How it works
The calculator divides the area by the coverage per ton for the chosen thickness and adds the extra; the joint and bed volumes are the area times their share and depth.
Area = Length × Width
Flagstone (tons) = Area × (1 + Extra ÷ 100) ÷ Coverage per ton
Joint fill = Area × Joint share × Joint depth
Setting bed = Area × Bed depth
The coverage per ton comes from a stone yard’s table, not from the stone’s density: irregular flagstone varies in thickness from piece to piece, and the yards’ figures describe what a ton typically covers when laid. Valley City Supply states the method directly – total square feet divided by the coverage for the stone’s thickness gives the tons.
Coverage by thickness
Valley City Supply lists irregular flagstone at 250 sq ft per ton for ½–¾ in, 150 for 1 in, 100 for 1½ in, 80 for 2 in and 40 for 3–4 in. Other yards publish different figures – Classic Rock Stone Yard gives roughly 120 sq ft per ton for 1 in and thinner, 100 for 1½ in and 80 for 2 in. The difference comes from the stone itself, so the presets are a starting point and your yard’s figure is better.
Joints and setting bed
Flagstone laid dry sits in a bed of sand or fine gravel, and the joints between the stones are filled with the same or with stone dust, gravel or plants. The joint volume depends on how tightly the stones fit, which only a sample layout shows; enter your estimate of the joint share. The bed depth is your installer’s or product’s figure – the compacted base below it is a separate layer.
Tons and tonnes
With US units the result is in short tons of 2,000 lb, the unit US yards sell by. With metric units it is in tonnes of 1,000 kg, and the presets are converted to square meters per tonne – 100 sq ft per short ton is about 10.2 m² per tonne.
Worked example
Inputs
- Enter the area as
- Length × width
- Length
- 12 ft
- Width
- 10 ft
- Flagstone thickness
- 1½ in (100 sq ft per ton)
- Extra
- 5 %
- Joint share of the area
- 10 %
- Joint depth
- 1.5 in
- Setting bed depth
- 2 in
Result
- Area
- 120 sq ft
- Coverage per ton used
- 100 sq ft/ton
- Flagstone
- 1.26 short tons (2,520 lb)
- Joint fill
- 1.5 cu ft (0.056 cu yd)
- Setting bed material
- 0.741 cu yd (20 cu ft)
Common mistakes
- Using one coverage figure for every thickness. A ton of 2 in stone covers about half the area of a ton of 1 in stone.
- Mixing short tons and metric tonnes. US yards sell by the 2,000 lb ton; a tonne is 1,000 kg (about 2,205 lb). The metric presets are converted from sq ft per short ton.
- Forgetting the joints and the bed. The stone covers the surface, but the joints and the setting bed need sand, stone dust or fine gravel on top of the stone order.
- Ordering exactly the area. Irregular pieces leave gaps and need trimming; stone yards suggest buying some extra.
- Building on a soft base. The compacted base under the setting bed is a separate layer; size it with the base or gravel calculator.
Assumptions and limits
- The area is a rectangle or the total you enter. Curves and steps are added up beforehand.
- Tons = area × (1 + extra) ÷ coverage per ton. The full area is used; whether a yard’s coverage figure allows for joints is not stated, so a share of joints is not subtracted from the stone.
- The presets are Valley City Supply’s coverage table for irregular flagstone (250, 150, 100 and 80 sq ft per ton for ½–¾, 1, 1½ and 2 in). Metric values are the same figures converted to square meters per tonne.
- Joint fill = area × joint share × joint depth; setting bed = area × bed depth. Compaction and settling are not added.
- Pieces, mortar and the compacted base are not counted.
Frequently asked questions
How much does a ton of flagstone cover?
It depends on the thickness. Valley City Supply’s table gives about 250 sq ft per ton for ½–¾ in stone, 150 for 1 in, 100 for 1½ in and 80 for 2 in. Classic Rock Stone Yard gives roughly 120, 100 and 80 sq ft per ton for 1 in, 1½ in and 2 in. Ask your yard for the stone you are buying.
How many tons of flagstone do I need for a 12 × 12 patio?
144 sq ft of 1½ in stone at 100 sq ft per ton is 1.44 tons, or about 1.51 tons with 5 % extra. Thinner stone needs less weight, thicker stone more.
How much sand do I need between the flagstones?
That depends on how wide the joints are. Estimate what share of the surface is joints, enter it with the joint depth, and the calculator gives the volume – for example, 10 % of 120 sq ft filled 1½ in deep is 1.5 cu ft.
Is flagstone sold by the ton or by the square foot?
Irregular (random) flagstone is usually sold by weight, and square-cut flagstone by the square foot, as Valley City Supply explains. This calculator gives the weight; for cut stone, buy the area plus your extra.
Sources
- Natural Landscape Stone Calculating and Estimating — Valley City Supply. Irregular flagstone sold by weight, "Total SQFT (÷) Coverage (based on flagstone thickness) =# of Tons"; sq ft per ton: 1/2–3/4 in 250, 1 in 150, 1 1/2 in 100, 2 in 80, 3–4 in 40; "We recommend purchasing 5% more" (waste default).
- Flagstone – Estimated Coverage — Classic Rock Stone Yard. "1″ thick and less flagstone will cover roughly 120 square feet per ton", 1.5 in roughly 100, 2 in roughly 80 – "These are estimated coverages" (cited to show how yards differ).
- NIST Handbook 44, Appendix C – General Tables of Units of Measurement — National Institute of Standards and Technology. Exact unit definitions: 1 in = 2.54 cm, 1 yd³ = 27 ft³, 1 US gal = 231 in³, 1 lb = 0.45359237 kg.
Method reviewed November 6, 2026