How the flagstone calculator works
Flagstone is a flat slab of solid stone, so the calculator uses the stone’s solid density rather than a loose-rock weight:
cubic feet = area (sq ft) × thickness (in) ÷ 12
tons = cubic feet × lb per cu ft ÷ 2,000 × (1 + extra %)
Worked example: a 12 × 12 ft patio
Sandstone flagstone, 1½ inches thick:
- Area: 12 × 12 = 144 sq ft
- Volume: 144 × 1.5 ÷ 12 = 18 cu ft of stone
- Weight: 18 × 145 lb = 2,610 lb = 1.31 tons
- With 10% for cutting and fitting: 1.44 tons
Worked example: a 10 × 14 ft dry-laid patio
Limestone flagstone, 2 inches thick: 140 sq ft × 2 ÷ 12 = 23.3 cu ft. × 160 lb = 3,733 lb = 1.87 tons, or 2.05 tons with 10% extra.
Square feet per ton of flagstone
The “1 ton covers” line in the calculator comes from the same math. These are solid-stone figures for tightly fitted pieces; wider joints stretch them a little.
| Thickness | Sandstone (145 lb/cu ft) | Limestone (160) | Quartzite (165) | Slate (172) |
|---|---|---|---|---|
| 1” | about 165 sq ft | about 150 sq ft | about 145 sq ft | about 140 sq ft |
| 1½” | about 110 sq ft | about 100 sq ft | about 97 sq ft | about 93 sq ft |
| 2” | about 83 sq ft | about 75 sq ft | about 73 sq ft | about 70 sq ft |
| 2½” | about 66 sq ft | about 60 sq ft | about 58 sq ft | about 56 sq ft |
Flagstone is rarely a uniform thickness. A pallet sold as “1½ inch” may run from about 1 to 2 inches, so use the average. Many yards will tell you the coverage per ton they see with their own stone; if they do, divide your area by their number for a quick check.
Which stone is it?
Flagstone is a use, not a rock type. Most of it is one of these:
| Sold as | Usually | About lb per cu ft |
|---|---|---|
| Arizona, Pennsylvania or Tennessee flagstone, bluestone | Sandstone | 145 (137–162) |
| Limestone flagging | Limestone | 160 (145–170) |
| Quartzite flagstone | Quartzite | 165 (162–168) |
| Slate flagging | Slate | 172 (168–175) |
Bluestone is a dense sandstone, so it sits toward the heavy end of the sandstone range. If you know your stone is dense, choose the next heavier option for a conservative order.
Joints, waste and the 10% allowance
Two things pull the quantity in opposite directions:
- Joints save stone. With pieces about 18 inches across, ½ inch joints take up about 5% of the area, 1 inch joints about 10% and 2 inch joints (for creeping plants or gravel) about 19%.
- Trimming wastes stone. Irregular flagstone has to be fitted, and some pieces break or are too thin to use.
For tight joints, 10% extra is a sensible default. For wide planted joints you can lower it to around 5%. For a first-time layout with lots of cutting, consider 15%.
The base under flagstone
Dry-laid flagstone usually sits on a few inches of compacted crushed rock with an inch or so of sand or stone screenings on top for bedding each piece. Mortared flagstone goes on a concrete slab. The base material is a separate order; your supplier can advise on depth for your soil and climate.
Stepping-stone paths
For a path of stepping stones you count stones rather than tons. Space them to a comfortable stride, often about 24 inches center to center, then divide the path length by the spacing and add one. Weigh an individual stone with the granite weight calculator set to Block or slab and the right stone type.
Flagstone thickness, density and the share lost to trimming vary with the quarry and the pallet. Use this as an ordering estimate and confirm coverage per ton with your stone yard.
Questions people ask
- How much flagstone do I need for a patio?
- Multiply the patio area in square feet by the stone thickness in feet to get cubic feet, multiply by the stone's density (about 145 pounds per cubic foot for sandstone), divide by 2,000 for tons and add about 10 percent. A 12 × 12 ft patio in 1½ inch sandstone needs about 1.31 tons, or 1.44 tons with the extra.
- How many square feet does a ton of flagstone cover?
- By the solid-stone math, a ton of 1½ inch sandstone flagstone covers about 110 square feet, 1 inch covers about 165 and 2 inch about 83. Denser stones such as quartzite and slate cover a little less per ton.
- What thickness of flagstone should I use?
- About 1 inch is used when the stone is mortared onto a concrete slab. For dry-laid patios on sand or gravel, 1½ to 2 inches is common so the pieces don't crack under foot traffic. Stepping stones set in soil or lawn are often 2 to 3 inches.
- Do wide joints mean I need less flagstone?
- Yes, a little. With pieces about 18 inches across, 1 inch joints leave about 10 percent of the area as joint and 2 inch joints about 19 percent. Irregular stone also needs trimming, which uses some of that back.