Rebar Calculator
This rebar calculator estimates the reinforcing steel for a concrete slab, footing, wall, or beam. Enter the dimensions, bar spacing, and bar diameter and the rebar calculator returns the total bar length, number of bars, and rebar weight. It works as a concrete rebar calculator, a footing rebar calculator, and a slab rebar calculator.
Results
The Rebar Calculator is part of our Rebar & Steel collection. Browse more rebar & steel to size, convert, and check your work alongside this tool.
What the rebar calculator does
The rebar calculator estimates how much reinforcing steel a concrete pour needs. Reinforcement gives concrete its tensile strength, and getting the quantity right is essential for ordering and budgeting. As a concrete rebar calculator it works out the total length of bar, the number of bars, and the rebar weight from your dimensions, spacing, and bar diameter, whether you are reinforcing a slab, footing, wall, or beam.
Formula for rebar weight
The number of bars in each direction is the span divided by the spacing, plus one. The total length adds both directions, and the formula for rebar weight uses the standard unit mass:
where d is the bar diameter in millimeters. This gives kilograms per meter for round reinforcing steel, which the rebar weight calculator multiplies by the total length.
Rebar calculator example
A 5 × 4 m slab with 20 cm spacing and 12 mm bars, run through the concrete slab rebar calculator:
At 0.888 kg/m for 12 mm steel, that is about 185 kg of rebar weight.
How to use the rebar calculator
Enter the slab or footing length and width in meters, the spacing between bars in centimeters, and the bar diameter in millimeters. The rebar calculator assumes a two-way mat and returns the total length, bar count, and weight. As a footing rebar calculator or slab rebar calculator, the same inputs cover each structure.
Reading the rebar calculator result
Use the length to order bar (sold in standard stock lengths) and the weight for costing, since rebar is priced per kilogram. Add an allowance for laps, overlaps at joints, and wastage, typically 10 to 15 percent on top of the concrete and rebar calculator figure. For metric jobs, the rebar calculator metric output keeps everything in kilograms and meters.
When to use the rebar calculator
- Ordering reinforcement for a slab with the concrete slab rebar calculator
- Sizing footing steel with the footing rebar calculator
- Estimating beam and column steel with the beam rebar calculator
- Budgeting by weight with the rebar weight calculator
Common rebar mistakes
- Forgetting laps and waste. Add 10 to 15 percent to the net length for a realistic order.
- Guessing spacing or bar size. Always take these from the structural drawings.
- Mixing units. Slab in meters, spacing in centimeters, diameter in millimeters.
Tips for accurate rebar results
- Round up to standard stock lengths when ordering.
- Add for laps (about 40× bar diameter) at every joint.
- Use the weight figure for costing since rebar sells by the kilogram.
Why the rebar calculator saves money
Reinforcement estimating is where a concrete project's budget is often won or lost, because steel is priced by weight and small changes in bar size have an outsized effect. Since the formula for rebar weight scales with the square of the diameter, moving from 10 mm to 12 mm bar increases the weight per meter by about 44 percent, and moving to 16 mm more than doubles it. Getting the bar size and spacing right from the structural drawings, never by guesswork, is the biggest driver of the cost, and the rebar calculator makes that visible.
From net length to a bar-bending schedule
The concrete slab rebar calculator gives the net length of a two-way mat, but a real bar-bending schedule adds several practical quantities. Bars overlap where they join, typically by forty times their diameter, which adds length at every lap. Starter bars, edge trim, and chairs to hold the mat at the right height all add steel. Experienced estimators take the net figure from the rebar weight calculator and add ten to fifteen percent for laps and waste, then round up to whole stock lengths. Treating the calculated figure as a solid starting point rather than a final answer is the key to ordering the right amount.
Rebar weight chart
Unit mass by bar size, used in the formula for rebar weight:
| Bar size | Mass (kg/m) |
|---|---|
| 8 mm | 0.395 |
| 10 mm | 0.617 |
| 12 mm | 0.888 |
| 16 mm | 1.578 |
Rebar Calculator FAQ
How is rebar weight calculated?
The formula for rebar weight is 0.006165 times the diameter in millimeters squared, giving kg per meter. Multiply by the total length. For 12 mm bar this is about 0.888 kg/m, which the rebar weight calculator applies.
How much rebar do I need for a slab?
The concrete slab rebar calculator divides the span by the spacing in each direction, adds the lengths, and applies the weight formula. A 5 × 4 m slab at 20 cm spacing needs about 209 m of 12 mm bar.
Should I add extra for laps and waste?
Yes. Add 10 to 15 percent to the concrete rebar calculator length to cover laps at joints, overlaps, and cutting waste. The tool gives the net mat length before those allowances.
Does the rebar calculator work in metric?
Yes. The rebar calculator metric output keeps length in meters and weight in kilograms, which suits most reinforcement ordering worldwide.