Site engineers and estimators use a universal derivation to find the unit weight of circular steel reinforcement (TMT) bars. The standard formula is D² / 162.28, where 'D' is the diameter of the bar in millimeters.
Where does 162.28 come from? It is derived from the standard density of steel, which is 7,850 kg/m³. By multiplying the cross-sectional area of the bar (πr²) by the density and a length of 1 meter, the calculation simplifies perfectly to the D²/162.28 constant.
Standard TMT Steel Weight Chart (IS 456)
For quick manual checking on site, here are the standard weights per meter for common bar diameters used in structural design:
Bar Dia (mm)
Weight (kg/m)
Per 12 m Rod
Rods / Tonne
6 mm
0.222 kg
2.66 kg
376
8 mm
0.394 kg
4.73 kg
211
10 mm
0.616 kg
7.39 kg
135
12 mm
0.887 kg
10.65 kg
94
16 mm
1.578 kg
18.93 kg
53
20 mm
2.465 kg
29.58 kg
34
25 mm
3.851 kg
46.22 kg
22
28 mm
4.831 kg
57.97 kg
17
32 mm
6.310 kg
75.72 kg
13
36 mm
7.986 kg
95.83 kg
10
40 mm
9.860 kg
118.31 kg
8
Worked Example: Steel for a One-Way Slab
Take a 5 m × 4 m one-way slab reinforced with 10 mm main bars at 150 mm centres spanning the 4 m direction, and 8 mm distribution bars at 200 mm centres running the other way. Working bar mark by bar mark is what keeps this reliable — the most common estimating error is adding lengths across different diameters before converting to weight.
Bar Mark
Count
Cut Length
Total Length
Weight
10 mm main
34
4.20 m
142.80 m
87.96 kg
8 mm distribution
21
5.20 m
109.20 m
43.02 kg
Total, before wastage
130.98 kg
With 5% cutting and lapping allowance
137.53 kg
Counts come from the slab geometry: 4,000 mm ÷ 150 mm gives 27 spaces, so 28 main bars, plus cover adjustments and the extra bars at each free edge bring the practical count to 34. Cut lengths include cover deductions and standard end hooks. Multiply each total length by the unit weight from the chart above (0.616 kg/m for 10 mm, 0.394 kg/m for 8 mm) and the slab needs roughly 138 kg, or about 0.14 tonnes.
Lap Length, Cover and Where Estimates Go Wrong
A bar longer than the 12 m stock length has to be lapped, and the lap is real steel that has to be paid for. Lap length is commonly taken as 50 times the bar diameter for bars in tension — 600 mm for a 12 mm bar, 1,000 mm for a 20 mm bar — though the governing figure comes from the development length provisions of IS 456 and depends on concrete grade, steel grade and whether the bar is in tension or compression. Ignoring laps entirely is one of the two reliable ways to under-order steel.
The other is measuring to the outside of the member instead of deducting cover. Clear cover is typically 20 mm for slabs, 25 mm for beams, 40 mm for columns and 50 mm for footings against earth. On a small slab that is a rounding error; across a whole structure it is a consistent over-measurement on every single bar mark.
Minimizing Wastage in BOQ Estimations
Steel is typically the highest-cost material on any structural project. While standard cutting wastage is usually restricted to 3% to 5%, inaccurate initial estimates or poorly optimized Bar Bending Schedules can severely impact contract margins.
For large-scale government contracts and multi-story structures, manual calculation becomes a liability. Moving your estimation to dedicated software ensures quantities are exact and dynamically linked to prevailing market rates.
Frequently asked questions
What is the formula for the weight of a steel bar?
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The unit weight of a round reinforcement bar is D² / 162.28 kilograms per metre, where D is the nominal diameter in millimetres. Multiply that by the total length in metres to get the total weight. For a 12 mm bar: 12² / 162.28 = 0.888 kg per metre.
Where does the constant 162 come from?
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It is derived, not arbitrary. The cross-sectional area of a bar is πD²/4 square millimetres. Multiplying by the density of steel (7,850 kg/m³) and converting units reduces the whole expression to D²/162.28. Many site engineers round it to 162, which overstates the weight by about 0.17% — negligible for an indent, but use 162.28 when you are reconciling against a supplier’s weighbridge slip.
How much does one 12 metre TMT rod weigh?
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Multiply the unit weight by 12. An 8 mm rod weighs 4.74 kg, 10 mm weighs 7.40 kg, 12 mm weighs 10.65 kg, 16 mm weighs 18.94 kg, 20 mm weighs 29.59 kg and 25 mm weighs 46.24 kg. TMT bars in India are supplied in 12 m standard lengths, so rod count matters as much as tonnage when you place an order.
Does the weight change with the grade — Fe415, Fe500 or Fe550?
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No. Grade describes yield strength, not density. An Fe415 and an Fe550 bar of the same diameter weigh exactly the same per metre. Grade changes how much steel your design needs, not what a given bar weighs.
How much wastage should I allow for reinforcement?
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Standard practice is 3% to 5% for cutting and lapping on normal building work. Complex bar bending schedules, heavy cranking or a lot of short members can push this higher. Because steel is usually the single largest material cost on a structural job, an unchecked wastage assumption moves the bottom line more than almost any other estimating decision.
How do I calculate the steel required for a slab?
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Work bar mark by bar mark rather than in bulk. For each mark, take the cut length from the bar bending schedule, multiply by the number of bars to get the running length for that diameter, convert to weight with D²/162, then total across all diameters. Summing lengths across different diameters first is the most common source of error.
Is nominal diameter the same as actual diameter?
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For weight calculation, use the nominal diameter — that is what IS 1786 tolerances and the D²/162 formula are built around. The ribs on a TMT bar make the measured outside diameter slightly larger, but the nominal size is what the mill supplies against and what your BOQ should quote.
