Key Takeaways
- A lasting driveway has three layers: sub base, base, and surface
- Sub base spreads the load over native soil; skipping it causes rutting on soft ground
- Base (crusher run) compacts firm and carries daily wheel loads
- Surface stone is chosen for looks and comfort, not structural strength
- Calculate each layer separately with its own material and density
This calculation follows standard aggregate industry methodology.
Cubic yard is a unit of volume equal to a cube measuring 3 feet on each side, commonly used in the US for ordering gravel and other aggregates.
Density in gravel calculations refers to the weight of material per unit volume, typically measured in tons per cubic yard or tonnes per cubic metre.
A gravel driveway that holds up is not a single depth of one material. It’s normally three layers, each doing a different structural job, and the most common failure (rutting and potholes within the first wet season) usually traces back to one of those layers being thin, wrong, or skipped entirely rather than to the gravel itself being poor quality.
The three layers, and what each one is for
Sub base. The bottom layer, and usually the deepest and coarsest. Its job is to spread the load from above over a wide enough area of the native soil that the soil doesn’t deform under it. This is the layer that actually determines whether a driveway holds up on soft or clay ground, and it’s the layer people skip most often because it’s invisible once the job is done. A large angular stone, something like #3, is typical here, because the large pieces interlock into a stable mass even before anything is compacted onto them.
Base. A denser, better-graded material over the sub base, typically crusher run. This is where the fines that were absent from the sub base matter: they let this layer compact into something firm and continuous, which is what actually carries the wheel loads day to day rather than letting them punch through to the sub base.
Surface. The layer you see and drive on. Usually a clean washed stone, chosen for how it looks and how it feels underfoot and under tyres, rather than for structural work, since the base underneath is doing that job.
Why calculating this as one average depth gets it wrong
If you take a driveway’s total depth, say nine inches across three layers, and calculate it as nine inches of one material, you get a tonnage that doesn’t correspond to anything you can actually order, because a supplier sells sub base stone, crusher run, and surface stone as different products at different prices and different densities. Worse, if you only use that single number to order one material, you’ll typically end up ordering surface stone in sub base quantities, which is expensive and structurally wrong: surface stone doesn’t compact the way sub base does.
The only way to get a driveway estimate that maps onto an actual order is to calculate each layer on its own material, its own depth, and its own density, then add the totals. That’s the entire design of the driveway calculator on this site: three independently configurable layers, each producing its own tonnage, summed into one total rather than averaged into a meaningless blend.
What depths are typical
Common starting points, not requirements, since the real answer depends on the soil underneath and what will be driving on it:
- Sub base: 4 inches, more on soft or clay ground.
- Base: 2 to 3 inches.
- Surface: 2 inches.
Nine inches, or 10 to 12 on poor soil, is a common total. For anything carrying more than ordinary car traffic, or on ground known to be soft, more sub base depth is the first thing to add, not more surface stone.
What happens when a layer is skipped
Skip the sub base on soft ground and the base layer above it slowly presses into the soil under wheel loads, and you get exactly the rutting pattern that shows up along the two wheel tracks first. Skip the base and put surface stone directly over sub base, and the surface never has anything solid to sit on, so it shifts and scatters instead of staying put. Neither failure is really about gravel quality; both are about a layer that wasn’t there.
Editorial note: This article is reviewed for accuracy and updated regularly. All calculations use standard industry formulas. For the latest density figures, consult your gravel supplier directly.
Frequently asked questions
How many layers does a gravel driveway need?
Three layers: sub base (4 inches or more), base (2 to 3 inches), and surface (2 inches). Each layer serves a different structural purpose and uses a different material. The total is typically 8 to 12 inches depending on soil conditions.
What material goes in each layer?
Sub base uses large angular stone like #3 that interlocks and spreads the load. Base uses crusher run that compacts firm. Surface uses clean washed stone like #57 chosen for appearance and comfort.
What happens if I skip the sub base?
On soft or clay ground, the driveway will rut and sink within the first wet season. The sub base is the layer that prevents the driveway from pressing into the native soil under wheel loads.
How do I calculate gravel for a driveway?
Use the driveway calculator which handles three layers independently. Enter each layer’s material, depth, and area separately, and it totals the tonnage. Do not calculate the driveway as one average depth of one material.