Rebar calculator
Counts the bars in a two-way mat for a spacing you have already been given. Rebar size, spacing and cover are engineer-specified under ACI 318 — the familiar 12 to 18 inch figures are typical residential practice, not a code requirement.
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This calculator needs JavaScript. The reference table below covers the common cases.
| Size | Diameter | Weight per ft |
|---|---|---|
| #3 | 0.375 in | 0.376 lb |
| #4 | 0.500 in | 0.668 lb |
| #5 | 0.625 in | 1.043 lb |
| #6 | 0.750 in | 1.502 lb |
The formula
bars = floor((span in inches − 2 × cover) ÷ spacing) + 1, in each direction
Worked example
A 20 × 24 ft slab at 18 in spacing with 3 in cover: 16 bars lengthwise, 19 crosswise, about 776 ft total — 39 twenty-foot sticks.
Reference table
| Size | Diameter | Weight per ft |
|---|---|---|
| #3 | 0.375 in | 0.376 lb |
| #4 | 0.500 in | 0.668 lb |
| #5 | 0.625 in | 1.043 lb |
| #6 | 0.750 in | 1.502 lb |
Assumptions
- Assumes a single two-way mat at uniform spacing.
- No allowance for laps. Lap length is specified by the engineer and adds materially to the total.
- Cover requirements come from ACI 318 and depend on exposure. General information, not engineering advice.