About Rebar Weight Calculator
The rebar weight calculator works out the mass of reinforcement steel from a bar bending schedule. For each row you enter the bar diameter, the length of one bar, and the number of bars; the tool computes the unit mass per metre, the mass of each row, subtotals grouped by bar size, and the grand total in kilograms and tonnes.
Unit mass per metre comes straight from the cross-sectional area and steel density: m = (pi/4 x d^2) x 7850 / 1,000,000 kg/m, where d is the nominal diameter in millimetres. This gives the familiar values of 0.888 kg/m for a 12 mm bar, 1.579 kg/m for 16 mm, 2.466 kg/m for 20 mm, and 3.854 kg/m for 25 mm. An optional lap and wastage allowance adds a percentage to cover laps, off-cuts, and tying.
How It Works
- Add a row to the schedule for each distinct bar size and length.
- Enter the nominal diameter (mm), the length of one bar (m), and the quantity.
- Optionally set a lap / wastage percentage and the steel density.
- The calculator returns unit mass per metre, subtotals by size, and the grand total in kg and tonnes.
Worked Example
A schedule has ten 12 mm bars each 6 m long and four 20 mm bars each 6 m long. The 12 mm unit mass is pi/4 x 12^2 x 7850 / 1e6 = 0.888 kg/m, so the 12 mm bars weigh 0.888 x 6 x 10 = 53.3 kg. The 20 mm unit mass is 2.466 kg/m, so the 20 mm bars weigh 2.466 x 6 x 4 = 59.2 kg. The grand total is 53.3 + 59.2 = 112.5 kg, or 0.112 tonnes, before any lap allowance.
Formulas
- Unit mass per metre
m = (pi / 4 * d^2) * rho / 1e6- Rule-of-thumb unit mass
m ~= 0.00617 * d^2- Entry mass
M = m * L * n- Total mass with lap allowance
M_total = sum(M) * (1 + lap/100) ; tonnes = M_total / 1000
Standards & References
- BS 8666 nominal bar masses
- Steel density 7850 kg/m^3
- Standard bar sizes 8, 10, 12, 16, 20, 25, 32, 40 mm
Frequently Asked Questions
What is the weight of a 12 mm rebar per metre?
A 12 mm bar has a unit mass of about 0.888 kg/m, from m = pi/4 x 12^2 x 7850 / 1,000,000. So a 6 m bar weighs roughly 5.33 kg. The same formula gives 1.579 kg/m for 16 mm, 2.466 kg/m for 20 mm, and 3.854 kg/m for 25 mm.
How is rebar unit weight calculated?
Multiply the cross-sectional area (pi/4 x diameter squared) by the steel density of 7850 kg/m^3. With the diameter in millimetres this reduces to m = 0.006165 x d^2 kg/m, often quoted as the d^2 / 162 rule when d is in mm and length in metres.
Should I add a lap allowance?
Yes, if your schedule lists net member lengths rather than cut lengths. Laps, end hooks, off-cuts, and tying wire typically add 3 to 10 percent. Use the lap / wastage field to apply a percentage to the whole order for procurement.
Does bar grade change the weight?
No. Mild steel and high-yield (deformed) bars have essentially the same density of 7850 kg/m^3, so the mass per metre depends only on the nominal diameter. Grade affects strength, not weight.