Mulch is sold by volume but spread by area, so the whole job comes down to one conversion: a 2 cubic foot bag covers 8 square feet at 3 inches deep, and a 3 cubic foot bag covers 12. Measure your bed area, pick a depth, and divide. A 100 square foot bed at 3 inches needs about 13 of the 2 cubic foot bags.
Here is the coverage math, the counts by bed size, and the point where bags stop making sense and bulk delivery wins.
Coverage per bag, by depth
Every figure below is one bag's volume divided by the depth. Pick your depth, then divide your bed area by the coverage to get the bag count:
| Depth | 2 cu ft bag covers | 3 cu ft bag covers |
|---|---|---|
| 1 in (light refresh) | 24 sq ft | 36 sq ft |
| 2 in (refresh) | 12 sq ft | 18 sq ft |
| 3 in (new bed) | 8 sq ft | 12 sq ft |
| 4 in (max useful) | 6 sq ft | 9 sq ft |
Three inches is the standard for a new bed, so the 8 and 12 square foot figures are the ones to memorize. Going deeper than 4 inches wastes material and can smother roots, so there is no row past it.
The mulch calculator runs your exact length, width, and depth and returns both bag counts plus the cubic yards for a bulk order, which is the number you actually want once the bed gets large.
Bags by bed size
Each row assumes a fresh 3 inch layer, using the 2 cubic foot bag since that is what most stores stock:
| Bed size | Area | Bags at 3 in |
|---|---|---|
| 4 x 10 ft | 40 sq ft | 5 bags |
| 10 x 10 ft | 100 sq ft | 13 bags |
| 4 x 25 ft (long border) | 100 sq ft | 13 bags |
| 20 x 10 ft | 200 sq ft | 25 bags |
| 30 x 15 ft | 450 sq ft | 57 bags |
Notice the 30 x 15 bed already needs 57 bags. That is a full SUV load twice over, and it is the size where hauling and lifting bags stops being worth it.
When to switch from bags to bulk
Bags win for small, scattered beds and for anyone without a truck. Bulk wins on price and effort once the quantity climbs. The crossover sits at roughly 2 cubic yards, about 25 to 27 of the 2 cubic foot bags.
There are two reasons to switch:
- Price. A cubic yard equals 13.5 of the 2 cubic foot bags. Bagged mulch at typical retail prices runs well above the per yard cost of bulk once you are buying by the dozen, and the gap widens the more you need.
- Handling. Twenty-five bags is 50 cubic feet you load, drive, unload, drag to the bed, and cut open one at a time. One bulk delivery of the same volume gets dumped in the driveway, and you move it with a wheelbarrow.
Below about a yard, bags are simpler and you buy only what you need with no leftover pile. Above two yards, order bulk. In between, it is a toss-up that usually comes down to whether you have a way to haul loose mulch.
Two measuring mistakes that leave you short
Rounding the depth down. Bags are thin when new, so a bed can look done at 2 inches when you meant 3. That half inch is a third of your material. Spread to a consistent, measured depth rather than by eye, especially near the end of the pile when the temptation to stretch it is strongest.
Forgetting beds settle and decompose. Organic mulch breaks down from the bottom, so a bed mulched to 3 inches in spring is often down to 1.5 by late summer. Measure the existing depth before a refresh: a bed still holding 2 inches needs only a 1 inch top up, which is half the bags of a full application.
Round the final count up to whole bags and add one extra. A single spare bag costs a few dollars and saves a second trip when the last stretch of bed comes up short, which it usually does.
Working on the rest of the yard? The gravel calculator sizes paths and borders, and the topsoil calculator covers the soil that goes under new plantings before the mulch goes on top.