Rebar Quantity Calculation: From Bar Diameter to Tonnes

Rebar quantity calculation converts the reinforcement shown on drawings into a weight, usually in tonnes, by measuring the total bar length for each diameter and multiplying it by the mass per metre of that diameter. The length must include laps, hooks and bends, not only the straight dimension of the concrete element. Adding up every diameter gives the rebar line of the bill of quantities.

How do you calculate the weight of rebar?

The weight of rebar is calculated as total bar length multiplied by mass per metre, and the mass per metre depends only on the bar diameter. The mass per metre is the bar's cross-sectional area multiplied by the density of steel, 7,850 kg/m³.

The exact formula is:

mass per metre (kg/m) = π × d² / 4 × 7,850, with d in metres.

With d in millimetres, it simplifies to a figure that is easy to work in your head: mass per metre ≈ d² / 162. For a Ø16 bar, 16² / 162 = 256 / 162 ≈ 1.58 kg/m, which matches the tabulated 1.578 kg/m.

Applied to a full set of drawings, the process looks like this:

1. Identify every bar mark or reinforcement callout and its diameter.
2. Determine the cut length of one bar, including bends, hooks and any lap.
3. Determine the number of bars, either from the callout or from spacing (distributed length ÷ spacing, rounded up, plus one).
4. Add up the total length for each diameter.
5. Multiply each total by its mass per metre and add the results.
6. Divide by 1,000 to convert kilograms to tonnes.

What is the weight per metre of each rebar diameter?

The weight per metre of rebar ranges from 0.222 kg/m for Ø6 to 6.313 kg/m for Ø32, based on a steel density of 7,850 kg/m³. These are nominal values: actual bars vary slightly within manufacturing tolerances, but nominal mass is the normal basis for measurement.

Because mass scales with the square of the diameter, doubling the diameter quadruples the weight. Ø16 at 1.578 kg/m weighs four times as much per metre as Ø8 at 0.395 kg/m, and a single wrong diameter in a callout can shift a line far more than a wrong length.

Why is rebar billed by weight and not by running metre?

Rebar is billed by weight because the price of reinforcing steel, its transport and much of its handling follow mass rather than length. A running metre of Ø32 contains about sixteen times as much steel as a running metre of Ø8, so a single price per metre would be meaningless without stating the diameter.

Weight also allows one rate to cover a mix of diameters. Some contracts split rates by diameter range or element type, so check the measurement rules before pricing. Length is what you measure; weight is what you bill.

How do laps, hooks and bends affect the rebar quantity?

Laps, hooks and bends add length to each bar beyond the straight dimension of the concrete element, and that extra length is part of the rebar quantity. Leaving them out is one of the most common reasons a rebar takeoff comes in light.

Laps

A lap is the overlap where two bars are joined so that force transfers from one to the other. Bars are supplied in limited stock lengths, so any run longer than the stock length needs laps. Lap length is usually specified as a multiple of the bar diameter, set by the design code and the drawing notes, and it varies with concrete strength, bar position and stress level.

Illustrative example: a 30 m run of Ø16 built from 12 m stock bars needs two laps. With an assumed lap length of 50 diameters (0.8 m), the run requires 30 + 2 × 0.8 = 31.6 m of bar, not 30 m.

Hooks and bends

Hooks at bar ends and bends in stirrups and links add length defined by the bending rules and the bar diameter. Stirrups are especially sensitive: they are numerous and each carries two hook allowances, so a small error per stirrup is multiplied many times over.

Anchorage and starter bars

Anchorage into supports and starter bars projecting from footings into columns usually appear only in sections or typical details, so a takeoff based on plans alone tends to miss them.

Illustrative example: how much rebar is in a 10 m beam?

Illustrative example: a single beam, 10 m long, 300 mm wide and 600 mm deep, with invented round reinforcement chosen only to demonstrate the method. A real design depends on loads, the design code and the engineer's detailing.

Assumptions:

• Bottom bars: 4 Ø20, each 10.6 m long (10.0 m plus a 0.3 m bend at each end).
• Top bars: 2 Ø16, each 10.6 m long.
• Stirrups: Ø8 at 200 mm spacing. With 25 mm cover, the stirrup perimeter is 2 × (250 + 550) = 1,600 mm, plus an assumed 160 mm for two hooks, giving a cut length of 1.76 m.
• Number of stirrups: 10.0 m ÷ 0.2 m + 1 = 51.

The beam contains about 173.5 kg of rebar, or 0.173 t. Two details in the result are worth noticing:

• The stirrups make up more than half of the total length (89.76 of 153.36 m) but only about a fifth of the weight. Measuring "metres of rebar" without separating diameters would badly misrepresent the steel.
• Leaving out the hook allowance would remove 51 × 0.16 m = 8.16 m of Ø8, or about 3.2 kg, roughly 9 percent of the stirrup weight. On one beam that is small; across every beam of a building it is not.

If the same beam is drawn on the floor plan and again on a beam detail sheet and both are measured, the bill shows about 347 kg for a beam that holds 173 kg.

Where do rebar quantities go wrong in drawings?

Rebar quantities go wrong mainly where reinforcement information is split across sheets: bar schedules, typical details, sections and general notes. A takeoff that reads only the plans misses information that is defined once and applies everywhere.

• Typical details, such as a typical slab edge or a typical column starter, apply to many locations but are drawn only once.
• General notes may define default cover, lap lengths or minimum reinforcement that never appears on the plan itself.
• Bar schedules and plan callouts can contradict each other, or the same bars can be counted once from each.
• Additional bars around openings and over supports are drawn in small local zones that are easy to overlook.

Each of these situations calls for a question, not a default value; see hidden assumptions in a bill of quantities and the guide to the automated bill of quantities. Quantap measures rebar by weight in tonnes, and when a diameter, spacing or detail is unclear or contradictory it asks through Quick Check, then updates the quantities. Every line can be checked back against the drawing, as the FAQ explains. For the concrete side of the same elements, see concrete quantity calculation.

Summary

Rebar quantity is total bar length per diameter multiplied by mass per metre, converted to tonnes. The mass per metre follows from the diameter alone (about d²/162 kg/m), so the hard part is getting lengths and counts right, including laps, hooks, bends and starter bars. Rebar is billed by weight because steel cost follows mass, which makes diameter errors more expensive than length errors. Most mistakes come from information split across schedules, sections, details and notes.