Rebar calculator and steel bar weight (D²/162)

How to use the rebar calculator

Choose Weight of bars to weigh a list of bars, or Mesh for a slab or footing to count bars at a spacing. Enter sizes and lengths from the drawings; the result is the steel to order in kg, tonnes or lb.

1. Pick Region presets at the top. The United States preset uses bar numbers (#3, #4…) and 20 ft stock bars; the others use millimetre sizes and 12 m bars. Any bar size can be chosen in either.
2. Weight of bars: add a line per bar size with the length of each bar and how many there are.
3. Mesh: enter the slab or footing size, the edge cover, and the bar size and spacing in each direction. Choose two layers for a top and bottom mesh.
4. Open Stock length, laps and wastage to match the bars sold near you and the laps on your drawings.
5. Enter your price per kg or per lb under Cost to see the steel cost.

Steel bar weight formula: D²/162

The weight of a steel bar in kg per metre is its diameter in mm squared divided by 162.2, usually rounded to D² ÷ 162. A 12 mm bar weighs 12² ÷ 162.2 = 0.888 kg/m, so a 12 m length weighs 10.65 kg.

kg per metre = density × area = 7,850 kg/m³ × π/4 × (D ÷ 1,000)²
             = 0.0061654 × D²  =  D² ÷ 162.2
Total weight = kg per metre × length × number of bars

The 7,850 kg/m³ density is the one IS 1786 uses for nominal mass, and the formula reproduces its table to within rounding. Dividing by 162 instead of 162.2 overstates the weight by about 0.1%. For weight per foot, multiply kg per metre by 0.3048.

US rebar sizes and weights

US bar numbers are the diameter in eighths of an inch up to #8: a #4 bar is ½ in across and weighs 0.668 lb per foot, so a 20 ft bar weighs 13.4 lb.

Weights are the ASTM A615 nominal values, as republished in Caltrans Bridge Design Details 13.1. Above #8 the diameters are no longer whole eighths of an inch: #9, for example, is 1.128 in.

Rebar for a slab: bar count, spacing and total length

Bars in one direction = (slab width − 2 × cover) ÷ spacing, rounded down, + 1; each bar is the slab length minus 2 × cover. A 5 × 4 m slab with 10 mm bars at 150 mm both ways and 25 mm cover needs 27 + 34 bars, 268 m and 165.2 kg of steel.

Bars along the length  = ⌊(W − 2c) ÷ s₁⌋ + 1,  each L − 2c long
Bars across the width  = ⌊(L − 2c) ÷ s₂⌋ + 1,  each W − 2c long
Weight = Σ (number × length × kg per metre) × (1 + wastage)

1. Bars along the 5 m length: (4 − 0.05) ÷ 0.15 = 26.3, rounded down to 26, + 1 = 27 bars, each 4.95 m: 133.7 m.
2. Bars across the 4 m width: (5 − 0.05) ÷ 0.15 = 33, + 1 = 34 bars, each 3.95 m: 134.3 m.
3. Total 268 m × 0.617 kg/m (10 mm) = 165.2 kg.
4. From 12 m stock bars: 268 ÷ 12 = 22.3, so order 23 bars before wastage.

In US units, a 20 × 15 ft slab with #4 bars at 12 in each way, 2 in from the edges, takes 15 + 20 bars, 588 ft of bar and 393 lb of steel, or 30 bars of 20 ft. These cover values are only for the example; cover is set by the engineer.

Sariya sizes in Pakistan: sooter to mm

Sariya (steel bar) sizes are sometimes given in sooter or sutar, which count eighths of an inch, the same idea as US bar numbers. 3 sooter is ⅜ in, 9.5 mm, sold as a 10 mm bar.

Zameen's saria rate guide lists the same pairs: 3 sutar as 10 mm, 4 sutar as 12 mm, 5–6 sutar as 16–20 mm and 7–8 sutar as 22–25 mm. Steel is sold by weight, so the useful number is kg per foot or per metre. If the bar or invoice gives a size in millimetres, weigh it at that size.

Laps, stock lengths and wastage

Where a run is longer than a stock bar, two bars overlap (lap) to carry the force across. The calculator adds a lap of 50 bar diameters by default, a round figure rather than a code value; your drawings set the real one.

Under IS 456 clause 26.2.5.1, a tension lap is the development length Ld or 30 diameters, whichever is more. Worked out from clause 26.2.1, Ld is about 47 diameters for Fe415 bars in M20 concrete and 57 for Fe500 in M20, which is why both are presets. In US practice the CRSI notes that lap length depends on concrete strength, bar grade, size and spacing; 48 diameters for Grade 60 bars is a common rule of thumb, not a code value.

Bars come in fixed stock lengths: Amreli Steels in Pakistan quotes 12 m as the standard length for its Maxima bars, and Gerdau gives typical US mill lengths of 20, 30, 40 and 60 ft. The calculator starts at 20 ft for the United States preset and 12 m elsewhere; set yours under Stock length. Offcuts, bends and hooks are why a wastage allowance is added on top.

Assumptions and limits

The calculator measures and weighs bars. It doesn't design reinforcement: it can't tell you which bar size, spacing or cover a slab needs.

• The mesh counts straight bars in a rectangle. Hooks, cranked bars, extra bars at openings, chairs and starter bars are extra lines.
• Weights are nominal. Actual bars can be slightly heavier or lighter, within the mass tolerance the bar standard allows.
• Steel per square foot or per square metre of slab varies with span and load. Take it from the bar bending schedule rather than a thumb rule.
• Costs appear only for prices you enter.

Pouring the slab too? The concrete calculator gives the volume, and its Next step link opens this calculator with the slab size filled in.

Sources

Constants and limits come from these standards, manufacturers' data and trade bodies. Where practice varies, the calculator lets you change the value.

• IS 1786:2008 High strength deformed steel bars (nominal mass per metre)Bureau of Indian Standards · 2008
• Bridge Design Details 13.1: reinforcement (ASTM A615 bar sizes and weights)California Department of Transportation · September 2025
• IS 456:2000 Plain and Reinforced Concrete: Code of Practice (Table 9, nominal mixes)Bureau of Indian Standards · 2000, reaffirmed
• Lap splicesConcrete Reinforcing Steel Institute · current
• Saria rates in Pakistan (sutar sizes and bar diameters)Zameen.com · January 2024

Found a value that doesn’t match your local practice? Tell us and we’ll review it.