Every bar size has a fixed mass per metre — 15M is 1.57 kg/m, 20M is 2.36 kg/m. So a rebar takeoff turns the drawings' reinforcing callouts into a weight. For a slab or wall reinforced with a mat, the shortcut is kilograms per square metre:
kg/m² = 4 × (1000 ÷ spacing in mm) × (kg/m of the bar) × lap factor
— the ×4 covers top & bottom mats, both directions.
Multiply that by the reinforced area and you have the weight.
Reinforcing is easy to under-read because a drawing can show several callouts — a base mat at 15M@200 plus heavier drop bars at 20M@200 over the columns. Read only the lightest, and the tonnage comes in low; treat the heaviest as the whole field, and it comes in high. The reliable approach locks onto the dominant base-mat spacing for the field and adds a top-steel allowance for the drops — rather than averaging them into a wrong single number.
Give Me The Cost reads the reinforcing callouts and computes the weight in tonnes — top and bottom, both ways, with laps — off the dominant spacing, and carries a top-steel allowance for drop bars. Its confidence read is deliberately honest that reinforcing runs light on heavily-detailed slabs, so it flags exactly where to verify the spacing against the drawings.
How is rebar measured in a takeoff?
By weight, in tonnes. Each bar size has a fixed mass per metre (15M = 1.57 kg/m), so spacing and area convert to kilograms and then tonnes — reinforcing is priced by mass, not length.
What is the biggest driver of rebar tonnage?
Bar spacing. Halving the spacing doubles the steel, so reading the slab's dominant spacing correctly matters more than any other single input.
Why do rebar takeoffs come in low?
Because slabs often show a light base mat plus heavier drop bars over supports. Reading only the base mat under-counts; the fix is the dominant spacing plus a top-steel allowance.
What lap allowance should a rebar takeoff use?
Bars overlap at splices, so a takeoff typically adds about 10–15% for laps and waste on top of the measured bar length.