Key Takeaways
- Depth multiplies straight through to cost; two inches versus three is a 50 percent difference
- Decorative beds and paths: 2 to 3 inches; driveways: 4 to 8 inches Asphalt Institute in layers
- Soil type, load, drainage, and climate all influence the right depth
- Driveways need multiple layers calculated separately, not one average depth
- Add 10 to 15 percent FHWA for compaction when loose gravel settles
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.
Depth is the number worth getting right before you order anything, because it multiplies straight through to what you pay. Two inches against three inches is a fifty percent difference in tonnage over the same area. Nothing else on the form has that kind of leverage.
Here is what people commonly build to, and, more usefully, what actually decides it.
Common depths, by job
[PERSONAL EXPERIENCE] These are starting points, not requirements. Every one of them changes with the site.
Decorative beds and borders: 2 to 3 inches. Enough to cover the ground and suppress light, not so much that it becomes a pit. Deeper is wasted material.
Paths and walkways: 2 to 3 inches over a prepared surface. Comfortable to walk on. Any deeper and loose gravel starts to feel unstable underfoot, particularly rounded stone like pea gravel.
Patio and paver base: 4 to 6 inches of compacted base, plus about an inch of bedding. More on soft ground, more again if anything will drive on it.
Gravel driveway: 4 to 8 inches total, in layers. Not one depth of one material. See below.
French drains and drainage trenches: 12 to 24 inches, though here the depth is set by where the water needs to go rather than by convention.
What actually decides it
Four things, roughly in order of how much they matter.
The load. A garden path carries a person. A driveway carries a car. A parking area might carry a delivery truck. Each step up needs more depth and, usually, a coarser material underneath.
The soil. This is the one people skip. Firm, well drained ground supports a shallower build than soft clay does. On poor ground the sub base is not optional depth, it is the layer that stops the whole thing sinking into the subsoil over the first wet season.
Drainage. Water is what destroys gravel surfaces, by washing out fines and by freezing. A site that holds water needs more thought than one that sheds it, and sometimes needs drainage before it needs gravel.
Climate. Freeze and thaw cycles move material. Regions that get them generally build deeper than regions that do not.
Why a driveway is a special case
The single most useful thing to understand about driveway depth is that it is not one depth. A driveway is normally built up in layers: a coarse sub base that carries and spreads the load, a binding layer over it, and a surface layer that is comfortable to drive and walk on.
Calculating that as one average depth of one material gets the sub base badly wrong, because the sub base is usually the deepest layer and the coarsest material. This is exactly why the driveway calculator works layer by layer and totals them rather than asking for a single depth.
Loose depth against finished depth
One more thing worth knowing, because it catches people out at the end of the job. Loose gravel occupies more volume than the same material does once it has been compacted. A layer that has to finish at four inches compacted needs more than four inches of loose material to start with.
How much more depends on the material and the equipment, which is why this site treats compaction as an allowance you can see and adjust rather than a hidden factor baked into the result. Ten to fifteen percent is a common allowance for a job that will be rolled.
A caution worth reading
None of the depths above is a specification. For anything structural, anything carrying vehicle loads, or anything your local building department has an opinion about, use what they require. This site produces shopping estimates, not engineering.
Once you have a depth you are happy with, the gravel calculator turns it into a quantity, and shows you the arithmetic it used to get 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 deep should gravel be for a driveway?
A typical driveway needs 4 to 8 inches total, built in layers: 4 inches of sub base (more on soft soil), 2 to 3 inches of base, and 2 inches of surface stone. The driveway calculator handles multi-layer calculations automatically.
How deep should gravel be for a walkway?
2 to 3 inches of decorative gravel over a prepared surface works for most walkways. Any deeper and loose gravel starts to feel unstable underfoot, especially rounded stone like pea gravel.
How deep should gravel be for drainage?
French drains and drainage trenches typically run 12 to 24 inches deep, but the exact depth depends on where the water needs to go. Use a clean, washed stone for drainage, not crusher run.
How much compaction allowance should I add?
Add 10 to 15 percent to your loose depth to account for compaction. A layer that needs to finish at 4 inches compacted will need about 4.5 to 4.6 inches of loose material to start with.