Gravel Driveway Depth & Layer Calculator

There is one piece of public thickness guidance we could source, and it is federal. Table 3 of FHWA's Gravel Roads Construction & Maintenance Guide gives a suggested minimum total gravel thickness — every layer added together — from two things you already know: how many heavy trucks use the road in a day, and how well the ground underneath supports a load.

Table 3, FHWA-OTS-15-002, Gravel Roads Construction & Maintenance Guide, August 2015, page 107. Metric values in the original are 165, 140, 115, 215, 180, 140, 290, 230, 180, 370, 290 and 215 mm. Two honest caveats: this is guidance for rural roads, not a design for a residential driveway, and the guide gives no numeric definition of low, medium and high subgrade support — no CBR value, no R-value. Read it as soft and wet, ordinary, and firm and well drained. If the driveway carries something that matters, an engineer and your local specification outrank this table.

FHWA's guide describes the arrangement in one sentence: build adequate base depth to carry the anticipated loads, then place a layer of surface gravel as a wearing course. Each layer does a different job, which is why they are different stone.

The materials in the calculator are the ones the WisDOT weight-volume study actually measured, so every one of them comes with a conversion factor and an expansion factor you can check. Decorative stone is not in the list because no primary source publishes a density for it.

• 3 inches minimum on the surface layer. FHWA's reason is practical, not structural: under 3 inches there is not enough material for a grader to work with when the surface needs reshaping. The minimum thickness of any single spread pass is 2 inches.
• 4 inches loose per lift, 8 with a real roller. The guide places material in horizontal layers "generally not exceeding a loose depth of 4 inches", and allows up to 8 inches where larger compaction equipment is available. It says outright that a total thickness over 4 inches should be placed in separate layers. Spread it all in one pass and you compact the top while the bottom stays loose. Work out the lifts with the compaction lift thickness calculator.
• 4% crown, which is ½ inch of fall per foot. Water that leaves the surface is what keeps the gravel where you put it. FHWA calls a straight-line 4% crown correct and singles out the rounded, parabolic shape as a problem, because the middle of the road stays flat and holds water.
• 2 feet back from any open excavation. If the stone is stockpiled beside a trench, 29 CFR 1926.651(j)(2) keeps materials at least 2 ft from the edge, or retained. A delivered pile of base is a surcharge load on the trench wall.

Volume of one layer

V (ft³) = length (ft) × width (ft) × layer thickness (in) / 12 → yd³ = ft³ / 27

Volume to buy

V order = V × (1 + extra % / 100)

Tons of one layer

tons = V order (compacted yd³) × conversion factor (tons per compacted yd³)

Measured conversion factors: ¾ in base 1.80–1.86, 1¼ in base 1.79–1.98, 3 in base 1.78–1.87, breaker run 1.82–1.87, select crushed 1.77–1.81, granular backfill 1.62–1.66, RAP 1.64–1.71, RCA 1.65–1.75. The midpoint is used and the range is shown with the result.

Loose cubic yards delivered

loose yd³ = V order (compacted yd³) × (1 + expansion %)

Measured compacted-to-loose expansion: 36–47% for ¾ in base, 34–52% for 1¼ in, 34–45% for 3 in, 27–30% breaker run, 27–29% select crushed, 38–52% granular backfill, 35–46% RAP, 37–60% RCA. Compaction changes the volume, never the weight.

Suggested minimum total thickness (comparison only)

FHWA Table 3: total gravel thickness (in) = f(daily heavy trucks, subgrade support)

0–5 trucks: 6.5 in low support, 5.5 medium, 4.5 high · 5–10: 8.5 / 7.0 / 5.5 · 10–25: 11.5 / 9.0 / 7.0 · 25–50: 14.5 / 11.5 / 8.5. This is guidance for rural roads, quoted for comparison — it is not a design and it does not replace an engineer or your local specification.

Crown

4% cross slope = 1/2 in of fall per foot, from the centerline to each edge

Water leaving the surface is what keeps the gravel in place. The FHWA guide calls a straight-line 4% crown correct and a parabolic crown a problem.