Flagstone calculator

Flagstone is sold by the ton, not by area. Enter the patio size, the thickness, and the stone, and get the tonnage to order — with the coverage-per-ton figure that changes every time the thickness does.

01 How the math works

Flagstone is priced and delivered by weight, so the whole calculation runs through weight. The plan area times the thickness gives a volume; the volume times the stone's density gives a weight; the weight divided by 2,000 lb gives the tons to order. Area never buys stone on its own — the thickness and the density decide how far a ton goes.

The default stone uses the trade's estimating rule: flagstone weighs about 14 lb per ft² for every inch of thickness, which is a density of 168 lb/ft³ (2,691 kg/m³) — the dense-sandstone and bluestone range. Sandstone runs lighter at 150 lb/ft³ (ASTM C616 Class II), quartzite and granite heavier at 160 lb/ft³ (ASTM C616 Class III). Pick the type and the calculator swaps the density. The coverage figure it reports is the "square feet per ton" a stone yard quotes — but it is only true at the thickness you entered, which is the part worth watching.

02 Worked example

Take a backyard flagstone patio: 10 ft × 12 ft, 120 ft² (11.15 m²), dry-laid in 2 in (51 mm) stone at the default 168 lb/ft³ (2,691 kg/m³).

Weight: 120 ft² × (2 ÷ 12) ft × 168 lb/ft³ = 3,360 lb = 1.68 tons (1,524 kg / 1.52 tonnes). Coverage at 2 in is 71.43 ft²/ton (7.32 m²/tonne), and 120 ÷ 71.43 lands on the same 1.68 tons.

Now the allowance. Irregular flag gets cut and chipped to fit, so add 15%: 1.68 × 1.15 = 1.93 tons (1.75 tonnes). Rounded up to the half-ton most yards sell in, that's 2.0 tons (2.0 tonnes). Go to 1 in stone and the same patio needs half the weight; go to 2.5 in and it needs a quarter more.

03 When this calculator is wrong

The default breakage allowance is 15%, and for irregular flag that is the floor, not padding. The flat 10% most calculators apply is calibrated for sawn, rectangular stone laid to a pattern; random flag that you cut to fit and trim around curves runs 15–20%, because the broken corners and the off-cuts that won't fit anywhere are discarded. Sawn-edge flag laid in a grid is the exception where 10% holds.

• As-quarried stone doesn't honor its nominal thickness. Flag sold "as quarried" runs a spread — a pallet called 2 in can hold stones from 1.5 to 2.5 in. Because it is billed by weight, a batch running 2.5 in against a 2 in nominal covers 2 ÷ 2.5 = 20% less area than the coverage chart promises. Gauged (calibrated-thickness) flag costs more per ton and removes the guesswork.
• Joint width moves coverage. The math assumes stones butt together. Dry-laid flag with 0.5–2 in (13–50 mm) joints of sand or planting mix covers more ground per ton — a 1.5 in joint on a field of 18 in stones adds roughly 5–8% to the area a ton reaches. That is coverage in your favor, but it is also joint-fill material you still have to buy.
• Density varies by quarry. The type presets are mid-range figures from the ASTM dimension-stone classes. A porous sandstone can sit below 150 lb/ft³ and a tight quartzite above 170, which shifts coverage by several percent. The supplier's own coverage chart for the specific stone beats any generic density.

04 What to do with the result

Order the rounded-up tonnage, and ask the yard two things before you pay: the actual average thickness of the pallet you're buying, and their own coverage figure for that stone. If their number and this one disagree, theirs wins — they've weighed and measured the specific lot. Flagstone is one of the few materials where buying a second batch later is a real problem: color and texture are set by the quarry run, and a replacement pallet rarely matches, so order the whole job at once from one lot rather than running short and chasing a match.

If the stone is sold by the pallet rather than loose by the ton, divide the tonnage by the pallet weight the yard quotes — pallets commonly run 1.5–2 tons but vary by stone, so use their figure, not a guess.

05 Common questions

How many square feet does a ton of flagstone cover?

It depends entirely on thickness. For typical 168 lb/ft³ stone, one ton covers 142.86 ft² at 1 in, 95.24 ft² at 1.5 in, and 71.43 ft² at 2 in (14.63, 9.75, and 7.32 m²/tonne). Halve the thickness and a ton goes twice as far.

How much does flagstone weigh per square foot?

About 14 lb/ft² per inch of thickness for typical flagstone — so 28 lb/ft² at 2 in. Lighter sandstone is nearer 12.5 lb/ft² per inch, quartzite about 13.33 lb/ft² per inch. It's the number to plan the delivery and the wheelbarrow trips around.

How thick should flagstone be for a patio?

For dry-laid flag on a compacted sand or stone-dust bed, 1.5–2 in (38–51 mm) is the usual range — thinner stone cracks underfoot, thicker is heavier to set and buys little. Mortared over an existing concrete slab, 1–1.5 in (25–38 mm) is enough. Confirm against your base and traffic.

How much extra flagstone should I buy for breakage?

15–20% for irregular flag you cut and fit, 10% for sawn rectangular stone in a simple pattern. The off-cuts and broken corners on random flag are real, and a dye-lot match later is harder than returning a few extra pieces now.

Is flagstone sold by the ton or the square foot?

By weight — the ton, or the pallet, which is priced by weight. Square-foot coverage is a derived figure the yard quotes off its own density and thickness. That is why two stones at the same price per ton cover different areas.