Rebar Calculator
Bars each way for a slab or footing from its size, spacing and cover; total length, laps at your stock length and weight from nominal bar mass. No design rules.
How many bars run each way, their lengths, the splices a stock length forces, the lap length you specify, and the weight and cost from labelled nominal kg/m for the bar size.
Example: A 20 × 12 ft slab with #4 at 12 in each way and 3 in cover takes 12 + 20 bars, 464 ft; with 5 % waste that is 147.6 kg (325 lb).
Count the bars,
weigh the length.
How bars are counted inside the cover, how stock length creates splices, where the weights come from, and which decisions belong to the engineer.
Counting bars
The slab's clear dimensions are its length and width less the cover on each edge. Bars running along the length are spaced across the clear width: ⌊clear width ÷ spacing⌋ + 1; bars across are spaced along the clear length the same way. Each bar is as long as the clear dimension it runs in. A different spacing across can be entered; 0 means the same as along.
Stock length, laps and waste
When a bar is longer than your stock length it is made of ⌈length ÷ stock⌉ pieces with a splice between each pair; every splice adds the lap length you enter (laps are specified by the design, commonly as a multiple of the bar diameter). Waste is your percentage; none is assumed. Stock bars to order = total length ÷ stock length, rounded up.
Weight
Weight is total length × the nominal mass per length of the bar size, from a small labelled dataset: imperial sizes #3–#11 as commonly published for ASTM A615, metric sizes from 0.006165 × d² kg/m (ISO 6935-2 / EN 10080 nominal). The standards themselves were not consulted; the mill certificate governs and can differ by a few percent.
What the number does not settle
Bar size, spacing, cover, lap length, chairs, mesh alternatives, dowels and edge details are the engineer's decisions; the page carries no design rules and states none. Inputs stay in your browser; the same four anonymous usage counts as the rest of the site apply.
SOURCES
- Nominal bar mass per length as commonly published for ASTM A615 (imperial) and ISO 6935-2 / EN 10080 (metric: 0.006165 × d² kg/m) — standards not consulted directly
Last reviewed 18 September 2026. How results are checked: How we verify.