Rebar calculator
For budgeting, reinforcement is often estimated as kilograms of steel per cubic metre of concrete. The ratio depends on the element: footings have the least, columns and beams the most.
This is an estimate for planning only — the actual bars, diameters and stirrups come from the structural drawings.
Your measurements
Result
- Reinforcing steel (estimate)
- 80 kg
A planning estimate only — the actual reinforcement comes from the structural engineer's drawings. Add 5–10% for laps and offcuts.
How the calculation works
- Steel (kg) = concrete volume × typical ratio for the element: ~80 kg/m³ for footings, ~100 kg/m³ for slabs and ~120 kg/m³ for columns and beams.
Worked example
For 1 m³ of concrete in footings the estimate is 1 × 80 = 80 kg of reinforcing steel.
What the calculator assumes
- The ratios are typical guide values for an ordinary house; real reinforcement comes from the structural design.
- Weight per metre of bar: Ø8 = 0.395 kg, Ø10 = 0.617 kg, Ø12 = 0.888 kg, Ø14 = 1.21 kg, Ø16 = 1.58 kg.
When to adjust the result
- For budgeting, not execution: diameters, number of bars and stirrups are read from the reinforcement drawings.
- Add 5–10% for laps, anchorages and offcuts, plus tying wire.
Placing rebar correctly
Reinforcement only works where the design puts it and with enough concrete cover around it. Use spacers or chairs to hold bars at the specified cover (often 3–5 cm, more where concrete touches soil), so the steel is protected from corrosion and sits in the tension zone.
Overlap bars by the lap length the engineer specifies (often around 40–50 bar diameters) and tie intersections firmly so the cage does not move during the pour. Order 5–10% extra for laps and cuts. The bar sizes and spacing in foundations and slabs come from the structural design, not from rules of thumb.
Frequently asked questions
How much rebar per cubic metre of concrete?
As a rough guide, around 80 kg/m³ in footings, 100 kg/m³ in slabs and 120 kg/m³ in columns and beams — the structural design gives the real figure.
How much does a 12 mm bar weigh?
About 0.888 kg per metre, so a 12 m bar weighs about 10.7 kg.
How much concrete cover does rebar need?
It depends on exposure and the design; typical values are about 2.5–3.5 cm indoors and 5 cm or more where concrete is cast against soil.
How do I convert metres of rebar to kg?
Multiply the length by the weight per metre: about 0.395 kg/m for 8 mm, 0.888 kg/m for 12 mm and 1.58 kg/m for 16 mm bar.