Stone coverage and depth by application

Coverage is depth-dependent, always

There is no single 'square feet per ton' figure for stone, because coverage halves when depth doubles. Any chart you use has to state the depth it assumes — most default to 2 inches.

The formula behind every chart: sq ft per ton = 324 ÷ depth in inches ÷ tons per cubic yard. For crushed stone at 1.4 t/yd³ and 2 inches, that is 324 ÷ 2 ÷ 1.4 ≈ 116 sq ft in theory, which the yards round down to 100 to account for spread loss and settlement.

Depth by what the surface has to do

Decorative gravel is a covering, so 2 inches is enough for appearance and 3 inches for weed suppression. Going deeper mostly wastes stone and makes the bed hard to walk on.

A trafficked surface is a structure, so depth follows load. Foot traffic needs 2-3 inches over a firm sub-base; a car needs a 4-6 inch sub-base plus a 3-inch top course; anything carrying trucks wants 8-12 inches in graded layers.

Drainage is about void volume rather than depth — a French drain is filled to whatever depth the trench needs to intercept water, with clean stone all the way.

Irregular shapes

Break odd areas into rectangles, triangles and circles and add them. A triangle is base × height ÷ 2; a circle is π × radius²; a ring or border is the outer circle minus the inner one.

For a curving bed, the fastest field method is to measure the length along the centre line and multiply by the average width — measure the width at four or five points and average them.

How much extra to order

Add 5% on decorative work for spread and settlement, 10% on driveways, and 20-25% on dense-graded base that will be plate-compacted.

Where the surface is being topped up rather than built new, measure the actual missing depth in several places rather than assuming the full spec depth — most refresh jobs need far less than people order.