Calculate reinforcement bar quantity, total length, steel weight, stock bars, laps, waste allowance and estimated material cost. Use separate bar sizes and spacing for each reinforcement direction.
Rebar Calculator
Enter your reinforcement details below. The calculator separates theoretical reinforcement from the estimated steel required for purchase.
Project Dimensions
Reinforcement Directions
Direction A and Direction B are calculated separately because reinforcement often uses different bar sizes or spacing in each direction.
Direction A
ADirection B
BKnown Bar Details
Stock Bars, Laps & Waste
Calculation Results
Purchasing Estimate
How to Calculate Rebar Quantity
Rebar quantity depends on the concrete dimensions, bar diameter, reinforcement spacing, number of layers and the amount of material required for laps and cutting waste.
For a reinforcement grid, the calculator first determines the usable concrete dimensions after deducting cover. It then calculates the number of bars required in each direction.
Rebar Weight Formula
A common theoretical formula for the weight of reinforcing steel is:
Weight per metre = diameter² ÷ 162
Diameter is measured in millimetres and the result is expressed in kilograms per metre.
Rebar Weight Table
| Bar Diameter | Approx. Weight per Metre | Approx. Weight of 12 m Bar |
|---|---|---|
| 6 mm | 0.222 kg/m | 2.66 kg |
| 8 mm | 0.395 kg/m | 4.74 kg |
| 10 mm | 0.617 kg/m | 7.41 kg |
| 12 mm | 0.889 kg/m | 10.67 kg |
| 16 mm | 1.580 kg/m | 18.96 kg |
| 20 mm | 2.469 kg/m | 29.63 kg |
| 25 mm | 3.858 kg/m | 46.30 kg |
| 32 mm | 6.321 kg/m | 75.85 kg |
How Rebar Spacing Affects Quantity
Closer reinforcement spacing requires more bars. For example, a slab using 12 mm bars at 150 mm centres requires more bars than the same slab using 12 mm bars at 200 mm centres.
The calculator uses the requested spacing to determine the number of spaces and then adds one bar so the reinforcement grid covers the usable dimension.
Why Direction A and Direction B Are Separate
Reinforcement grids often use different bar diameters or spacing in the two directions. Keeping the directions separate gives a more useful estimate than applying one bar size and spacing to the entire slab.
Rebar Laps and Stock Lengths
When a required cut length exceeds the available stock-bar length, additional stock pieces are required. A lap allowance accounts for the overlap between adjoining pieces.
The purchasing estimate therefore differs from the theoretical reinforcement quantity. Theoretical weight represents the reinforcement length installed in the calculation, while purchasing weight accounts for stock lengths, laps and waste.
What This Rebar Calculator Estimates
- Number of reinforcement bars
- Required reinforcement length
- Bar weight per metre
- Theoretical steel weight
- Stock-bar requirements
- Lap allowance
- Waste allowance
- Estimated purchasing weight
- Estimated material cost
- Approximate tie-wire requirement
Rebar Calculator FAQ
How do I calculate how many rebars I need?
Determine the usable dimension after deducting concrete cover, divide the dimension by the reinforcement spacing, round the number of spaces upward and add one bar. Repeat the calculation for the other direction.
How much does a 12 mm rebar weigh?
A 12 mm reinforcing bar weighs approximately 0.889 kg per metre using the standard theoretical weight formula. A 12 metre bar weighs about 10.67 kg.
How many 12 mm bars do I need for a slab?
The number depends on the slab dimensions, concrete cover, reinforcement spacing and number of reinforcement layers. Enter those details in the calculator to estimate the quantity.
What does 12 mm rebar at 150 mm centres mean?
It means 12 mm diameter reinforcing bars are positioned at a maximum nominal spacing of 150 mm from centre to centre, subject to the structural design and detailing requirements for the project.
Does the calculator include rebar waste?
Yes. Enter a waste percentage to estimate additional purchasing quantity. Waste is kept separate from the theoretical reinforcement requirement.
Does this calculator design structural reinforcement?
No. The calculator estimates quantities from reinforcement information supplied by the user. Structural reinforcement size, spacing, cover, laps and detailing should come from approved engineering design.
