🧲 Rebar Calculator

Reading the bar grid result

This tool estimates a simple orthogonal grid layout for material take-off purposes; actual bar size, spacing and cover requirements must follow the project's structural drawings.

• Bar spacing, size (diameter) and required concrete cover for a given slab are structural design decisions set by an engineer or the applicable building code — this calculator only estimates quantity for a specified spacing.
• This estimate does not include lap splices at bar joins, which add extra length for slabs larger than standard bar stock lengths (commonly 6 m or 12 m); add a lap allowance where bars must be joined.

What is rebar spacing and edge clearance?

Rebar (reinforcing bar) is typically placed in a two-way grid inside a concrete slab, with bars running in both directions at a set on-center (o.c.) spacing — the distance measured from the center of one bar to the center of the next. Edge clearance is the gap left between the outermost bars and the slab edge, commonly required for concrete cover over the steel.

This calculator lays out a simple rectangular grid: it works out how many bars fit at the specified spacing within the usable slab area (slab dimensions minus edge clearance on both sides), then sums the lengths of all bars running in both directions.

How to use this rebar calculator

1. Enter the slab length and width in meters.
2. Enter the bar spacing (on-center) in centimeters — the distance between adjacent parallel bars.
3. Enter the edge clearance in centimeters — the gap left between the bars and the slab edge.
4. Read the total number of bars, the total combined length, and the bar count running in each direction.

The formula behind the rebar estimate

Usable slab dimensions equal the slab length and width minus twice the edge clearance (clearance on both sides). The number of bars running lengthwise equals the usable width divided by the spacing, rounded down, plus 1 (for the bar at each end of the range); the same logic gives the number of bars running widthwise from the usable length. Total length sums each direction's bar count multiplied by its usable run length.

Worked example: a 6 m × 6 m slab with 30 cm (0.3 m) spacing and 8 cm (0.08 m) edge clearance gives a usable dimension of 6 − 2(0.08) = 5.84 m in each direction. Bars per direction = floor(5.84 ÷ 0.3) + 1 = 20 bars, for 40 bars total, each 5.84 m long, giving a total length of 40 × 5.84 = 233.6 m.

Common mistakes

• Using a spacing or bar size that was not specified by the structural engineer or drawings — spacing directly affects slab strength.
• Forgetting to add extra length for lap splices where standard-length bars must be joined across a large slab.
• Entering edge clearance greater than half the slab dimension, which leaves no usable area for bars.
• Treating this planning estimate as a substitute for an engineer-approved reinforcement drawing.

자주 묻는 질문

참고 자료

1. American Concrete Institute (ACI 318) — Building Code Requirements for Structural Concrete: reinforcement spacing and cover provisions.
2. Concrete Reinforcing Steel Institute (CRSI) — reinforcing bar detailing and placement reference.
3. International Residential Code (IRC) / International Building Code (IBC) — general slab reinforcement provisions.