The gravel formula
Gravel starts as a volume and finishes as a weight. Work out the volume in cubic yards, then multiply by the stone's density to get tons.
volume (cu ft) = length × width × ( depth ÷ 12 )
cubic yards = volume ÷ 27
tons = cubic yards × density
Depth is in inches, so divide by 12 for feet; divide cubic feet by 27 for cubic yards. The density (tons per cubic yard) varies by stone — about 1.4 is a safe default for crushed gravel.
Worked example
A 30 ft × 10 ft driveway, 4 inches deep, gravel at 1.4 tons/yd³:
Volume: 30 × 10 × (4 ÷ 12) = 100 cu ft.
Cubic yards: 100 ÷ 27 = 3.70 cu yd.
Tons: 3.70 × 1.4 = 5.18 tons.
Tip: order about 5-10% extra. Gravel compacts as it settles and as vehicles drive over it, so the finished depth is always a little less than what you spread.
How deep should gravel be?
A driveway typically needs 4 to 6 inches, often laid as a coarse base layer topped with finer surface stone. Walking paths and decorative beds can be shallower, around 2 to 3 inches. Deeper is more stable but costs more — match the depth to the load the surface will carry.