Crushed limestone is area times depth times density — the same as any aggregate. What makes it its own question is the grade: #57, crusher run and screenings pack differently and do different jobs. Here is the method, and which grade goes where.
The method
Volume = area × depth, then multiply by the density for tonnage. A 40 × 10 ft driveway (400 sq ft / 37 m²) of #57 at 3 in (75 mm) is about 3.7 yd³ (2.8 m³); at roughly 1.35–1.5 US tons per cubic yard that is about 5 US tons — order around 5.5 with a 10% compaction allowance.
Limestone Calculator
Pick a grade (#57, crusher run, screenings) and enter the area for the tons and cubic yards.
#57 vs crusher run
#57 is ¾ in “clean” stone with no fines — it drains freely and won’t compact hard, so it’s used as a surface, in drains and around pipe. Crusher run (#411) mixes stone with stone dust, so it compacts to a solid, load-bearing base. Use crusher run underneath and #57 on top.
The best driveway build-up
Two layers: a compactable base of crusher run, topped with a 2–3 in (50–75 mm)wearing course of #57 for drainage and a clean look. Size and price each layer separately at its own depth — the calculator lets you switch grade and depth for each.
How deep?
For a driveway, about 4 in (100 mm) of compacted crusher run base plus 2–3 in (50–75 mm) of #57 on top. Paths and drainage need less; heavy-traffic or soft-ground jobs need more base, laid and compacted in two lifts over a geotextile.
Tons per yard
About 1.35–1.5 US tons per cubic yard of crushed limestone (roughly 1.5–1.65 tonnes per m³) — it weighs around 95–100 lb per cubic foot. Crusher run packs a little denser than clean #57; your quarry can give the exact figure for their stone.