Rebar Quantity for Concrete Beams: How to Calculate

To calculate the rebar in a concrete beam, split it into bar groups (bottom bars, top bars, extra top bars over the supports and stirrups), work out the length of one bar in each group including hooks, multiply by the number of bars, then convert metres to kilograms with the mass per metre for each size. A 6 m beam with three 20M bottom bars, two 15M top bars, extra 20M top bars at each support and 10M stirrups works out to about 130 kg of steel, as the worked example below shows.

All bar sizes, spacings and cut-off lengths come from the structural drawings. The example uses illustrative values so the method is clear; it is not a design.

What to take off the beam drawings

Before counting anything, collect these from the beam elevation, the sections and the general notes:

• Beam size and length. Width, depth, clear span and the support widths, so you know the overall bar length.
• Concrete cover, from the notes or CSA A23.1. Interior beams are commonly 30 mm (Class N); other exposures need more. Our concrete cover guide has the table.
• Each bar group: number, size, length or cut-off dimension, and the hook at each end.
• Stirrups: size, shape, spacing and the length of each spacing zone.
• Laps where bars are spliced, and any side (skin) bars in deep beams.
• How many identical beams share the same mark.

Then use the mass per metre for each size:

The hook allowances are the extra length added to the detailing dimension for a standard 90° end hook, from RSIC's standard hook table. RSIC estimating practice is to add a hook only where the drawings, specification or typical details call for one, and to assume a 90° hook if the type is not stated.

The bar groups in a typical beam

A continuous beam over several supports has extra top bars at every interior support and may have bottom bars lapped or stopped at the supports. Our post on lap length and splice zones for beams and columns covers where those laps go.

Worked example: one 6 m beam

Beam: 300 mm wide by 500 mm deep, 6.0 m clear span between two 300 mm wide concrete supports, so 6.6 m out to out. 40 mm end cover. Grade 400W bar.

Mark B1, bottom bars: 3-20M continuous, 90° hook each end

• Straight detailing length: 6,600 minus 2 × 40 = 6,520 mm
• Plus two 20M hooks: 2 × 300 = 600 mm
• One bar: 7.12 m; three bars: 21.36 m
• Weight: 21.36 × 2.355 = 50.3 kg

Mark T1, top bars: 2-15M continuous, 90° hook each end

• 6,520 + 2 × 250 = 7.02 m per bar; two bars: 14.04 m
• Weight: 14.04 × 1.570 = 22.0 kg

Mark T2, extra top bars: 2-20M at each support, extending 1,800 mm past the support face (from the drawing), 90° hook into the support

• In the support: 300 minus 40 = 260 mm; into the span: 1,800 mm; hook: 300 mm
• One bar: 2.36 m; four bars (two at each end): 9.44 m
• Weight: 9.44 × 2.355 = 22.2 kg

Mark S1, stirrups: closed 10M, 135° hooks, @ 150 mm for 1,200 mm from each support, @ 250 mm in the middle

• Stirrup size: RSIC practice, when the drawings do not say otherwise, makes stirrups 80 mm smaller than the beam each way: 220 by 420 mm out to out.
• Length: 2 × (220 + 420) = 1,280 mm, plus two 135° hooks at about 100 mm each for 10M (RSIC Table 5D) = 1.48 m.
• End zones: first stirrup 50 mm from the support face, then 1,200 ÷ 150 = 8 spaces, so 9 stirrups in each end zone, 18 in total.
• Middle zone: 6,000 minus 2 × 1,250 = 3,500 mm at 250 = 14 spaces, which adds 13 stirrups between the end zones.
• Count: 18 + 13 = 31 stirrups; 31 × 1.48 = 45.9 m
• Weight: 45.9 × 0.785 = 36.0 kg

Summary

Every bar is shorter than an 18 m stock length, so no laps are needed in this beam. If ten beams share the same mark, multiply by ten.

Adding out-to-out legs and full hook allowances slightly overstates the true bent length of a stirrup, which keeps an estimate on the safe side. A detailer's bar list uses the exact developed lengths. For the stirrup calculation in detail, see how to calculate stirrup cut length.

Extra top bars: where the "formula" comes from

Searches for a "steel beam top extra bar formula" usually turn up rules of thumb such as extending top bars a quarter or a third of the span from the support. Those are shortcuts from other design traditions, not CSA requirements.

In Canada, the engineer sets bar cut-off points from the bending moment diagram, then checks them against the anchorage rules in CSA A23.3 Clause 12 (clause numbers below are from the 2004 edition; your engineer works to the edition the Ontario Building Code references). Two rules shape the result:

• Negative moment (top) bars must be anchored in or through the support, and at least one-third of the top bars at a support must extend past the point of inflection by the greatest of the effective depth, 12 bar diameters or one-sixteenth of the clear span (Clause 12.12).
• Positive moment (bottom) bars: at least one-third in simply supported members, and one-quarter in continuous members, must run into the support, at least 150 mm where the beam is cast with the support (Clause 12.11).

For a quantity calculation, you do not need to apply these rules yourself. Use the cut-off dimensions on the drawings. If the drawings do not show them, the beam is not detailed enough to order from, and the question goes back to the engineer.

Setting it up in a spreadsheet

A beam steel quantity sheet only needs one row per bar mark:

For stirrups, add a count column calculated as the zone length divided by the spacing, rounded up (the CEILING function), plus one. Total the weight by bar size at the bottom, because suppliers and fabricators work by size.

A spreadsheet like this is good for checking a bar list or comparing quotes. Order from a detailed take-off of the actual drawings.

What hand calculations usually miss

• Laps in continuous beams and in beams longer than stock length. Each lap adds a full lap length of steel.
• Steel in the joints. Bars hooked into columns, and stirrups or ties that continue through beam-column joints where the details call for them.
• Different stirrup zones. Closer spacing near supports, around point loads and at openings.
• Hanger bars and extra bars at beams that support other beams.
• Side bars in deep beams and any torsion steel shown on the sections.
• Identical beams. The single most common error is calculating one beam correctly and forgetting there are twelve.

Our general rebar estimating guide covers the same method for slabs and footings.

From beam quantities to a bar list

A hand calculation tells you roughly how much steel a beam holds. A bar list tells the shop exactly what to cut and bend. We prepare detailed take-offs from your drawings free of charge, then detail, cut, bend and tag beam bars and stirrups to match. Send your drawings for a quote, or see our rebar products.