Rebar Calculator
From slab dimensions or a bar list to the order: bars, meters, kilograms, laps and a cutting list per stock length
A reinforcement order is three numbers apart from the drawing: how many bars fit at the given spacing, how much the lap splices add when bars are longer than the stock, and how many stock bars you have to buy once every piece is cut from a 6 m or 12 m length. This calculator does all three and prints the cutting list.
For a slab, enter length, width, cover and the spacing in each direction — the number of bars in each direction is (clear dimension / spacing) rounded up, + 1, and each bar is the clear dimension in the other direction. For beams, footings or any bar list, switch to straight bars and enter count and length. Pick the bar size from the metric Ø6–Ø40 or US #3–#18 list; the weight per meter comes from the same tables as our rebar datasheets.
Any bar longer than the stock is split into pieces with a lap splice at each joint (default 40 diameters, editable — the value belongs to the design). All pieces are then packed onto stock bars with a first-fit-decreasing algorithm, full-length pieces are counted as uncut bars, and the remnant of every cut bar is listed. Try 6 m against 12 m stock on the same slab: the difference in offcuts is often the largest single saving on the order.
Frequently Asked Questions
How many rebars do I need for a slab?
Per direction: (clear dimension / spacing) rounded up, + 1 — the spacing is a maximum, so a partial space still needs a bar. A 6.0 × 4.0 m slab with 30 mm cover and Ø12 at 200 mm both ways needs 3.94 / 0.2 = 19.7 → 20 spaces + 1 = 21 bars of 5.94 m along the length and 5.94 / 0.2 = 29.7 → 30 spaces + 1 = 31 bars of 3.94 m across — 52 bars, 246.9 m, 219 kg in place.
How is rebar weight calculated?
Weight per meter = d² / 162 with d in millimeters (Ø12: 144 / 162 = 0.888 kg/m); US bars use the ASTM A615 nominal lb/ft (#4: 0.668 lb/ft = 0.994 kg/m). Multiply by the total length of all bars. The ribs are already included — the nominal diameter is defined by mass per length.
What lap length should I enter?
Whatever the design specifies. Typical tension laps are 40–50 bar diameters for common concrete grades and B500 / Grade 60 steel, longer for top bars in deep members and in seismic detailing. The calculator applies the lap only where a bar is longer than the stock bar, once per joint.
Why is the order heavier than the steel in the slab?
Two reasons: lap splices add steel wherever a bar is spliced, and offcuts are the part of each stock bar you cannot use. The stock length decides the second part — the same 6 × 4 m slab needs 21 bars of 12 m = 224 kg (2% offcuts, because two 5.94 m pieces or three 3.94 m pieces fill a bar almost exactly) but 52 bars of 6 m = 277 kg (over 20%), because a 3.94 m piece leaves a 2.06 m remnant from every 6 m bar.
Does it work with US bar sizes and 20 ft / 40 ft stock?
Yes. Choose #3–#18 as the bar size and 20 ft (6.096 m) or 40 ft (12.192 m) as the stock; dimensions stay in meters and millimeters, weights are shown in kilograms and pounds.
Related guides
- Rebar Size Chart: Diameter, Weight & Area (US #3–#18 vs Metric Ø6–Ø40)
- How to Calculate Metal Weight: Formulas and Examples