How to use it
- Enter the slab length and width.
- Choose the bar size and the center-to-center spacing from your drawings.
- Enter the edge cover, the distance from the slab edge to the first bar.
- Choose one mat, or two for top and bottom reinforcement.
- Keep an allowance of 10 to 15% for laps and offcuts.
Worked example
A 20 × 12 ft slab with #4 bars at 12 in each way and 2 in edge cover: the bars running the long way sit within 12 ft minus 4 in, so 11.67 ÷ 1, rounded down, plus one gives 12 bars, each 19.67 ft long. The other direction takes 20 bars, each 11.67 ft long. That is 469 ft of bar, or 516 ft with 10% for laps and offcuts. At 0.668 lb/ft, the mesh weighs about 345 lb, and you would buy 26 bars of 20 ft.
Rebar weight tables
| US bar | Diameter | Weight |
|---|---|---|
| #3 | 0.375 in | 0.376 lb/ft |
| #4 | 0.500 in | 0.668 lb/ft |
| #5 | 0.625 in | 1.043 lb/ft |
| #6 | 0.750 in | 1.502 lb/ft |
| #7 | 0.875 in | 2.044 lb/ft |
| #8 | 1.000 in | 2.670 lb/ft |
| #9 | 1.128 in | 3.400 lb/ft |
| #10 | 1.270 in | 4.303 lb/ft |
| #11 | 1.410 in | 5.313 lb/ft |
| Metric bar | Weight |
|---|---|
| 8 mm | 0.395 kg/m |
| 10 mm | 0.617 kg/m |
| 12 mm | 0.888 kg/m |
| 16 mm | 1.578 kg/m |
| 20 mm | 2.466 kg/m |
| 25 mm | 3.853 kg/m |
| 32 mm | 6.313 kg/m |
Frequently asked questions
How do you calculate the weight of rebar?
For metric bars, weight in kg per metre is the diameter in millimetres squared, divided by 162. A 12 mm bar weighs 144 ÷ 162 = 0.888 kg/m. US bars use the nominal weights in ASTM A615, listed in the table above.
How long should rebar laps be?
Lap lengths are set by your design code and engineer, and depend on bar size, concrete strength and where the bar sits. Figures around 40 to 60 bar diameters are common, which is why the calculator adds a percentage allowance rather than a fixed length.
What is concrete cover?
Cover is the concrete between the bar and the surface. It protects the steel from corrosion and fire. Where concrete is cast directly against the ground, 3 in (75 mm) is commonly required.
How many 20 ft bars do I need?
Divide the total bar length, including the allowance, by 20 and round up. The calculator does this for you.