About this tool
A rebar grid keeps a concrete slab from cracking under load. Slab area, the largest spacing you allow between bars, and bar size determine how many bars and how much linear footage you need to buy. The calculator also shows the count of grid crossings (where you tie wires) and an approximate steel weight for ordering.
How it works
The grid runs both directions. Bar count in each direction equals slab dimension minus edge cover, divided by max spacing, rounded up plus one. Total linear feet sums both directions. Multiply by lb-per-foot for that bar size to get weight.
How to use the Rebar Spacing Calculator
The Rebar Spacing Calculator is a browser-based tool that runs entirely on your device. Inputs you enter never reach a server - all calculations happen client-side in JavaScript. This means:
- Privacy: nothing is logged, sent, or stored by 3Tej. Inputs disappear when you close the tab.
- Speed: results update as you type. No network round trip.
- Offline use: once the page is cached, it works without internet.
- No signup: no account, no email, no rate limits.
Step by step
- Enter your inputs in the form above. Each field is labeled with its unit (currency, percent, kg, etc.) and the expected range.
- Read the result as it updates. The number reflects the formula commonly accepted in Rebar Spacing-related calculations.
- Adjust to see sensitivity: change one input at a time and watch how the output moves. This is the fastest way to understand which variable matters most.
- Copy or screenshot the result for later reference. The page state persists for the session if your browser allows it.
When you would use this
- Quick estimates: when you need a number now and don't want to open a spreadsheet.
- Sensitivity analysis: testing how a result changes as inputs vary, before committing to a real-world decision.
- Comparison: running the same calculation with different inputs to compare options side by side.
- Learning: building intuition for how the underlying math behaves.
- Documentation: capturing a snapshot of inputs and outputs at a point in time.
The formula explained
This calculator uses the following formula:
bars_long = ceil((wid - 2 × cover) / spacing) + 1; total_lf = bars_long × len + bars_short × wid
The reason this formula works is rooted in the underlying physics, finance, or biology of the problem. Behind every calculator is a published, peer-reviewed equation or a widely accepted convention. We do not invent formulas; we apply standard ones from textbooks, government tables, professional bodies, and academic literature.
If you are curious about the math, the simplest way to verify is to plug in two known numbers and compare against a known result. The calculator should match published examples to within rounding precision.
Frequently asked questions
What size rebar for a slab?
Number 3 (3/8 inch) for sidewalks and patios, number 4 (1/2 inch) for driveways, number 5 (5/8 inch) for footings under load.
How far apart should rebar be?
12 inch grid for residential slabs, 16-18 inch grid for sidewalks and light loads, 6-8 inch for heavy loads or footings.
Should rebar go up or down in slab?
Place rebar in the middle to upper third of the slab thickness, supported on chairs. Avoid rebar resting on grade where it can rust.
How much overlap on rebar splice?
Lap splice 40 times bar diameter as a rule of thumb. For number 4 rebar, that is about 20 inches.
Is the Rebar Spacing Calculator accurate?
The Rebar Spacing Calculator applies the standard formula for rebar spacing. Accuracy is limited only by your input precision. For decisions with material consequences, use the result as a starting point and verify with a qualified professional or the relevant official source.
Is the Rebar Spacing Calculator free?
Yes. 100% free, no signup, no payment, no API key. The site is funded by display ads that appear around the tool but not inside the calculation flow.
Are my inputs saved?
No. Inputs stay in your browser tab. Closing the tab discards them. The site uses Google Analytics for traffic measurement (anonymized) but does not see what you type into the form.
Can I use the Rebar Spacing Calculator on my phone?
Yes. The tool is responsive and tested on iOS Safari, Android Chrome, and major desktop browsers. Touch targets meet Apple's 44pt and Google's 48dp minimum guidance.
How do I report a bug or suggest improvement to the Rebar Spacing Calculator?
Email hi@3tej.com with the URL of this page and a description of what you saw vs expected. We typically respond within 72 hours and update calculators when rules or formulas change.
How accurate is the Rebar Spacing Calculator?
It applies the standard formula. Accuracy is limited only by your input precision. For decisions with material consequences (taxes, medical, legal, structural), use the result as a starting point and verify with a qualified professional in the relevant field.
Is the Rebar Spacing Calculator free to use?
Yes. 100% free, no signup, no payment, no API key. The site is funded by display ads around the tool but not inside the calculation flow.
Are my inputs saved anywhere?
No. All inputs stay in your browser tab. Closing the tab discards them. The site uses Google Analytics for traffic measurement (anonymized) but the analytics never see what you type into the form.
Does the Rebar Spacing Calculator work offline?
Yes. Once the page has loaded, it works without internet. The calculation runs in JavaScript on your device.
Can I share results from the Rebar Spacing Calculator?
Take a screenshot or copy the output. The page doesn't generate shareable URLs for specific calculations - inputs stay in your browser only.
Why are the results different from another rebar spacing tool?
Most likely: different formula assumptions, different default values, different rounding rules, or different applicable rates. Check the methodology if both tools document it. Both can be valid for different scenarios.