Landscape material is the one area of DIY estimating where the units fight you. Depth is in inches, area is in feet, the material is sold in cubic yards or bags or by the ton, and none of those convert without knowing what you are buying.
Start with cubic yards
Volume is the base unit, and everything else converts from it.
Cubic yards = (length ft × width ft × depth inches) ÷ 12 ÷ 27
The two divisions are unit conversions. Dividing by 12 turns depth in inches into feet, and dividing by 27 turns cubic feet into cubic yards (a yard is 3 feet, and 3³ = 27).
Worked example. A bed 20 feet by 8 feet, mulched 3 inches deep:
- 20 × 8 × 3 = 480
- ÷ 12 = 40 cubic feet
- ÷ 27 = 1.48 cubic yards
Round up when ordering bulk. Suppliers deliver in half- or quarter-yard increments, and being slightly over beats paying a second delivery fee for a quarter yard.
How deep
Depth matters more than most people expect because it multiplies directly. Going from 2 inches to 3 is a 50% increase in material.
| Material and purpose | Depth |
|---|---|
| Mulch, weed suppression | 3 in |
| Mulch, around trees | 3-4 in, never against the trunk |
| Mulch, refresh over existing | 1-2 in |
| Decorative gravel | 2-3 in |
| Gravel walkway | 3-4 in |
| Gravel driveway, base layer | 4-6 in |
| Gravel driveway, top layer | 2-3 in |
| Playground mulch | 6-12 in (check safety guidance) |
Two practical notes. Mulch deeper than about 4 inches starts causing problems: it holds too much moisture against roots and can go anaerobic. And never pile mulch against a trunk. The “mulch volcano” you see around suburban trees rots the bark and eventually kills them, so pull it back a few inches.
Bags versus bulk
Bagged mulch is almost always 2 cubic feet per bag. A cubic yard is 27 cubic feet, so:
Bags = ⌈ cubic yards × 27 ÷ 2 ⌉, about 13.5 bags per cubic yard
Our 1.48-yard bed needs 20 bags.
The economics break at roughly one cubic yard. Below that, bags win: no delivery fee, no minimum order, and you can carry them in a car. Above it, bulk wins and the gap widens fast. Bulk mulch commonly runs a third to a half the per-yard price of bagged, and 40 bags is an unpleasant afternoon.
Also consider where the pile goes. Bulk arrives as a heap on your driveway and has to be moved by wheelbarrow, which for two or three yards is real work. Bags can be dropped where they are needed.
Gravel is sold by weight
This conversion catches people out. Gravel is quarried, hauled and sold by the ton, not by volume, but you calculated a volume.
Density is the bridge, and it varies by material:
| Material | Approximate tons per cubic yard |
|---|---|
| Crushed stone, common gravel | 1.4 |
| Pea gravel | 1.3-1.4 |
| River rock | 1.3-1.4 |
| Crushed limestone | 1.4-1.5 |
| Sand | 1.3-1.5 |
| Topsoil | 1.0-1.3 |
| Mulch, wood chips | 0.4-0.5 |
So tons = cubic yards × tons per cubic yard, with 1.4 as the default for common gravel, which is what the gravel calculator uses.
A 30 × 10 foot area at 3 inches deep is 2.78 cubic yards, which at 1.4 is about 3.9 tons.
Look at how far mulch sits from stone in that table. Wood is about a third the density of rock, which is why one person can move a yard of mulch and not a yard of gravel.
Ask your supplier for their figure. Density varies with stone type, size and moisture content, and they know what they are selling.
Cost and delivery
Bulk material pricing has a structure worth knowing:
- Delivery is usually a flat fee within a radius, regardless of quantity. One order of four yards costs much less than two orders of two.
- Many suppliers have a minimum, often one or two yards.
- The truck needs access and somewhere to tip. Confirm where it will go before it arrives, and put down a tarp if it is landing on a driveway you care about.
- Order slightly over. Running a quarter yard short means either a second delivery fee or an unfinished bed.
Estimating an irregular bed
Curved and irregular beds don’t have a length and a width, so approximate them:
- Cover the shape with rectangles on a sketch and add the areas.
- For a rough circle, radius × radius × 3.14.
- For a long curved border, average width × total length along the curve.
A few percent over is fine. You spread this material by eye, and rounding up to whole yards absorbs the error.
The checklist
- Volume first: length × width × depth ÷ 12 ÷ 27 = cubic yards.
- Pick the depth from the purpose, not by habit.
- Under a yard, buy bags (13.5 per yard). Over a yard, buy bulk.
- For gravel, convert to tons at your supplier’s density, around 1.4.
- Order slightly over. Delivery is the expensive part, not the material.
The mulch calculator and the gravel calculator handle the conversions and state which density they used.