Guide · 6 min read
Calculating fastener tonnage for a project bill of quantities
How to convert a drawing or bill of quantities into the tonnage and piece counts a bulk supplier can price, with practical allowances for waste and loss.
Work from connections, not from guesswork
Structural fastener quantities come out of the connection schedule: number of connections, bolts per connection, plus one nut and one or two washers per bolt. Count the connections from the drawings, multiply, then add the allowance below. Estimating fasteners as a percentage of steel tonnage is a habit worth breaking — it is accurate only until the connection detail changes.
Use weight per thousand
Every fastener size has a nominal weight per thousand pieces. Convert your piece count with it and you have the tonnage a supplier quotes against, and the freight weight your logistics team needs.
- — M12 x 50 hex bolt, grade 8.8 — roughly 70 kg per thousand
- — M16 x 60 hex bolt — roughly 150 kg per thousand
- — M20 x 80 hex bolt — roughly 290 kg per thousand
- — M16 hex nut — roughly 33 kg per thousand
- — M16 flat washer — roughly 12 kg per thousand
- — 12-14 x 65 roofing screw — roughly 22 kg per thousand
Add a real allowance
Fasteners are lost, dropped, over-driven and stolen. On structural bolting, five percent over the calculated count is a working allowance; on roofing screws driven by hand at height, ten percent is closer to reality, and on small screws consumed by a production line, fifteen percent is not unusual.
The allowance is cheap. Running out of an M16 bolt size mid-erection and air-freighting a carton is not.
Roofing and cladding: work per square metre
Roofing fixings are counted per square metre of sheet, driven by profile and purlin spacing. A common IBR or corrugated roof runs roughly six to eight fixings per square metre including side-lap stitching. Multiply by roof area, add the side-lap and flashing screws separately, then apply the waste allowance.
Round to packing quantities
Suppliers ship in cartons, kegs and bulk bags. Ordering 7,350 screws means paying for 7,500 and receiving a broken carton. Round up to the packing quantity, note it in your bill of quantities, and the arriving pallet will match the paperwork.