How many bags of mulch
A bed of 100 square feet at 3 inches deep is 100 × 0.25 = 25 cubic feet. With 10% for spill and settling, 27.5. That is 14 bags of 2 cubic feet, 10 bags of 3, or 19 bags of 1.5 — and at the reference prices here, $62.72, $59.80 and $75.62. The bigger the bag, the less you pay for the same mulch, which is the one thing a bag count hides until you see all three.
The arithmetic is square feet × depth in feet ÷ bag size, rounded up, because a store sells whole bags. Three inches is 0.25 feet; two inches is 0.1667. Change the depth box and every bag count moves with it. Change the waste box and they move again — the 10% here is for what spills off the shovel and what settles in the first week, and it is a convention rather than a measurement, so it is yours to set to zero.
Bags by bed size
At 3 inches, with the 10% waste box:
| Bed | Square feet | 1.5 ft³ bags | 2 ft³ bags | 3 ft³ bags |
|---|---|---|---|---|
| 5 × 10 ft | 50 ft² | 10 | 7 | 5 |
| 10 × 10 ft | 100 ft² | 19 | 14 | 10 |
| 10 × 15 ft | 150 ft² | 28 | 21 | 14 |
| 10 × 20 ft | 200 ft² | 37 | 28 | 19 |
| 10 × 30 ft | 300 ft² | 55 | 42 | 28 |
| 10 × 40 ft | 400 ft² | 74 | 55 | 37 |
Your bed is not on the list: type its own measurements above and the page works it out. An L round a corner, a circle under a tree, a bed with a path taken out of it — add a part for each piece, and subtract the pieces that get no mulch.
When bags stop being the answer
This page answers in bags on purpose, however big the bed gets. Past about 2 cubic yards — roughly 196 square feet at 3 inches — a delivered load is on offer too, and the block under the list compares the two for you rather than deciding for you.
Bags stay the cheaper way for longer than people expect, because delivery is charged by the trip. At 600 square feet the bags come to 55 of the 3 cubic foot size, $328.90, against a 6.25 cubic yard load at $322.50: the load wins by $6.40, and only there. Below that the trip charge eats the difference. The prices here are reference figures you can type over, and your own numbers may move that crossing point a long way.
What bags buy you that a load does not: you can carry them one at a time, store what you do not use, and spread the job over two weekends. What a load buys you: one trip, no plastic, and no stacking 55 bags in a car.
Three inches, and why
The University of Illinois Extension recommends 2 to 4 inches of mulch on a bed. Thinner than 2 and weeds come through; deeper than 4 and you start keeping air from the roots. Three inches is the middle, and the depth this page starts at. Type something outside that range and the page says so beside the depth box rather than quietly working it out.