Working out how much mulch to buy
Mulch is bought as a volume but spread as a depth, so the only two numbers that matter are the area of the bed and how thick you want the layer. Everything else follows. The trap is that bagged and bulk mulch are priced in different units, which makes them almost impossible to compare in your head at the yard gate.
- bed area — square feet of ground you are covering, not the plants
- depth — finished thickness of the layer in inches
- bag size — 2 ft³ and 3 ft³ are the two common sacks
- 27 — cubic feet in a cubic yard, the unit bulk is sold in
What a cubic yard actually spreads
| Depth | One cubic yard covers | 2 ft³ bags per yard | One 2 ft³ bag covers |
|---|---|---|---|
| 2 in | 162 sq ft | 13.5 | 12 sq ft |
| 3 in | 108 sq ft | 13.5 | 8 sq ft |
| 4 in | 81 sq ft | 13.5 | 6 sq ft |
Worked example: a 500 square foot bed at 3 inches
Sixty-three sacks is roughly four and a half yards. That is a pallet's worth, several car loads, and the point at which a bulk delivery almost always wins on price even after the delivery charge.
Why there is no waste allowance on mulch
Nothing is lost between the pile and the bed. You barrow it, tip it and rake it, and any surplus simply makes the layer a little thicker or goes round another shrub. What does change the quantity is settlement: bark compacts over the first few weeks, and shredded material more than nuggets, so a bed raked to three inches will read closer to two and a half by the end of the season.
Common mistakes
- Measuring the planting, not the ground. You mulch the soil between the plants. Beds with mature shrubs need noticeably less than their outline suggests.
- Comparing bag price to yard price directly. A yard is 13.5 of the 2 ft³ sacks. Multiply before you decide which is cheaper.
- Going deeper than four inches. Thick mulch holds water against stems and starves roots of air. It costs more and does harm.
- Mulching to the trunk. Piling it against bark rots the collar. Pull it back a couple of inches and you will need slightly less.
When these numbers do not apply
- Topping up an existing bed. Measure the extra depth you want, not the total, or you will buy twice what you need.
- Sloping beds. Mulch creeps downhill and usually needs a deeper layer at the top to end up even.
- Rubber and stone mulches. Those are sold by weight and this volume maths does not carry across.
- Playground depths. Safety surfacing is specified far deeper than a garden bed and follows its own standards.