Concrete Reinforcement Quantities: A Take-Off Method for QSs

You work out concrete reinforcement quantities by taking bar lengths and numbers from the engineer's drawings or bar bending schedules, adding cutting-length allowances for bends, laps and hooks, and converting each bar size to weight using its mass per metre. A percentage-of-concrete-volume check then tells you whether the total is plausible. This article sets out that method step by step, with a worked slab example, the formulas for common shape codes and the drawing details to confirm before you issue a figure.

Quantiflow

Turn Drawings Into Priceable Takeoffs

QuantiFlow automates NRM2-aligned quantity takeoffs and Bills of Quantities from architectural drawings, while preserving your professional judgement.

Explore QuantiFlow

TL;DR

Reinforcement is measured by weight, separately for each bar type and diameter, so every quantity starts as a count and a length.
Mass per metre in kg is the bar diameter in mm squared, divided by 162, based on a steel density of 7,850 kg/m³.
Cover, laps, hooks and bends change cutting lengths. Leaving them out is a common cause of underestimates.
Use kg of steel per m³ of concrete as a sense check on the finished total, never as the take-off itself.
Record assumptions and drawing revisions against each line so the figure can be defended later.

Start with what the drawings actually give you
Unit weights: from bar diameter to kilograms
Cutting lengths: where cover, bends, hooks and laps come in
A worked example: a ground-bearing slab
Measuring slabs, beams, columns and foundations
Using the reinforcement ratio as a sense check
Drawing checks before you issue the quantity
Common mistakes in reinforcement take-off
Where software fits
FAQ
Sources

Start with what the drawings actually give you

UK structural drawings reach you in one of three states, and each changes how you measure.

Full bar bending schedules. Bar marks, sizes, shape codes, numbers and cutting lengths are already scheduled. Your job is to check and total them by diameter.
Reinforcement shown on layout drawings only. You see sizes and spacings (for example H12 at 200 mm centres each way) but no schedule. You build the lengths yourself.
Concept or tender-stage information. Only concrete sizes and an indicative note. You are estimating, and the quantity is provisional.

The third case is common because the total quantity is usually only known once detailing is finished. The Concrete Society's guide to contractor detailing of reinforcement describes two ways UK contracts deal with this: a lump sum, where the specialist concrete contractor estimates and includes reinforcement in its tender, or remeasurement, where the contractor submits drawings and schedules for approval and the work is remeasured. Know which applies before you spend time on precision, because it decides whether your number is a final quantity or a provisional allowance.

The same guide says the consulting engineer should specify cover, lap and anchorage lengths and the positions of designed joints, rather than leaving them to the detailer. If those are missing from the drawings, raise a query rather than assuming values.

Unit weights: from bar diameter to kilograms

Reinforcement is billed by mass, so every length has to become kilograms. Steel density is taken as 7,850 kg/m³. The cross-sectional area of a round bar is π × d² ÷ 4, so mass per metre works out at roughly d² ÷ 162, with d in millimetres. The Rospower bending schedule calculator uses this same relationship and lists the following values:

Bar diameter	Mass per metre
12 mm	0.888 kg/m
16 mm	1.578 kg/m
25 mm	3.853 kg/m

For other sizes, apply the formula and keep the same number of decimal places throughout the job, so that rounding does not drift between sheets. Total mass for a bar mark is then the number of bars, multiplied by the cutting length in metres, multiplied by mass per metre.

Reinforcing steel versus reinforced concrete

These two densities are often confused, and mixing them up is an easy way to produce a wildly wrong figure.

Reinforcing steel: 7,850 kg/m³. You use this to convert bar volume or length to weight.
Reinforced concrete: a much lower figure, because most of the volume is concrete. BS EN 1991-1-1 gives plain normal-weight concrete as 24 kN/m³ and adds 1 kN/m³ for reinforced or prestressed concrete, giving 25 kN/m³, about 2,500 kg/m³. Some reference tables round reinforced concrete to 2,400 kg/m³, so state which value you used when the figure feeds a load or haulage calculation.

Neither of these numbers is used to price the reinforcement itself. Reinforced concrete density matters for loads and transport. Steel density matters for converting bars to tonnes.

Cutting lengths: where cover, bends, hooks and laps come in

A bar rarely runs the full dimension of the member. Four details decide its true length.

Cover. Cover is deducted from the structural dimensions to find the geometry of the bar inside the member. A 6 m slab with 50 mm cover at each end holds a bar of 5.9 m, not 6 m.
Bends and hooks. These add length beyond the straight dimensions, according to the shape code in BS 8666.
Laps. Where one bar is shorter than the member, the overlap with the next bar is extra steel. The length is specified by the engineer, commonly expressed as a multiple of bar diameter.
Anchorage. Bars entering supports, walls or adjoining members need an anchorage length that also adds to the total.

The Concrete Society guide is clear that failing to account for these extensions often leads to significant underestimates of total reinforcement weight. For shapes with several bends, BS 8666:2020 also requires a minimum straight length of at least 4d between curved portions so the bar can be bent on site or in the factory, which is worth remembering when a scheduled length looks too short for its shape.

Shape code formulas

For scheduled bars, cutting length is calculated from the shape code dimensions. Heaton Manufacturing's rebar weight calculator gives two common ones, where A, B and C are the dimensions on the schedule, r is the bend radius and d the bar diameter:

Shape code 11 (a bar with one bend): L = A + (B) − 0.5r − d
Shape code 21 (a bar with two bends): L = A + B + (C) − r − 2d

BS 8666:2020 is the British Standard that governs bar bending schedules, so when a supplier's schedule and your own cutting length disagree, check the shape code and the dimension convention first. Most disputes come from one dimension being read as an inside rather than outside measurement.

A worked example: a ground-bearing slab

The numbers below are illustrative, not taken from a real project. Assume a rectangular slab 6.0 m × 4.0 m × 200 mm thick, with a single layer of H12 bars at 200 mm centres each way and 50 mm cover at all edges.

Step 1: bars in the 6 m direction. These are spaced across the 4 m width. Count: 4.0 ÷ 0.2 + 1 = 21 bars. Length of each: 6.0 − 0.05 − 0.05 = 5.9 m. Total: 21 × 5.9 = 123.9 m.

Step 2: bars in the 4 m direction. These are spaced along the 6 m length. Count: 6.0 ÷ 0.2 + 1 = 31 bars. Length of each: 4.0 − 0.10 = 3.9 m. Total: 31 × 3.9 = 120.9 m.

Step 3: total length. 123.9 + 120.9 = 244.8 m.

Step 4: weight. An H12 bar weighs 0.888 kg/m, so 244.8 × 0.888 = 217.4 kg.

Slab reinforcement take-off totals and weight

Step 5: allow for laps. If the longest stock bar is shorter than the slab, add laps. As an illustration only, a lap of 40 bar diameters on a 12 mm bar is 480 mm. Take the actual lap length from the engineer's notes, count the number of laps, and add their length to the total before weighing.

Step 6: sense check. Concrete volume is 6.0 × 4.0 × 0.2 = 4.8 m³. The steel in the example, before laps, is 217.4 ÷ 4.8 = 45.3 kg/m³. That is the kind of figure you compare against comparable jobs, not a pass mark.

Notice what the example leaves out: chairs, spacers, tying wire, starter bars, edge trim and any extra top steel over supports. Whether these are measured or deemed included depends on the measurement rules your bill is written under, so check them before the sheet goes out.

Measuring slabs, beams, columns and foundations

The arithmetic is the same for every element. What changes is what to look for on the drawings.

Slabs and ground beams

Look for top and bottom layers, additional top steel over supports, trimming bars around openings, and edge details such as U-bars. Measure each layer separately by diameter. If the slab is on a ground-bearing detail, mesh may be specified instead of loose bars. Mesh is normally measured by area with laps allowed for, so keep it on a different line from bar.

Beams

Beams combine straight bars (top and bottom), links, and sometimes bent-up bars. Links are numerous and short, so small errors in cutting length multiply quickly. Calculate link length from the beam section less cover on each face, add the hook allowances from the shape code, then multiply by the number of links, which is the span divided by spacing plus one, adjusted for closer spacing near supports where the drawing shows it.

Columns

Columns carry vertical bars with laps at each lift, plus links. Check the starter bar projection from the level below, as lap length at every floor adds up across a multi-storey frame. Links usually change spacing near beam junctions, so read the elevation notes rather than applying one spacing for the full height.

Foundations

Pad and strip footings are often detailed with bars in both directions and L-shaped starters for columns or walls. The minimum cover to earth-face concrete is usually larger than for internal members, so do not reuse a slab cover value. Take the specified cover from the drawing notes.

For concrete itself, formwork and the full elemental sequence, our practical guide to concrete quantity takeoff covers how reinforcement fits alongside the other concrete items.

Using the reinforcement ratio as a sense check

The reinforcement ratio is the cross-sectional area of steel divided by the cross-sectional area of concrete. A ratio of 0.01 means the steel area is 1% of the concrete section. Because steel has a density of 7,850 kg/m³, a 1% ratio corresponds to roughly 78.5 kg of steel per m³ of concrete (0.01 × 7,850), and other ratios scale in proportion. In the slab example above, 12 mm bars at 200 mm centres give 565 mm² per metre width against 200,000 mm² of concrete, so the ratio is about 0.28%.

The Concrete Society explains that minimum reinforcement for crack control is calculated as f_ctm ÷ f_yk, the mean tensile strength of the concrete over the characteristic strength of the reinforcement. This is a design requirement, so use it to understand why a member carries the steel it does, not as a take-off shortcut.

Practical rules for using ratios:

Compare like with like. Slabs, beams, columns and footings sit at different ratios. Only compare a total against the same element type.
Use ratios to catch large errors. A missing bar layer or a decimal slip will show up. A ratio will not reveal a smaller omission such as starter bars.
Do not price from a percentage. A quantity derived from volume is an allowance. Label it as one in the bill, and replace it when schedules arrive.

Be wary of rule-of-thumb percentages quoted online for each element. They vary from page to page and rarely say what cover, loading or code they assume. If you use an in-house benchmark, build it from your own completed jobs and note the source of each.

Drawing checks before you issue the quantity

Work through this list before the reinforcement figure leaves your desk:

Revision. Confirm the structural drawing and the schedule are the same revision, and that the architectural drawing used for concrete dimensions matches. Our guide to drawing revision control sets out a workflow for this.
Cover, lap and anchorage. Confirm these are specified by the engineer, not assumed.
Bar types. Keep different types and sizes of steel on separate lines.
Shape codes. Check each against BS 8666 and recompute a sample of cutting lengths by hand.
Openings and voids. Deduct or trim according to the measurement standard for the project, and avoid double deductions where elements meet.
Provisional status. Mark any quantity that is a ratio-based allowance rather than a measured one.
Contract route. Note whether the contract treats reinforcement as lump sum or remeasure.

One practitioner workflow guide says the take-off workbook should keep a clear audit trail by element, marking dimensions, assumptions and revisions, so tender and site control can both rely on it. Our article on take-off audit trail fields lists what to capture, and the drawings to BoQ process explains how to link each line to its drawing revision.

Common mistakes in reinforcement take-off

Using structural dimensions as bar lengths. This ignores cover and under- or over-counts, depending on the bar's position.
Forgetting laps on long runs. Stock bar lengths are limited, so long walls and slabs always need laps.
Counting bars as span divided by spacing. You need one more bar than the number of spaces.
Mixing steel and concrete density. See above: they are different numbers for different jobs.
Not recording the assumption. If a cover or lap value is yours rather than the engineer's, say so on the sheet.
Rounding too early. Round at the total, not at each bar mark.

Where software fits

Any draft quantity, from a spreadsheet or from software, still needs a surveyor to review it. Our piece on why QSs must audit every draft covers how to do that.

QuantiFlow is in development. It reads construction drawings and produces a draft Bill of Quantities for a quantity surveyor to review and sign off. Published plans are Solo at £69 per month, Business at £299 per month, and a custom Enterprise option. You can see them on the QuantiFlow site.

FAQ

How do I calculate reinforcement weight for a slab?

Count the bars in each direction (span divided by spacing, plus one), multiply by each bar's cutting length after cover and laps, and add the lengths by diameter. Multiply by mass per metre, which is the diameter squared divided by 162. Check the result against kg of steel per m³ of concrete for similar slabs.

What is the unit weight of reinforcing steel?

Reinforcing steel is taken as 7,850 kg/m³. Per metre of bar, that gives about 0.888 kg for 12 mm, 1.578 kg for 16 mm and 3.853 kg for 25 mm, as listed by Rospower. Reinforced concrete as a whole is much lighter per m³ because most of its volume is concrete.

Can I estimate reinforcement as a percentage of concrete volume?

Yes, as an early allowance or a sense check, not as a measured quantity. The ratio varies by element, loading and design, so a percentage cannot replace schedules. Label ratio-based figures as provisional and replace them once the engineer's details are issued.

Which British Standard covers bar bending schedules?

BS 8666:2020 is the British Standard for bar bending schedules, and it defines the shape codes and the minimum straight lengths between bends. Use it to check the cutting lengths on any schedule you receive.

Sources

This article is for general information only and is not professional, legal or commercial advice. Quantity surveying decisions should be taken by a qualified professional with reference to the specific project, drawings and contract in question. Content is produced with AI assistance and reviewed before publication. QuantiFlow Ltd accepts no liability for reliance on it.