How the driveway gravel calculator works
The two-layer system
A surviving gravel driveway is two compacted layers: 4–6" of crusher run (fines lock it into a pavement-like base) topped with 2" of clean crushed stone (water sheds, tires grip). Single-layer driveways — one deep pile of round stone — rut, migrate, and re-order themselves annually. The layers do different jobs; skipping one is how driveways fail.
Fabric, crowns, and drainage
Geotextile fabric between soil and stone stops the migration that buries gravel into mud — mandatory over clay, cheap insurance elsewhere. The surface should crown 2–3 inches toward the edges so water exits; flat driveways collect the water that turns stone soup. Ditches or swales at the edges complete the hydrology.
Compaction discipline
Base stone compacts in 2" lifts with a plate compactor ($60/day rental) — one deep loose layer never compacts evenly. The surface layer stays loose by design (it is the wear surface). Annual inspection finds the potholes while they are shovel-sized; the 2-inch refresh every 2–4 years is the whole maintenance contract.