Rebar Calculator
Work out the rebar for a slab, footing, column or beam: how many bars each way, how many stock bars to buy, what the steel weighs and what it costs.
Often 150–300 mm (6–12 in). Your drawing wins.
Gap from the concrete edge to the ends of the bars.
For hooks or bends. 0 for straight bars.
2 for a top and a bottom mat.
Leave blank for no cross bars.
Concrete cast against the ground usually gets 75 mm (3 in).
10 × diameter is a common allowance for a 135° hook.
For a lap into the next floor or a bend into the footing.
40 × 12 mm = 480 mm. Typical rule of thumb; your drawing wins.
For example 12 matching columns.
Each way. About 1 m (3 ft) is typical.
Steel prices change week to week. Use your dealer’s price; cost is on the bars you buy.
Bar bending schedule
Cut length includes laps and hooks. Weights use the length you need, not the offcuts.
How your rebar is worked out
Count the bars each way, add up their length with laps, cut it from stock bars, then weigh it.
- 1
Count the bars
(4 − 0.1) ÷ 0.2 = 19.5→ 19 + 1 = 20 barsand 30 the other way
- 2
Add up the length
20 × 5.9 m+ 30 × 3.9 m= 235 m of bar
- 3
Bars to buy
5.9 m: 2 per bar3.9 m: 3 per bar20 bars + 5% extra= 21 bars of 12 m
- 4
Weigh the steel
12² ÷ 162 = 0.889 kg/m235 m × 0.889= 208.9 kg
Read more: the formula, a worked example, bar weights, cover and laps
Rebar is counted bar by bar. Find how many bars fit each way at your spacing, how long each one is once the cover is taken off and any laps are added, how many stock bars you need to cut them from, and what the steel weighs.
The formula
bars in a row = (width − 2 × cover) ÷ spacing, rounded down, + 1
length of each bar = length − 2 × cover + hooks at the ends
lap = lap multiple × bar diameter, added each time a run is longer than one stock bar
pieces in a run = (run − lap) ÷ (stock length − lap), rounded up
total length = number of bars × cut length
stock bars = pieces cut from bars of the stock length, + extra %
metric weight (kg/m) = diameter in mm² ÷ 162 · US weight = lb/ft from ASTM A615
tie length = 2 × (width − 2 × cover + depth − 2 × cover) + 2 hooks − bend deductions
Worked example
A slab 6 m long and 4 m wide with 12 mm bars at 200 mm both ways, 50 mm cover and 12 m bars. Lengthwise bars: (4 − 2 × 0.05) ÷ 0.2 = 19.5, rounded down to 19, plus 1 = 20 bars, each 6 − 0.1 = 5.9 m long. Widthwise bars: (6 − 0.1) ÷ 0.2 = 29.5, so 30 bars, each 3.9 m. Total length = 20 × 5.9 + 30 × 3.9 = 118 + 117 = 235 m. No bar is longer than 12 m, so there are no laps. Two 5.9 m pieces come out of one 12 m bar (10 bars) and three 3.9 m pieces come out of another (10 bars), so 20 bars, plus 5% extra = 21, rounded up: 21 bars of 12 m. A 12 mm bar weighs 12² ÷ 162 = 0.889 kg/m, so the steel needed weighs 235 × 0.889 = 208.9 kg and the 21 bars you buy weigh 21 × 12 × 0.889 = 224 kg. There are 20 × 30 = 600 crossings; at 30 cm of wire each that is 180 m of tie wire.
With laps (US example): a strip footing 40 ft long with 3 in cover needs runs of 39.5 ft, but the bars are 20 ft. A #4 bar is ½ in thick, so a 40 × diameter lap is 20 in (1.67 ft). Pieces = (39.5 − 1.67) ÷ (20 − 1.67) = 2.06, rounded up to 3, and each run is 39.5 + 2 × 1.67 = 42.83 ft long.
Bar sizes and weights
| Metric bar | kg per m | One 12 m bar | US bar | lb per ft |
|---|---|---|---|---|
| 8 mm | 0.395 | 4.74 kg | #3 (⅜ in) | 0.376 |
| 10 mm | 0.617 | 7.41 kg | #4 (½ in) | 0.668 |
| 12 mm | 0.889 | 10.67 kg | #5 (⅝ in) | 1.043 |
| 16 mm | 1.580 | 18.96 kg | #6 (¾ in) | 1.502 |
| 20 mm | 2.469 | 29.63 kg | #7 (⅞ in) | 2.044 |
| 25 mm | 3.858 | 46.30 kg | #8 (1 in) | 2.670 |
| 32 mm | 6.321 | 75.85 kg |
Metric weights use d² ÷ 162, which comes from steel at 7,850 kg/m³ (π ÷ 4 × d² × 7,850 ÷ 1,000,000 = d² ÷ 162.2). US weights are the nominal weights in ASTM A615 (US), Table 1. Rolled bars can be a few percent off the nominal weight.
Cover, laps and hooks
- Cover protects the steel from rust and fire. Typical values: ACI 318 (US): 3 in (75 mm) for concrete cast against the ground, ¾ in (20 mm) for slabs inside, 1½ in (40 mm) for beams and columns. IS 456 (India): 20–30 mm for slabs depending on exposure, at least 40 mm for columns and 50 mm for footings.
- Laps: 40 × bar diameter is a common rule of thumb. Your drawing may ask for more; follow it.
- Hooks and bends: 10 × diameter for each 135° tie hook is a common allowance. Bar bending schedules often take off about 2 × diameter for each 90° bend and 3 × diameter for each 135° bend, because a bent bar stretches a little.
- Chairs hold the steel up at the right height. About one every 1 m (3 ft) each way is typical.
This is an estimate for planning and buying. It is not a structural design: bar sizes, spacing, cover and laps must come from your engineer or your local building code. Cover and lap figures checked 12 October 2026. The ASTM A615 weights and ACI 318 / IS 456 cover figures are summarised and have not yet been checked line by line against the official standards (12 October 2026).
Pouring the concrete too? The concrete calculator works out the volume, bags or ready-mix trucks for the same slab, footing or column.
Common questions
How do I work out how many rebar bars a slab needs?
Take off the cover at both edges, divide by the spacing, round down and add one bar. Do it both ways. A 6 m × 4 m slab with 50 mm cover and bars at 200 mm needs (4 − 0.1) ÷ 0.2 = 19.5, so 20 bars 5.9 m long, and 30 bars 3.9 m long the other way: 235 m of bar.
How do you calculate the weight of a steel bar?
For metric bars, weight in kg per metre = diameter in mm squared ÷ 162. A 12 mm bar is 144 ÷ 162 = 0.889 kg/m, so a 12 m bar weighs about 10.67 kg. US bars use the nominal weights in ASTM A615: #3 is 0.376 lb/ft, #4 is 0.668 lb/ft and #5 is 1.043 lb/ft.
How many 12 mm bars are in a tonne?
A 12 m length of 12 mm bar weighs 12 × 0.889 = 10.67 kg, so one tonne is 1,000 ÷ 10.67 = 93.75 bars: about 93 full bars. Dealers in India and the Gulf often sell by weight, so the calculator shows the weight of the bars you buy as well as the steel you need.
How long should a rebar lap be?
A common rule of thumb is 40 times the bar diameter: 480 mm for a 12 mm bar or 20 in for a #4 bar. Many drawings ask for more, depending on the concrete, the steel and where the lap is, so always follow your engineer. The calculator lets you set the lap and adds it every time a run is longer than one bar.
How much tie wire do I need?
Count the crossings you tie and allow about 30 cm (12 in) of wire for each. The default slab has 20 × 30 = 600 crossings, so 180 m of wire. At 1.2 mm (18 gauge) that is about 1.6 kg. Tying every other crossing halves it.
What lengths does rebar come in?
Typical stock lengths are 12 m (and sometimes 6 m or 9 m) in India, the Gulf and most metric countries, and 20 ft, 30 ft, 40 ft or 60 ft in the US. Longer bars mean fewer laps; shorter bars are easier to carry. You can also enter your own length.
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