Slab Rebar Calculator
Calculate how many reinforcement bars you need for a concrete slab from dimensions and bar spacing, free right in your browser.
Enter Values
Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.
How to use this calculator
- Enter the slab length and width (m) and the bar spacing (mm).
- Read the bars each way and total length.
How it works
Bars each way = slab dimension ÷ spacing + 1; total length sums both directions of a single mat.
Worked example
5 × 4 m slab at 200 mm. 21 + 26 bars, ~209 m total.
Common mistakes
- Forgetting laps, trim/edge bars and chairs.
- Using this for structural design — bar size and spacing must come from an engineer.
Frequently asked questions
How much rebar does a slab need?
From the slab size and bar spacing, the tool counts bars in each direction and totals the length and weight for your mesh or bar layout.
What rebar spacing is typical for a slab?
Around 200 mm centres each way is common for domestic slabs, but always follow the engineer's design for the job.
Does this size the bars?
No — it counts a mat at your chosen spacing. Bar diameter and spacing should be specified by a structural engineer.
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Cite this tool
Zerdly. (2026). Slab Rebar Calculator. Retrieved from https://www.zerdly.com/tools/slab-rebar-calculator/
Tip: Enter any known values to calculate the remaining results.
All calculations run in your browser. Your inputs are never saved or transmitted.