Rebar Calculator
Estimate reinforcing-bar counts, length, weight and optional cost in Imperial or Metric units.
Use this rebar calculator to prepare a planning takeoff for a rectangular, single-layer reinforcing grid in a concrete slab, footing or foundation area. Choose Imperial or Metric units, then enter the dimensions, native spacing and bar designation to estimate bars in both directions, total length, waste-adjusted length, standard weight and optional material cost.
Your Estimate
Bars Running Lengthwise
9 bars
Bars Running Widthwise
16 bars
Total Bars
25 bars
Total Linear Feet
340.0 ft
Waste-Adjusted Linear Feet (10%)
374.0 ft
Estimated Total Weight (4)
249.8 lb
Assumes a rectangular, single-layer grid with a bar along each edge. Weight and cost use waste-adjusted linear feet. Bar designations and required reinforcement vary by country, code and approved structural plans.
Results Actions
Standard Rebar Sizes and Weights
Imperial weights are standard nominal U.S. values. The 10M–25M designations and weights are used in Canadian metric practice and are not universal worldwide standards. Confirm the designation, grade, mass and stock length used in your country and project specifications.
How to Use the Rebar Calculator
- 1Choose Imperial or Metric units; changing systems loads sensible native defaults.
- 2Enter the slab or footing length and width in feet or meters.
- 3Select an Imperial spacing in inches or a native Metric spacing in millimeters.
- 4Choose #3–#6 rebar in Imperial mode or 10M–25M in Metric mode.
- 5Select a 5%, 10% or 15% waste allowance.
- 6Optionally enter the price per linear foot or linear meter.
- 7Review bars in each direction, total bars, total length, waste-adjusted length, weight and optional cost.
Rebar Grid Formulas
- 1Bars running lengthwise = ceiling(project width ÷ spacing) + 1, using inches in Imperial mode or millimeters in Metric mode.
- 2Bars running widthwise = ceiling(project length ÷ spacing) + 1, using inches in Imperial mode or millimeters in Metric mode.
- 3Total bars = lengthwise bars + widthwise bars.
- 4Total bar length = lengthwise bars × project length + widthwise bars × project width.
- 5Waste-adjusted length = total bar length × (1 + waste percentage).
- 6Estimated weight = waste-adjusted length × standard weight per foot or meter for the selected designation.
- 7Estimated material cost = waste-adjusted length × price per linear foot or meter when a price is entered.
Example Calculation
Imperial example: a 20 ft by 10 ft slab with #4 bars at 16 in spacing needs 9 lengthwise bars and 16 widthwise bars, totaling 340 ft before waste and 374 ft with 10% waste; estimated weight is 249.8 lb. Metric example: a 6 m by 3 m slab with 15M bars at 400 mm spacing also needs 9 lengthwise and 16 widthwise bars, totaling 102 m before waste and 112.2 m with 10% waste; at 1.570 kg/m, estimated weight is 176.2 kg.
Accuracy & Assumptions
- Results are planning estimates only and do not design reinforcement.
- The calculator assumes one rectangular grid layer with a bar along each edge.
- Bar counts round up so spacing does not exceed the selected maximum across the entered dimensions.
- Every lengthwise bar is assumed to span the full slab length and every widthwise bar the full slab width.
- Weight and optional cost are calculated from waste-adjusted linear feet in Imperial mode or linear meters in Metric mode.
- Waste can help cover cuts and handling but may not fully cover required lap splices, hooks, bends or development length.
- Stock-length optimization, chairs, tie wire, supports, openings, edge cover and multiple reinforcing mats are not included.
- Bar designations, grades, standard sizes and stock lengths vary by country; 10M–25M options are not universal worldwide standards.
- Rebar size, spacing, grade, cover, lap length and placement must follow approved structural drawings and applicable local code.
Frequently Asked Questions
How do I calculate the number of rebar bars in a slab?
Divide the perpendicular slab dimension by spacing using consistent units—inches for Imperial or millimeters for Metric—round up, and add one for the opposite edge. Repeat in the other direction.
Why does the calculator add one bar?
The spaces between bars are intervals. A run divided into eight spaces requires nine bars to create those spaces, including bars at both edges.
What rebar size should I use?
Rebar size depends on loads, slab or footing thickness, concrete strength, soil and structural design. This calculator estimates quantities only; use the size specified on approved drawings.
What rebar spacing should I use?
Spacing must come from structural plans or applicable project requirements. Imperial and native Metric options are takeoff inputs, not recommendations for a particular load.
Does the estimate include lap splices?
Not explicitly. The waste allowance provides a general buffer, but required lap lengths depend on bar size, grade, concrete, location and design. Add splice length from the approved details.
How is rebar weight calculated?
Imperial mode multiplies waste-adjusted feet by pounds per foot for #3–#6. Metric mode multiplies waste-adjusted meters by kilograms per meter for the selected 10M–25M designation.
Are 10M, 15M, 20M and 25M used worldwide?
No. These are Canadian metric bar designations and are included as native Metric options for this calculator. Other countries use different diameter-based or national designations, so follow the project specifications.
Does this calculator work for footings?
It can estimate a rectangular single-layer grid area. Continuous strip footings often use longitudinal bars and stirrups rather than a simple two-way grid, so follow the actual footing reinforcement detail.
Does the cost include labor and accessories?
No. Optional cost is rebar footage multiplied by price per foot. It excludes fabrication, delivery, labor, chairs, tie wire, couplers and equipment.
Can I use this for two rebar mats?
Run the estimate for one mat and multiply the takeoff as required, or calculate each mat separately when layouts differ. The calculator itself assumes one layer.
This calculator provides a preliminary material takeoff only and does not design concrete reinforcement. Bar designations and standards vary by country, and the included 10M–25M options are not universal worldwide. Required size, spacing, grade, cover, lap splices, hooks, development length and placement depend on structural loads, concrete design, soil, exposure, approved drawings and applicable code. Confirm the reinforcing schedule with a licensed design professional or the project’s structural documents before purchasing or installation.