Quick answer
Bars along the length = ⌈width × 12 ÷ spacing⌉ + 1, bars along the width = ⌈length × 12 ÷ spacing⌉ + 1. A 20 × 10 ft slab at 12 inch spacing needs 11 bars of 20 ft and 21 bars of 10 ft: 430 ft of #4 bar weighing 287.2 lb.
Enter the slab size, spacing and bar size. The calculator returns bar counts, total length and weight. Use this rebar quantity calculator to check your own figures.
Related tools: Sonotube Calculator: Concrete volume and bags for round column forms. Post Hole Concrete Calculator: Concrete per post hole after the post's volume. Concrete Steps Calculator: Concrete volume for solid stacked steps.
How to use the Rebar Quantity Calculator
- Enter slab length (ft) (above 0).
- Enter slab width (ft) (above 0).
- Enter bar spacing (on center) (in) (above 0).
- Choose bar size: #3, #4, #5, #6, #7 and #8 and 5 more.
- Press Calculate to see the result, and use Reset to start again.
| Input | What to enter | Allowed values |
|---|---|---|
| Slab length | A number, in ft | > 0 |
| Slab width | A number, in ft | > 0 |
| Bar spacing (on center) | A number, in in | > 0 |
| Bar size | Choose one: #3; #4; #5; #6; #7; #8; #9; #10; … | Starts at #4 |
Formula and method
Formula
bars along length = ⌈width × 12 ÷ spacing⌉ + 1; bars along width = ⌈length × 12 ÷ spacing⌉ + 1
IEEE-754 double precision; shown to the decimals indicated.
Does not include laps, hooks or cover; add those from your drawings. Plan geometry only; confirm quantities with your supplier and local code before ordering.
Worked examples
These examples are verified: each one is an automated test case that the live calculator must reproduce exactly.
Example 1
Inputs: Slab length = 20; Slab width = 10; Bar spacing (on center) = 12; Bar size = #4.
| Result | Value |
|---|---|
| Total rebar length | 430 ft |
| Bars along length | 11 |
| Bars along width | 21 |
| Weight | 287.2 lb |
Example 2
Inputs: Slab length = 24; Slab width = 24; Bar spacing (on center) = 16; Bar size = #5.
| Result | Value |
|---|---|
| Total rebar length | 912 ft |
| Bars along length | 19 |
| Bars along width | 19 |
| Weight | 951.2 lb |
The first line is the headline result (total rebar length). Below it the rebar Quantity Calculator lists bars along length, bars along width and weight, which describe the same case from other angles. IEEE-754 double precision; shown to the decimals indicated.
The grid
A 12 inch grid across 10 ft needs 10 + 1 = 11 long bars. Across 20 ft it needs 21 short bars. #4 bar weighs 0.668 lb per foot, so 430 ft is 287.2 lb.
Laps and chairs
The count does not include laps (overlaps of about 30 bar diameters), cover at the edges or chairs to hold the bar up. Add 10% for laps and cuts. The structural design decides the size and spacing.
How the result changes with slab length
Holding every other input at the first worked example, this table changes only slab length. It shows how sensitive the answer is to that one value; each row was produced by running this calculator.
| Slab length | Total rebar length | Bars along length | Bars along width |
|---|---|---|---|
| 10 | 220 ft | 11 | 11 |
| 20 | 430 ft | 11 | 21 |
| 40 | 850 ft | 11 | 41 |
| 80 | 1,690 ft | 11 | 81 |
What happens with invalid input
These messages are produced by the calculator itself, so you know what to fix.
- Slab length: entering "0" shows "Slab length must be greater than 0." beside that field and no result.
- Bar spacing (on center): entering "0" shows "Bar spacing (on center) must be greater than 0." beside that field and no result.
Choosing between the Rebar Quantity Calculator and similar calculators
Each row shows what a related calculator on this site asks for and what it returns, so you can pick the one that matches your question.
| Calculator | You enter | It returns |
|---|---|---|
| [Sonotube Calculator](/construction/sonotube/) | tube diameter, height and number of tubes | Concrete volume |
| [Post Hole Concrete Calculator](/construction/post-hole-concrete-calculator/) | hole diameter, hole depth and post shape | Concrete per hole |
| [Concrete Steps Calculator](/construction/concrete-steps-calculator/) | number of steps, riser height and tread depth | Concrete volume |
| [Concrete Reinforcing Mesh Calculator](/construction/concrete-reinforcing-mesh-calculator/) | slab area, sheet width and sheet length | Sheets needed |
| [Concrete Volume Calculator](/construction/concrete-volume-calculator/) | length, width and thickness | Concrete needed |
Background: construction estimates
Material quantities are geometry plus a margin. Calculate the exact volume or area, add an allowance for waste, spillage, cuts and uneven ground (calculator pages commonly mention 5 to 10 percent, a rule of thumb that varies by material and job) and round up to whole units. One cubic yard is exactly 27 cubic feet.
Suppliers often have minimum order sizes and may charge for small loads, so confirm quantities, product yields and local code with your supplier or a qualified estimator before ordering.
Assumptions and limitations
- Add laps and chairs separately
Quantities are estimates. Add allowances for waste and site conditions, and follow local codes and a qualified professional's instructions for structural or safety-critical work.
If a required value is empty, not a number, or outside its allowed range, the form marks that field with a message and shows no result instead of a misleading number. The automated checks deliberately try invalid entries for slab length and bar spacing (on center) to confirm this.
Frequently asked questions
What does #4 mean?
A bar 4/8 inch (½ inch) in diameter.
How far apart should rebar be?
Often 12 to 18 inches for slabs; follow the design.
Where does rebar sit in the slab?
In the middle or upper third, on chairs.
What does the Rebar Quantity Calculator calculate?
Bars and total length of rebar in a grid across a slab. It returns Total rebar length, Bars along length, Bars along width and Weight.
What formula does the Rebar Quantity Calculator use?
bars along length = ⌈width × 12 ÷ spacing⌉ + 1; bars along width = ⌈length × 12 ÷ spacing⌉ + 1. Does not include laps, hooks or cover; add those from your drawings.
What do I need to enter in the rebar Quantity Calculator?
You enter Slab length (ft), Slab width (ft), Bar spacing (on center) (in) and Bar size.
How is the result rounded or limited?
IEEE-754 double precision; shown to the decimals indicated. Add laps and chairs separately.
What is an example of a rebar Quantity result?
With slab length 20, slab width 10, bar spacing (on center) 12 and bar size #4, the total rebar length is 430 ft.
Can I rely on this calculator for a decision?
Quantities are estimates. Add allowances for waste and site conditions, and follow local codes and a qualified professional's instructions for structural or safety-critical work. For this calculator in particular: Add laps and chairs separately.
Related calculators
- Sonotube Calculator — Concrete volume and bags for round column forms.
- Post Hole Concrete Calculator — Concrete per post hole after the post's volume.
- Concrete Steps Calculator — Concrete volume for solid stacked steps.
- Concrete Reinforcing Mesh Calculator — Mesh sheets for a slab, allowing for overlaps.
- Concrete Volume Calculator — Cubic yards of concrete and bags needed for a slab, footing or pad.
- Concrete Weight Calculator — Weight of a volume of concrete from its density.
Sources
- OpenStax, Prealgebra 2e (Rice University): perimeter, area, volume, circumference and the Pythagorean theorem
- NIST Special Publication 811, Guide for the Use of the International System of Units (SI), Appendix B conversion factors (international foot 0.3048 m, inch 25.4 mm, US gallon 3.785411784 L)
Content last reviewed 2026-11-04. Formulas and worked examples are checked by automated tests against independently computed values.