Rebar Calculator
Calculate the number and total length of rebar needed for a concrete slab in a grid pattern.
This free online rebar calculator provides instant results with no signup required. All calculations run directly in your browser — your data is never sent to a server. Enter your values below and see results update in real time as you type. Perfect for everyday calculations, homework, or professional use.
Minimum: 0
Length of the slab
Minimum: 0
Width of the slab
Center-to-center spacing (12 or 18 in typical)
Minimum: 0
Length of each rebar stick (20 ft standard)
Results
Bars (length direction)
21
Bars (width direction)
21
Total Linear Feet
840 ft
20-ft Sticks to Buy
42
How to Use This Calculator
Enter your input values
Fill in all required input fields for the Rebar Calculator. Most fields include unit selectors so you can work in your preferred unit system — metric or imperial, whichever matches your problem.
Review your inputs
Double-check that all values are correct and that you have selected the right units for each field. Incorrect units are the most common source of calculation errors and can produce results that are off by factors of 2, 10, or more.
Read the results
The Rebar Calculator instantly computes the output and displays results with units clearly labeled. All calculations happen in your browser — no loading time and no data sent to a server.
Explore parameter sensitivity
Try adjusting individual input values to see how the output changes. This is a quick and effective way to develop intuition about how different parameters influence the result and to identify which inputs have the largest effect.
Formula Reference
Rebar Calculator Formula
See calculator inputs for the governing equation
Variables: All variables and their units are labeled in the calculator interface above. Input fields accept values in multiple unit systems — select your preferred unit from the dropdown next to each field.
When to Use This Calculator
- •Use the Rebar Calculator when you need accurate results quickly without the risk of manual computation errors or unit conversion mistakes.
- •Use it to verify calculations made by hand or in spreadsheets — an independent check can catch errors before they lead to costly decisions.
- •Use it to explore how changing input parameters affects the output — a quick way to develop intuition and identify the most influential variables.
- •Use it when collaborating with others to ensure everyone is working from the same numbers and applying the same assumptions.
About This Calculator
The Rebar Calculator is a free, browser-based calculation tool for engineers, students, and technical professionals. Calculate the number and total length of rebar needed for a concrete slab in a grid pattern. It implements standard formulas and supports both metric (SI) and imperial unit systems with automatic unit conversion. All calculations are performed instantly in your browser with no data sent to a server. Use this calculator as a quick reference and sanity-check tool during design, analysis, and learning. Always verify results against primary engineering references and applicable standards for any safety-critical application.
About Rebar Calculator
Reinforcing bar (rebar) strengthens concrete by resisting tensile forces that concrete alone cannot handle. For slabs, rebar is typically laid in a grid pattern with bars running in both directions. This calculator determines how many bars you need in each direction, the total linear footage, and how many standard 20-foot sticks to purchase. Common rebar spacings are 12 inches on center for driveways and structural slabs, and 18 inches on center for lighter-duty applications. The rebar size (gauge) depends on the structural requirements -- #4 (1/2 inch diameter) is the most common for residential slabs.
The Math Behind It
Formula Reference
Bars per Direction
Bars = floor(Perpendicular Dimension / Spacing) + 1
Variables: Spacing converted to feet
Worked Examples
Example 1: 20 x 20 ft slab, 12 in spacing
Slab = 20 x 20 ft, Spacing = 12 in, Bar Length = 20 ft
You need 42 twenty-foot rebar sticks for the grid.
Common Mistakes & Tips
- !Forgetting to add the +1 bar at the starting edge of the grid.
- !Not accounting for lap splice lengths at bar joints.
- !Using rebar that is too small for the structural requirements.
- !Setting rebar at the wrong depth within the slab thickness.
Related Concepts
Used in These Calculators
Calculators that build on or apply the concepts from this page:
Frequently Asked Questions
What size rebar for a driveway?
Most residential driveways use #4 rebar (1/2 inch) at 12-inch spacing in both directions. For heavy vehicle loads, upgrade to #5 rebar or reduce spacing to 8 inches.
How much overlap at splice joints?
The minimum lap splice is 40 bar diameters. For #4 rebar, that is 40 x 0.5 = 20 inches. For #5 rebar, it is 40 x 0.625 = 25 inches.