Flagstone calculator

How the flagstone calculator works

Flagstone coverage depends on how thick the stone is: thinner stone covers more ground per ton. The coverage figures come from the weight of the stone. Most flagstone (sandstone, limestone, bluestone, slate and similar) weighs very roughly 150 to 170 lb per solid cubic foot. A ton of stone 1½ in thick then covers about 2,000 ÷ 160 ÷ (1.5 ÷ 12) = 100 sq ft. Because a pallet of flagstone varies in thickness from piece to piece, the calculator gives a range.

Worked example

A 12 × 14 ft patio (168 sq ft) in 1½ in flagstone, with 10% extra for fitting the irregular pieces: 168 × 1.1 = 184.8 sq ft. At 110 sq ft per ton that is 1.68 tons; at 90 sq ft per ton, 2.05 tons. Order about 2 tons, and pick the stones yourself if the yard allows it so you get pieces that fit together.

Under it, a 4 in compacted base running 6 in past the edges is (14 + 1) × (12 + 1) × 4 ÷ 12 × 1.2 ÷ 27 = 2.9 cu yd, and 1½ in of bedding sand is 168 × 1.5 ÷ 12 × 1.1 ÷ 27 = 0.86 cu yd.

Stepping stones

Example: a 30 ft path with stones 24 in center to center needs 360 ÷ 24 + 1 = 16 stones. With 18 in stones, that leaves 6 in of lawn or gravel between them.

Spacing is personal. Walk the route at a normal pace, mark where your feet land, and measure. Paths used mostly by children, or on a slope, may want closer stones. Each stone sits in a shallow hole on 1 to 2 in of sand so its top is level with or just above the surrounding ground; the calculator allows 2 in of sand under each stone.

Laying a flagstone patio

1. Base: dig out and compact the soil, add about 4 in of crushed base in lifts, compacted, sloped about ¼ in per foot away from the house. See the paver base calculator and the slope calculator.
2. Bedding: spread sand or fine screenings. Because flagstone varies in thickness, the bed is usually a little deeper than under pavers, about 1 to 2 in, and you add or scoop out under each stone to level it.
3. Fit the stones: lay out the big pieces first, then fill in, aiming for joints of a fairly even width. Trim edges with a chisel or a saw where needed.
4. Joints: fill with sand, stone dust, a wide-joint polymeric sand, or soil and creeping plants. The polymeric sand calculator explains joint fill for narrower joints.

Buying flagstone

Flagstone is sold by the ton or by the pallet, and the price and coverage depend on the stone type, its color and how thick and regular it is. Patio-grade stone is usually sorted by thickness; ask what range of thickness the pallet contains and what coverage the yard sees with it. Irregular stone also comes in sizes, from small pieces that fill gaps to large slabs; a mix is easier to fit.

Coverage per ton is an estimate from typical stone weights. Stone yards often know the coverage they get with each stone they sell; use their figure if it differs.