Rebar Calculator
Area 1
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Grid and bars
This counts the material for the grid you choose. It does not decide bar size, spacing, edge clearance or lap length: those depend on the load, the soil and your local code. Ask a structural engineer or your local building department.
Starts at 20 ft: Nucor lists 20 ft among its cut lengths and The Home Depot sells #4 bar in 20 ft lengths. Shorter bars are sold too; type the length your supplier sells.
Distance from the slab edge to the first bar, on every side. Leave empty for 0.
How far two bars overlap where they join. Leave empty for 0.
Bar sizes are US sizes and the weights are US figures in pounds per foot.
How this is calculated
How the rebar count is worked out
Lay the grid out on paper first. In each direction, take the side the bars are spaced across, subtract the edge clearance at both ends, divide by the spacing, drop the fraction and add one. That is how many bars run that way. Each of them is as long as the other side minus the clearance at both ends.
A bar longer than the stock length is made of several pieces that overlap where they join. The number of joins, or splices, is the bar length minus one lap, divided by the stock length minus one lap, rounded up, minus one. Each splice adds one lap of steel to that run.
Add up every run in both directions, laps included, to get the total length. Divide it by the stock length and round up to get the bars to buy. That count assumes the offcuts go into other runs and adds nothing for cutting loss, so a job with many short pieces may need a few more. The weight is the bars to buy times the stock length times the weight per foot of the bar size. Ties are one per crossing: the bars one way times the bars the other way.
The calculator does not choose the bar size, spacing, clearance or lap. Those come from the load, the soil and the local code, so get them from a structural engineer or your building department before you buy.
Examples
Shed pad: 10 ft × 10 ft
- Bars along the length: 10 ft ÷ 12 in, rounded down, + 1 = 11 bars each 10 ft long
- Bars along the width: 10 ft ÷ 12 in, rounded down, + 1 = 11 bars each 10 ft long
- Bar length, laps included: 11 × 10 ft + 11 × 10 ft = 220 ft = 67.06 m
- Ties: 11 × 11 bars, one tie per crossing = 121
- Bars to buy: 220 ft ÷ 20 ft, rounded up = 11 bars
- Weight: 11 bars × 20 ft × 0.668 lb/ft = 146.96 lb
11 bars #4 bars, 12 in grid. Weight: 146.96 lb. Total bar length: 220 ft. Along the length: 11 × 10 ft. Along the width: 11 × 10 ft. Ties, one per crossing: 121.
Garage slab with clearance and laps: 24 ft × 24 ft
- Bars along the length: (24 ft − 2 × 3 in) ÷ 16 in, rounded down, + 1 = 18 bars each 23 ft 6 in long
- Splices per bar along the length: (23 ft 6 in − 24 in) ÷ (20 ft − 24 in), rounded up, − 1 = 1
- Bars along the width: (24 ft − 2 × 3 in) ÷ 16 in, rounded down, + 1 = 18 bars each 23 ft 6 in long
- Splices per bar along the width: (23 ft 6 in − 24 in) ÷ (20 ft − 24 in), rounded up, − 1 = 1
- Bar length, laps included: 18 × (23 ft 6 in + 1 × 24 in) + 18 × (23 ft 6 in + 1 × 24 in) = 918 ft = 279.81 m
- Ties: 18 × 18 bars, one tie per crossing = 324
- Bars to buy: 918 ft ÷ 20 ft, rounded up = 46 bars
- Weight: 46 bars × 20 ft × 0.668 lb/ft = 614.56 lb
46 bars #4 bars, 16 in grid. Weight: 614.56 lb. Total bar length: 918 ft. Along the length: 18 × 23 ft 6 in. Along the width: 18 × 23 ft 6 in. Ties, one per crossing: 324.
Bars to buy by slab size: #4 bar, 16 in grid, 20 ft stock, no clearance or lap
| Slab | 20 ft bars to buy | Weight | Total bar length | Along the length | Along the width | Ties, one per crossing |
|---|---|---|---|---|---|---|
| 10 ft × 10 ft | 8 bars | 106.88 lb | 160 ft | 8 × 10 ft | 8 × 10 ft | 64 |
| 12 ft × 12 ft | 12 bars | 160.32 lb | 240 ft | 10 × 12 ft | 10 × 12 ft | 100 |
| 16 ft × 12 ft | 16 bars | 213.76 lb | 316 ft | 10 × 16 ft | 13 × 12 ft | 130 |
| 20 ft × 20 ft | 32 bars | 427.52 lb | 640 ft | 16 × 20 ft | 16 × 20 ft | 256 |
| 24 ft × 24 ft | 46 bars | 614.56 lb | 912 ft | 19 × 24 ft | 19 × 24 ft | 361 |
| 30 ft × 20 ft | 47 bars | 627.92 lb | 940 ft | 16 × 30 ft | 23 × 20 ft | 368 |
Weight used: 0.668 lb/ft for #4 bar, the nominal weight its sources publish. Bars to buy assumes the offcuts are used in other runs and adds nothing for cutting loss or mistakes.
Questions
Does this tell me what rebar my slab needs?
No. It counts the material for the grid you pick. Bar size, spacing, edge clearance (cover) and lap length depend on the load the slab carries, the soil under it and your local code, and this site does not decide any of them. Ask a structural engineer or your local building department, then enter their numbers here.
Where do the spacing, clearance and lap values come from?
From you. The spacing buttons are a choice of four grids, not a recommendation, and edge clearance and lap start at 0 because the site has no value of its own for them. Enter what your drawings, engineer or building department give you.
Why is there one more bar than the side divided by the spacing?
A grid has a bar at both ends of a row. A 10 ft side at 12 in spacing has 10 gaps and 11 bars. When the side does not divide evenly, the leftover is less than one spacing and gets no extra bar.
How are the bars to buy worked out?
The calculator adds up every bar run, laps included, divides that total by the stock length and rounds up. That assumes the pieces left over after cutting are used in other runs. If your offcuts are too short to use, you will need more bars than this, so check the count against your cutting plan.
What does the lap do to the count?
Where a run is longer than one stock bar, two bars overlap by the lap length you enter, and that overlap is extra steel. The breakdown shows how many splices each run needs and adds the lap for each one. The lap has to be shorter than the stock length.
How is the weight found?
Bars to buy × stock length × the nominal weight per foot of the bar size, as Nucor and CRSI publish it. It is the weight of the bars you bring home, useful for a trailer or a delivery. In metric mode it is also shown in kilograms.
How many rebar ties do I need?
One at every place where two bars cross. Every bar one way crosses every bar the other way, so the ties are the bars along the length times the bars along the width. A 10 ft square at 12 in has 11 × 11 = 121 crossings. Ties at laps, chairs and anything else you tie are not in the count, and how much tie wire a coil holds depends on the coil, so check the label.
Can I use bars shorter than 20 ft?
Yes. Type the length your supplier sells in the stock bar length field, for example 10 ft. Runs longer than that get splices, and the bar count goes up.
My slab is not a rectangle. What do I do?
Split it into rectangles and add each one with "Add another area". Each area gets its own grid with the same spacing, clearance and lap, and the lengths are added before the bars are counted.
Related tools
Sources
The bar and tie counts are plain arithmetic on the numbers you enter. The weight per foot of each bar size is a reference value published by both Nucor and CRSI. The 20 ft starting stock length is a length Nucor and The Home Depot both sell; change it to yours. Spacing, edge clearance and lap length have no source on this site because they are your choices. The unit definitions are from NIST.
- Nucor, Steel Rebar, product page, accessed October 8, 2026
- CRSI, Product Catalog, Spring 2025, page 16, Desktop Detailing Chart, table of standard reinforcing bars (nominal weight, lb/ft), accessed October 8, 2026
- The Home Depot, 1/2 in. x 20 ft. #4 Rebar, product page, accessed October 8, 2026
- NIST Handbook 44 (2024), Appendix C, General Tables of Units of Measurement, accessed October 8, 2026
- NIST Special Publication 811, Appendix B.9, conversion factors listed by kind of quantity, accessed October 8, 2026
Last reviewed: October 8, 2026