or border instead of a planted bed, the gravel calculator uses the same area-and-depth approach, just converted to tons using material density instead of bags. Filling in the lawn itself is a different calculation again, see the grass seed calculator. If you are surfacing a play area rather than a decorative bed, depth is a safety requirement, not a style choice, use the playground surfacing calculator instead. In the southern US, pine straw is a common alternative to wood mulch, sold by the bale rather than the cubic yard, see the pine straw calculator if you are weighing the two. If the bed needs new soil before it needs mulch, the topsoil calculator handles that layer, including the tons figure for bulk delivery, and the compost calculator handles working organic matter into that soil rather than just topping it. If you are weighing wood mulch against a material that never decomposes, the rubber mulch calculator covers landscape use of recycled rubber mulch, including the smaller bag size and weight it requires.