Transparent method
The formula
Worked example
Example inputs
How to interpret the result
Concrete, gravel, mulch and topsoil all settle, compact or spread unevenly once actually placed, so an order calculated from the mathematically precise volume of the space alone tends to fall short in practice. Real ground isn't perfectly flat, and material doesn't pack with zero gaps the way the geometry assumes.
Before you rely on it
What to check
Round the calculated volume up and add a margin for uneven ground and settling, rather than ordering the exact calculated figure with nothing extra.
The common error
Where people go wrong with concrete, gravel & mulch calculator
Ordering the precise calculated volume with no margin added. Uneven ground and natural settling routinely mean the exact geometric calculation runs short of what the project actually needs.
Sensitivity evidence
How length (ft) changes the cubic yards needed
| Length (ft) | Cubic yards needed | Bags needed |
|---|---|---|
| 16 | 2 | 88.9 |
| 18 | 2.2 | 100 |
| 20worked example | 2.5 | 111.1 |
| 22 | 2.7 | 122.2 |
| 24 | 3 | 133.3 |
Every input, tested
Which input moves the cubic yards needed most
| Input | Tested from | To | Cubic yards needed at each end | Swing |
|---|---|---|---|---|
| Depth (in) | 3.6 | 4.4 | 2.2 to 2.7 | 0.494 (20%) |
| Length (ft) | 18 | 22 | 2.2 to 2.7 | 0.494 (20%) |
| Width (ft) | 9 | 11 | 2.2 to 2.7 | 0.494 (20%) |
Two variables at once
Cubic yards needed by length (ft) and width (ft)
| Length (ft) \ Width (ft) | 8 | 10 | 12 |
|---|---|---|---|
| 16 | 1.6 | 2 | 2.4 |
| 18 | 1.8 | 2.2 | 2.7 |
| 20 | 2 | 2.5 | 3 |
| 22 | 2.2 | 2.7 | 3.3 |
| 24 | 2.4 | 3 | 3.6 |
Step by step
The worked example, input by input
| Input | Value used | What it means |
|---|---|---|
| Length (ft) | 20 | |
| Width (ft) | 10 | |
| Depth (in) | 4 | |
| Material | Concrete (80lb bags) | Sets the bag-coverage estimate. |
| Cubic yards needed | 2.5 | |
| Bags needed | 111.1 | |
Inputs, definitions and assumptions
Length (ft)
Width (ft)
Depth (in)
Material
Sets the bag-coverage estimate.
Where the inputs come from
Enter the length and width of the area in feet, the depth in inches and the material: concrete, gravel, mulch or topsoil, which sets the coverage per bag. Measure the actual area, breaking irregular shapes into rectangles, and choose a depth that suits the job: 4 inches is common for a concrete slab, 2 to 3 inches for mulch and 4 to 6 inches for a gravel base. Round up the result, since ordering short is costly.
When the result stops applying
Ground is rarely perfectly level, and compaction of gravel reduces volume. Concrete slabs need reinforcement, a base and forms that add to the material list. Bag yields vary by product: an 80-pound bag of concrete yields about 0.6 cubic feet, while a 60-pound bag yields about 0.45. Order a bit extra, since a short pour is worse than a leftover bag.
What to compare it with
See the cubic yard and cubic feet calculators for other volumes, the rectangle area calculator for the surface, and the paver, tile and flooring pages for surface materials. The unit conversion pages convert cubic units, and the fence and paint calculators cover other projects.
People also ask
Frequently asked questions
How many bags of concrete are in a cubic yard?
About 45 bags of 80 pounds or 60 bags of 60 pounds, since a cubic yard is 27 cubic feet.
How deep should a concrete slab be?
Four inches is standard for patios and walkways, and driveways and garage floors are often 5 to 6 inches thick.
Should I order extra material?
Yes, about 5% to 10% extra to cover uneven ground, spillage and settling.