Rebar Calculator

About the Rebar Calculator

Concrete is strong when it is squeezed and weak when it is pulled apart, which is why almost every slab that carries real loads has steel in it. Reinforcing bar, usually called rebar, gives the slab tensile strength and helps hold cracks tight when they form. For a typical residential slab, such as a garage floor, a shed foundation, or a patio that will carry a vehicle, a flat grid of bars laid in both directions is a common arrangement. The Rebar Calculator estimates how much steel that grid takes, so you can order the right number of sticks before the pour.

The method follows how a grid is laid out. Across each dimension of the slab, bars sit at regular intervals, with one at each edge. The tool divides the slab dimension by your spacing, rounds up to a whole number of intervals, and adds one to include the final bar. It does this for both directions, multiplies each bar count by the length of the bars in that direction, and adds the two totals. Dividing by the stick length, 20 feet in U.S. mode or 6 meters in metric mode, gives the number of sticks to buy.

The output is a quantity for planning and budgeting, not a structural design. The size of the bar, the spacing, the position in the slab, and the amount of overlap at joints are decisions set by plans, local code, or an engineer. What the calculator does well is turn those decisions into a shopping list, so that a homeowner, a contractor, or a supplier can check a quote against the geometry of the slab.

When to Use It

How to Use It, Step by Step

1. Select U.S. or metric units. In U.S. mode, slab size is in feet, spacing in inches, and stick length in feet. In metric mode, slab size is in meters, spacing in centimeters, and stick length in meters.
2. Enter Slab length as the longer dimension of the pour. Use the dimension of the slab itself, and remember that bars sit inside the forms, so your plans may call for them to stop a few inches short of the edge.
3. Enter Slab width in the same units. If the pour is not a simple rectangle, divide it into rectangles and run each one separately, taking care not to count a shared edge twice.
4. Set the Grid spacing from your plans or engineer, measured center to center between parallel bars. The default is 12 inches (30 cm). Typical residential slabs use somewhere from 12 to 18 inches, but the plans govern.
5. Choose the Rebar stick length that your supplier stocks. The default is 20 feet (6 m), and the allowed range runs from 10 to 40 feet in U.S. mode or from 3 to 12 meters in metric mode.
6. Click Calculate Rebar Needed and read the number of bars running in each direction, the total linear length, and the number of sticks. The stick count is the highlighted line.
7. If your plans call for two layers, such as a top and bottom mat, double the totals. Add steel for laps where bars are spliced, for corners, and for any thickened edges or footings that sit around the slab.
8. Add a small allowance for cutting waste and mistakes, then confirm the bar size, spacing, and placement with your plans or engineer before ordering.

Understanding Your Results

The first two lines tell you how many bars run in each direction. Bars labeled as running lengthwise are as long as the slab and are spaced across its width, while bars running widthwise are as long as the width and are spaced along the length. Compare these to a sketch of your slab. A 12 foot by 20 foot slab at a 12 inch grid should show 13 bars in one direction and 21 in the other, because each count includes both edge bars.

The total length is the sum of all bar lengths, and the stick count is that total divided by your stick length and rounded up. Remember that the calculation treats the total as one continuous quantity, so it assumes offcuts from one bar can be reused somewhere else. In practice, a bar that is slightly longer than a stick needs a splice or a second stick, and not every leftover piece fits. Real orders often come out a few sticks above the displayed number.

Weight can help you check a supplier's invoice. Common deformed bars weigh about 0.376 pounds per foot for number 3 bar, 0.668 for number 4, and 1.043 for number 5, where the number is the diameter in eighths of an inch. Multiply by total linear feet to estimate pounds. Use the results as a planning guide, and have the engineer or the building department confirm the bar size and layout.

More Worked Examples

Tips for Better Results

• Support the grid on chairs or bricks so the steel sits in the middle or upper part of the slab as specified, because bars lying on the ground do almost nothing once the concrete is poured.
• Overlap bars at splices by the length your plans require, which is commonly expressed as a multiple of the bar diameter, and add that extra length to your order rather than relying on the base total.
• Buy tie wire and plastic or metal chairs along with the rebar, since they are inexpensive but essential, and a shortage on pour day can hold up the entire crew.
• Plan your cuts on paper before ordering. Sketch which sticks are cut for which bars, and see how offcuts can be reused, because the best layout can save several sticks on a mid-sized slab.
• Keep rebar clean and off the ground before the pour. Light surface rust is generally acceptable, but heavy scale, mud, or oil can reduce the bond between steel and concrete.
• Schedule the inspection if your jurisdiction requires one, because many building departments want to see reinforcement in place before concrete is placed and may refuse to approve a pour afterward.

Common Mistakes to Avoid

• Forgetting the extra bar at the edge is the classic error. The count is the span divided by the spacing plus one, so a slab 10 feet wide on a 12 inch grid needs 11 bars, not 10.
• Using the spacing from a different bar layout, such as wire mesh, produces the wrong count. Use the center-to-center spacing from your own plans.
• Leaving out overlaps leaves the grid short. Splices need extra length, so add allowance for laps at every joint between sticks.
• Ignoring a second layer when the plans call for top and bottom steel halves the order. Double the totals if the design includes two mats.
• Treating the result as a design is a serious mistake. The tool does not choose bar size, spacing, or cover, so those must come from drawings, code, or an engineer.

Assumptions and Limits

• The calculator models one flat mat of bars laid in a perpendicular grid with a bar at each edge. It does not include a second layer, dowels, chairs, tie wire, corner bars, or additional steel in footings and thickened edges.
• The stick count divides the total length by the stick length and rounds up, which assumes offcuts can be reused. Real jobs lose steel to awkward lengths and laps, so the practical number can be higher.
• Bars are assumed to run the full dimension of the slab. In practice, plans leave clear cover between the steel and the edge of the concrete, so actual bar lengths are slightly shorter.
• Reinforcement requirements depend on slab thickness, soil, loads, and climate. For any structural slab, retaining element, or heavy load, have a licensed engineer specify the reinforcement, and follow local building code.

Key Terms

Rebar

Short for reinforcing bar, a steel rod with ridges on its surface that bonds to concrete and carries tensile loads the concrete cannot.

Stick

A single length of rebar as sold by the supplier, commonly 20 feet in the United States or 6 meters in metric countries.

Grid spacing

The center-to-center distance between parallel bars in a mat, which determines how many bars the slab needs.

Lap splice

An overlap of two bars used to join them, with the overlap length typically set as a multiple of the bar diameter.

Chair

A small support, made of plastic or wire, that holds rebar at the correct height above the ground so concrete can flow beneath it.

Bar number

In U.S. sizing, the diameter of a bar in eighths of an inch, so number 4 bar is half an inch across and number 3 is three eighths.