Rebar Calculator
Enter the slab or footing length and width, the bar spacing and the bar size. A 20 by 10 foot grid of #4 bars at 12 inch spacing needs 473 ft of rebar including waste.
How the rebar calculator works
The calculator lays out a simple grid of bars in two directions, the way a slab or a wide footing is often reinforced. Bars run the length of the pour at a set spacing across the width, and a second set runs across the width at the same spacing along the length. It counts the bars, adds up their length, adds a waste and lap allowance, and converts the total length to weight.
Enter the length and width in feet and the spacing in inches, measured center to center. The tool adds one bar to each count so the last bar sits at the far edge. It does not subtract concrete cover at the edges, so the result is slightly generous. The count of 20 foot sticks assumes standard 20 foot lengths and rounds up.
The formulas
Example: For a 20 by 10 foot slab with #4 bars at 12 inch spacing, the grid has 11 bars running the length and 21 bars running the width. That is 430 ft before waste and 473 ft with the allowance, or 24 sticks. The weight is 316 lb at 0.668 lb/ft. The dimensions are an illustration only.
Weight per foot
| Bar | Weight, lb per ft |
|---|---|
| #3 | 0.376 |
| #4 | 0.668 |
| #5 | 1.043 |
| #6 | 1.502 |
These are the standard nominal weights for U.S. imperial reinforcing bar, as tabulated by Engineering ToolBox. Mill weights vary slightly.
Limits
The spacing, bar size and number of layers must come from your plans, engineer or local code, not from this tool. Footings with stirrups, dowels, bends or continuous bars need a bar schedule. Use this for quick budgeting and ordering estimates. For the concrete itself, try the cubic yard calculator, and see concrete cost for pricing.
Choosing the spacing
Common spacings are 12, 16 and 18 inches on center, but the right choice depends on the load, the soil and the design. A closer spacing means more bars and more steel. The calculator lets you compare spacings quickly: change the spacing from 12 to 16 inches and watch the total length and weight drop. Always confirm the final spacing against the drawings or with the local building department.
Lap splices and waste
Bars are sold in standard lengths, and a run longer than one stick needs splices where the bars overlap. The overlap length is set by the design and code, and it adds to the total length. The waste and lap allowance box covers this along with offcuts. Ten percent is a reasonable starting point for a plain slab, but a job with many splices may need more. Raise the number if your plans call for long laps.
For a footing, enter the footing length and width and use the spacing from the plans. If the footing has only two or three continuous bars, count those by hand: length times number of bars, plus laps. The grid method in this tool fits slabs, pads and wide mats best.
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Frequently asked questions
How much rebar do I need for a slab?
It depends on the design. For a 20 by 10 foot slab at 12 inch spacing, a two-way grid needs 473 ft, including a waste allowance.
How many 20 foot sticks is that?
The default layout needs 24 sticks, rounded up.
How much does rebar weigh per foot?
#3 weighs 0.376 lb per foot, #4 0.668, #5 1.043 and #6 1.502.
Does this replace an engineer’s design?
No. Spacing and bar size come from plans or local code. The tool only estimates quantity.
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