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Rebar Calculator

Estimate the number of reinforcing bars, total bar length, and total steel weight needed for a rectangular slab or foundation mat, based on bar spacing, diameter and number of layers.

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These calculators provide estimates and educational results. Always verify critical engineering calculations against applicable standards and qualified professional review.

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What Is It

Rebar placement in a slab or footing is typically specified as a grid at a fixed spacing in each direction. This calculator converts slab dimensions and spacing into a bar count and material quantity takeoff for estimating and ordering steel.

How to Calculate

Enter the slab's length and width, the bar spacing (commonly 150-300mm for slabs), the bar diameter, and whether the design uses a single mat or a top-and-bottom double mat. The calculator computes the number of bars running in each direction, total length, and total weight using the standard D²/162 bar weight formula.

Example

A 5m × 4m slab with 200mm spacing and 10mm bars, single mat: bars running the 5m direction spaced every 200mm across the 4m width need about 21 bars, plus about 26 bars running the other way, for roughly 47 bars totaling around 210m of bar and 130kg of steel.

Recommendations

This calculates the main grid quantity only - always add extra for laps/splices (typically 40-50 bar diameters of overlap at each splice), edge trimmers, and chairs/spacers, none of which are included in this base takeoff.

Frequently Asked Questions

What does the D²/162 formula mean?

It's the standard worldwide approximation for deformed steel reinforcing bar unit weight: dividing the bar diameter in millimeters² by 162 gives the weight in kilograms per meter, based on steel's density and the bar's nominal cross-section.

Should I use single or double mat reinforcement?

Single mat suits slabs on grade with light loads; double mat (top and bottom) is used for structural slabs, elevated decks, or where both positive and negative bending moments need reinforcement - follow your structural drawings.

Does this account for lap splices?

No - this gives the base grid quantity from slab dimensions and spacing. Add roughly 5-10% extra length for typical lap splices in a real material order, more for longer spans that require multiple bar lengths per run.

Last reviewed: 2026-09-12