Flagstone is natural stone split into flat, irregular slabs: sandstone, bluestone, slate, limestone or quartzite. In the United States it is sold by the ton, so the key question for a patio or walkway is how much weight of stone covers your area. This calculator gives the tonnage, the sand and gravel for the base, the joint sand and the cost.
The formula
Weight = area × thickness × stone density × (1 + waste)
Most flagstones weigh about 2,500 to 2,700 kg per m³, or roughly 155 to 170 lb per cu ft. As a rule of thumb, a ton of 1 to 1.5 in flagstone covers about 100 to 120 sq ft, and a ton of 2 in stone about 70 to 80 sq ft.
Example
A 200 sq ft patio in 1.5 in sandstone, with 10% waste for cutting and fitting, needs about 2.1 tons: roughly 103 sq ft per ton. Under it, a 1 in sand bed takes 0.62 cu yd and a 4 in compacted gravel base 2.47 cu yd. Joints take about 4 bags of polymeric sand. At $300 a ton, the stone costs about $644.
Typical base
| Use | Gravel base | Setting bed |
|---|---|---|
| Garden path, light foot traffic | 3 to 4 in | 1 in of sand |
| Patio | 4 to 6 in | 1 in of sand or stone dust |
| Driveway | 8 in or more, or a concrete slab | mortar |
Wet-laid flagstone, set in mortar on a concrete slab, is more durable in freeze-thaw climates but costs more and needs a solid slab underneath.
Laying flagstone
- Excavate to the depth of the base, bed and stone, and slope the surface about 1/8 in per foot away from the house.
- Compact the gravel in layers with a plate compactor.
- Screed the sand bed, then lay the largest stones first, fitting smaller pieces in between.
- Keep joints 1/2 to 1 in wide, check that the stones are stable and level with a rubber mallet.
- Sweep polymeric sand into the joints, then mist with water to set it.
Limits
Flagstone thickness varies from piece to piece, so the weight is an estimate. Order 10% extra for cutting, and up to 15% for complex shapes. Pallets are heavy: check delivery access and where the truck can unload.