Rebar Calculator for a Concrete Slab

Spacing is the center-to-center distance between parallel bars, not the clear gap between their edges. A 12 inch grid puts bar centers one foot apart in both directions, forming roughly square openings across the slab. Enter it in inches and the tool converts internally to feet for the count.

A mat needs a bar on both outer edges, not just the openings between them. So the count is the perpendicular span divided by spacing, rounded down, plus one for the closing edge bar. That is why a 10 ft width at 12 in spacing gives 11 bars rather than 10.

Bars lengthwise run the full length of the slab and are spaced out across its width, so their count comes from the width. Bars widthwise run the full width and are spaced along the length. The tool reports each count and length separately so the linear-foot total stays accurate.

Patios and walkways often use 16 to 18 inch grids, while driveways and structural slabs use 12 inches or tighter. Spacing is an engineering decision that depends on load, slab thickness and bar size, so follow your structural engineer or local building code rather than a rule of thumb.

No. The grid deliberately assumes a bar sitting on each edge and does not subtract an edge clearance, which keeps the estimate on the safe side. In practice bars are held back a couple of inches from the form and lifted onto chairs for cover, but that does not change the bar count meaningfully.

No, the linear feet assume single full-length bars with no overlap. Where bars are spliced end to end you should add a lap, commonly around 40 bar diameters, and add extra for any hooks or bends. Treat the tool output as the baseline before splice and waste allowances.

Divide the total linear feet by the length of rebar you buy, often 20 feet per stick, then round up. For the 430 foot example that is about 22 sticks before you add margin for laps, offcuts and the occasional damaged bar.

No, it reports counts and linear feet only, because weight depends on bar size. To convert, multiply the linear feet by the per-foot weight of your bar: a #4 bar is about 0.668 lb/ft and a #5 bar about 1.043 lb/ft.

Yes. The calculation runs entirely in your browser with no dimensions sent to a server, so your slab sizes never leave your device. You can use it offline once the page has loaded.