Rebar Calculator
Count the bars, splices, ties and weight of reinforcement for a slab, at any spacing and bar size.
The Rebar Calculator runs entirely in your browser. Slab dimensions and prices you enter are computed on your device and never uploaded.
Open the Concrete Calculator
About Rebar Calculator
Reinforcement is laid as a grid: bars running the length of the pour spaced across its width, and bars running the width spaced along its length. Counting them is one formula per direction — usable span divided by spacing, plus one for the bar that starts the run — and that plus one is the step most estimates quietly forget, which is why deliveries arrive a bar short in each direction. This calculator counts both directions, trims each bar by the concrete cover at either end, adds a lap wherever a run is longer than the stock bar you are buying, and totals the weight, the tie wire and the cost.
Features
- Bar counts in both directions, with the cover subtracted from each span
- Imperial #3 to #11 sizes with soft-metric equivalents and nominal weights
- Lap splices added automatically for runs longer than the stock bar
- Number of stock bars to order at your chosen length
- Total weight in pounds and kilograms
- Tie intersections and the tie wire they need
- Waste allowance and material cost
- Cover check against the usual minimum for each exposure
How to use the Rebar Calculator
- Enter the slab length and width in feet
- Choose a bar size and the spacing you want in each direction
- Set the concrete cover and the stock bar length you can buy
- Read the bar counts, weight and number of stock bars to order
Example
Input
20 × 20 ft slab · #4 bar · 16 in centres · 3 in cover
Output
15 bars each way · 585 ft of bar
30 × 20 ft stock bars · 391 lb · 225 ties
Wider spacing saves bar quickly — moving from 12 to 16 inch centres cuts roughly a quarter of the steel.
Common errors & troubleshooting
- The delivery is one bar short in each direction. — Bar count is the span divided by the spacing plus one, because the run needs a bar at both edges. Dividing alone leaves out the closing bar.
- Bars end up too close to the slab edge. — Cover is measured from the concrete face to the steel, and the calculator already trims it from both ends. Three inches is the usual minimum where concrete is cast against earth — less and the steel eventually rusts and spalls the slab.
- The ordered length looks higher than the grid needs. — Laps. Any run longer than the stock bar is spliced, and each splice overlaps by 40 bar diameters, which is added to the ordered length rather than the net length.
Frequently asked questions
- How much rebar do I need for a concrete slab?
- It depends on spacing far more than area. As a guide, a 20 by 20 foot slab on 16 inch centres takes about 585 feet of #4 bar. Enter your own dimensions and spacing above for the exact count.
- What spacing should rebar be at?
- Twelve to eighteen inches covers most residential work: 12 inches for heavily loaded slabs, 16 for driveways and garage floors, 18 to 24 for sidewalks and patios. Structural slabs should follow the engineer's drawing rather than a rule of thumb.
- How long should a rebar lap splice be?
- Forty bar diameters is the common Class B tension lap, so a #4 bar laps about 20 inches and a #8 bar about 40. Codes vary with concrete strength and bar coating, and this tool uses the 40-diameter figure.
- How much does rebar weigh per foot?
- A #3 bar is 0.376 lb per foot, #4 is 0.668, #5 is 1.043 and #8 is 2.67. The tool multiplies the nominal weight for your chosen size by the total ordered length.
- How much concrete cover does rebar need?
- Three inches where concrete is cast directly against earth, two inches for surfaces exposed to weather, and an inch and a half for protected interior slabs. Cover is what keeps water and chlorides away from the steel.
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