Rebar Spacing Calculator
Estimate two-way reinforcing bar length for a rectangular slab.
Calculate rebar spacing calculator from clear inputs
Rebar Spacing Calculator gives a focused worksheet for home & diy questions where a quick deterministic estimate is more useful than a spreadsheet. The rebar spacing calculator fields mirror the variables in its formula, and the output separates the main result from supporting values so you can sanity-check this calculation before reusing it.
A 20 by 12 slab at 18 inch spacing needs 9 lengthwise bars, 14 widthwise bars, and 348 feet.
Keep the estimate private in your browser
For Rebar Spacing Calculator, every calculation runs in your browser with no upload, account, cookie, live lookup, or server-side storage. The rebar spacing calculator result is best treated as a planning number because the real-world constraints behind this estimate can change the answer. Adjust the rebar spacing calculator assumptions and confirm important decisions against the relevant source of truth for that exact situation.
Frequently Asked Questions
How is the number of bars calculated?
For each direction, the distance across is divided by the spacing, rounded down, plus one bar for the far edge. A 20 × 12 ft slab at 18 inches needs 9 bars running lengthwise and 14 running widthwise, 348 ft in total.
Does the total include overlaps?
No. Where bars are joined they must overlap (lap splice), often about 40 bar diameters, and bars are usually kept 2 to 3 inches in from the edges. Add 10 to 15% for laps and waste.
What spacing is typical?
12 to 18 inches on center is common for residential slabs and patios, but required spacing depends on slab thickness, load, and local code or an engineer's drawings.
Related tools
Browse the full set of free, private, in-browser tools.