Rebar Calculator
Enter the size of your slab and the spacing of the rebar grid to see how many bars you need, the total length and the weight. Choose the bar size to include the weight per foot.
How to use this rebar calculator
- Enter the length and width of the slab in feet.
- Enter the bar spacing in inches, measured center to center. The same spacing is used in both directions.
- Choose the bar size. The weight per foot is built in.
- Enter the number of layers, an allowance for laps and cuts, and the stock length you buy.
- Press Calculate to see the bars, total length, pieces to buy and weight. Use Copy result or Export CSV to keep it.
How to calculate how much rebar you need
The calculator lays out a grid of bars in both directions and adds them up.
- Bars running along the length = width (in) / spacing, rounded down, plus 1; each is as long as the slab length
- Bars running along the width = length (in) / spacing, rounded down, plus 1; each is as long as the slab width
- Total length = (bars x bar length) x layers x (1 + allowance % / 100)
- Stock pieces = total length / stock length, rounded up
- Weight = total length x weight per foot (#3 0.376, #4 0.668, #5 1.043, #6 1.502 lb per ft)
Worked example
A slab is 20 ft by 10 ft with #4 bars at 12 inch spacing. Bars along the length: 120 / 12 = 10, plus 1 gives 11 bars of 20 ft, or 220 ft. Bars along the width: 240 / 12 = 20, plus 1 gives 21 bars of 10 ft, or 210 ft. The total is 430 ft. With a 10% allowance for laps and cuts, that is 473 ft. With 20 ft stock bars you buy 24 pieces. The weight is 473 x 0.668 = 316.0 lb.
Assumptions & limits
- The grid is a simple mat with the same spacing in both directions. Edge cover, chairs, ties and bends are not planned.
- The weight per foot values are the standard nominal weights for deformed bars #3 to #6.
- The lap and cut allowance is an editable placeholder. Required lap lengths depend on the bar size, the concrete and local code.
- The calculator does not choose a bar size or spacing. Those come from your engineer, plans or code.
- Footings, columns and stirrups need their own bar counts.
Frequently asked questions
How do I calculate how much rebar I need for a slab?
Count the bars each way across the grid, multiply by their length, add an allowance for laps and cuts, and divide by the length of the stock bars you buy.
How much does #4 rebar weigh?
The standard nominal weight of #4 bar is 0.668 lb per foot. #3 is 0.376, #5 is 1.043 and #6 is 1.502 lb per foot.
What spacing should I use?
It depends on the slab, the load and local code. Follow your plans or ask an engineer. The calculator counts the bars for the spacing you enter.
How long is a piece of rebar?
Rebar is often sold in 20 ft lengths, and some yards offer other lengths. You can enter your own stock length.
What is a lap splice?
A lap splice is an overlap where two bars join. Lap lengths depend on bar size and code, so the calculator uses an editable percentage allowance.
Does this work for footings or walls?
It counts a flat grid. For footings use the length and width of the footing, and ask your engineer about the right bar layout.
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Last updated: November 2026