Reinforcement is the only major material on a building site that is ordered by weight and fixed by number. You buy tonnes and you place bars, and the conversion between the two is where estimates either become accurate or become an argument with the supplier.
The reliable method is to work from the bar bending schedule, not from a rule of thumb about kilograms per cubic metre of concrete. Thumb rules are useful for a sanity check and useless for an order. This guide explains what a schedule contains, how to convert bar lengths into weight, how laps and offcuts change the quantity, and how to check the answer before the steel arrives.
What a bar bending schedule contains
A schedule is a table, one row per bar mark, with:
Weight, derived from the total length and the unit mass
If your drawings come without a schedule, you can build one: measure each distinct bar run, count how many times it occurs, and record the diameter. That is an hour of work on a small house and it eliminates almost every ordering error.
Unit mass: the number that does the work
Steel reinforcement has a standard mass per metre for each diameter, derived from a density of 7,850 kg/m³:
Diameter (mm)
Mass (kg/m)
12 m length (kg)
|---|---|---|
6
0.222
2.66
8
0.395
4.74
10
0.617
7.40
12
0.888
10.66
16
1.579
18.95
20
2.466
29.59
25
3.854
46.25
32
6.313
75.76
The quick formula for any diameter is diameter squared divided by 162, which gives kilograms per metre: 12 × 12 ÷ 162 = 0.889. That is accurate enough for estimating and easy to check in your head.
From lengths to weight
Weight = total length × unit mass
Worked example. A ground beam schedule with 12 bars of 16 mm at 6.4 m and 40 links of 10 mm at 1.6 m:
16 mm: 12 × 6.4 = 76.8 m × 1.579 = 121.3 kg
10 mm links: 40 × 1.6 = 64.0 m × 0.617 = 39.5 kg
Total for the beam = 160.8 kg
Multiply by the number of identical beams. Eight beams of that mark is 1,286 kg, or 1.29 tonnes, before laps and waste.
Laps: the quiet multiplier
Every bar that continues beyond a stock length needs a lap, and every lap is extra steel. Lap length is commonly 40-50 times the bar diameter for tension laps, depending on the concrete and the code.
A 16 mm bar with a 40-diameter lap adds 640 mm to each continuation. On a 25 m beam made from 12 m stock, there are two continuations per bar, so 1.28 m of extra bar per line. With four top and four bottom bars, that is 8 lines × 1.28 = 10.2 m of additional 16 mm steel, or 16 kg per beam, on a beam that weighed 160 kg. That is a 10% increase that a schedule without laps will not show.
Laps are the single most common reason a reinforcement order comes up short. Add 5-10% for laps on a typical frame, and more where stock lengths are short relative to the spans.
Stock lengths and the offcut problem
Reinforcement is sold in stock lengths of 6, 9 or 12 m depending on the market. A bar of 7.4 m cut from 12 m stock leaves 4.6 m of offcut, which is either used on another mark or scrapped.
The estimating approach that works is a cutting list: list every required length in ascending order, then combine lengths to stock bars to minimise offcut. On small jobs this is done by hand; on large jobs, software does it and the saving is measured in percent of total tonnage.
If you cannot be bothered with a cutting list, add 8-12% waste and accept the cost. Do not add 3% and hope.
Mesh reinforcement
Slabs are often reinforced with fabric mesh rather than loose bar. Mesh is specified by its designation, which encodes the bar spacing and diameter, and it is ordered by sheet.
A standard sheet is 4.8 × 2.4 m, or 11.52 m². Sheets = area ÷ 11.52, plus laps, which are typically one full mesh pitch at every edge, and that means 400 mm or more. On a 100 m² slab, laps and cutting typically add 8-12%, so 10 sheets of 11.52 m² cover about 103 m² of net slab with allowances.
Mesh also needs support: chairs at roughly 1 per square metre for a single layer, and spacers at edges and around openings.
Tying, spacing and the labour line
Fixing reinforcement is slow, skilled work that is often priced per tonne or per square metre:
Slabs: 0.15-0.25 tonnes per operative per day on straightforward mesh
Beams and columns: 0.1-0.2 tonnes per day, less where access is poor
Tie wire: about 8-10 kg per tonne of reinforcement
Chairs and spacers: 1 per square metre for slab mesh, more for double layers
Cover blocks: at 1 per square metre of formwork face
The labour line is often 30-40% of the reinforcement cost and it is invisible in a per-tonne material price. When comparing quotes, check whether fixing is included.
Ordering, delivery and certificates
Steel is heavy and awkward. A tonne of 12 m bars is a lorry load that needs a crane or a hiab, and it needs somewhere to be stored off the ground. Delivery and handling costs scale with tonnage but not linearly, because the first tonne carries the trip.
Ask for mill certificates with each delivery. The certificate shows the grade, the heat number and the mechanical properties, and it is the only evidence that the steel is what the specification requires. On a project with any inspection regime, missing certificates mean the steel has to be replaced.
Common errors, and the checks that catch them
Counting one direction of bars in a slab and forgetting the other
Omitting laps, which is a 5-10% shortfall
Measuring bar lengths without bend allowances, so cutting lengths are short
Mixing mesh and loose bar quantities in the same line
Forgetting link spacing changes at the ends of beams, where links are closer
Ordering by bar count instead of weight, which is how suppliers price
A quick check on any slab: reinforcement is typically 60-120 kg per cubic metre of concrete for domestic work. If your takeoff gives 25 kg/m³, you have probably missed a layer; if it gives 250 kg/m³, you have probably double-counted.
A worked takeoff from a small slab
A suspended slab 8.0 m × 6.0 m, 150 mm thick, with a bottom mesh of 10 mm bars at 200 mm both ways and a top mesh over supports.
Bottom steel, short direction (8.0 m span, bars running 8.0 m at 200 mm centres across the 6.0 m width):
Number of bars = 6.0 ÷ 0.2 + 1 = 31 bars
Length each = 8.0 m plus two 90° bends, say 8.4 m of cutting length
Total = 31 × 8.4 = 260.4 m of 10 mm
Weight = 260.4 × 0.617 = 160.7 kg
Bottom steel, long direction (bars running 6.0 m across the 8.0 m width):
Number of bars = 8.0 ÷ 0.2 + 1 = 41 bars
Length each = 6.0 + 0.4 = 6.4 m
Total = 41 × 6.4 = 262.4 m of 10 mm
Weight = 262.4 × 0.617 = 161.9 kg
Top steel over supports, taken as 25% of the bottom steel for a simple domestic slab: about 81 kg.
Total slab reinforcement = 160.7 + 161.9 + 81 = 403.6 kg, or 0.40 tonnes, before laps and waste. Add 8% for laps and offcuts: 0.44 tonnes.
As a check, the slab is 8.0 × 6.0 × 0.15 = 7.2 m³ of concrete, so the reinforcement rate is 403.6 ÷ 7.2 = 56 kg/m³. That is at the low end of the 60-120 kg/m³ range, consistent with a lightly reinforced domestic slab.
What the takeoff tells the programme
Steel is the material that decides whether a pour happens. Reinforcement must be delivered, cut, bent, placed, tied, checked and inspected before the concrete arrives, and an inspection that finds a missing bar stops the pour.
Two estimating habits protect the programme. Order steel with a two-day buffer before the fixing gang starts, and keep the bar schedule with the delivery note so the checker can match what is on site against what is drawn. The cost of a delayed pour, with a concrete wagon waiting, is far higher than any saving from a late steel order.
Cover, exposed steel and the cost of getting it wrong
Cover is the distance from the outside of the bar to the face of the concrete. It protects the steel from moisture and fire, and it is set by spacers and cover blocks, which are cheap items that are chronically under-ordered.
For domestic work, 25-40 mm of cover is typical for slabs and beams, and 40-50 mm for foundations in contact with the ground. Order cover blocks at roughly one per square metre of formwork face, and expect to break a third of them.
Exposed or under-covered steel rusts, expands and spalls the concrete off. The repair is a structural repair, not a cosmetic one, and it always costs more than the cover blocks would have.
Grade, weldability and the certificates that matter
Reinforcement grade is written as a yield strength, commonly 500 MPa for high-yield deformed bar or 250 MPa for mild steel. The grade affects the lap length as much as the diameter does: a lower grade needs a longer lap for the same bar.
Mill certificates carry the heat number, the chemical composition and the mechanical test results. Keep them with the delivery note and the bar schedule. On any project with third-party inspection, steel without traceable certificates is treated as unproven and may be rejected, which is an expensive way to learn that paperwork is part of the material.
Storing steel on site
Rebar arrives lightly rusted, which is normal and harmless. It does not survive being stored in mud or standing water, because corrosion at the bar surface reduces the bond with the concrete.
Store bars on bearers clear of the ground, sorted by diameter so the fixing gang is not searching for a size in a bundle, and covered where rain is heavy. Offcuts should be kept in a marked rack rather than scattered, because on most jobs the offcuts are enough to complete the last few marks.
Frequently asked questions
How do I calculate the weight of a rebar? Diameter squared divided by 162 gives kilograms per metre. A 12 mm bar is 0.89 kg/m.
How much does a 12 m length of 12 mm rebar weigh? About 10.7 kg.
How long should a lap be? Typically 40-50 times the bar diameter, and the exact figure comes from the design code and the concrete class.
How much steel is in a cubic metre of concrete? For domestic slabs and beams, 60-120 kg per cubic metre is a reasonable range for checking, not for ordering.
Do I need chairs for slab mesh? Yes, at about one per square metre, or the mesh sits on the membrane and has no cover beneath it.
Can I cut rebar with a disc cutter? Yes, but not with a wood blade and not without eye protection. Cutting generates heat and sparks and it is worth doing away from combustible material.
Estimate it with FRELUX
The FRELUX reinforcement calculator converts a bar schedule into weight by diameter, adds laps and offcuts, and prices the result per tonne in your market's currency. Enter the schedule once and the order quantity, cutting list allowance and fixing time follow.