Prepare a preliminary slab takeoff
Estimate the grid length before checking engineered drawings and supplier stock lengths.
Rebar Calculator
Enter slab dimensions, spacing, cover, layers, and waste to estimate rebar counts and total purchase length.
How to Use Rebar Calculator
1
Enter Slab Dimensions
Use the overall rectangular slab length and width in feet.
2
Set the Grid
Enter the spacing and edge cover required by the approved project documents.
3
Review the Takeoff
Choose layers and waste, then compare installed and rounded purchase length.
Rebar Calculator Features
- Live rectangular grid estimate
- Directional bar counts for one or two layers
- Adjustable spacing, cover, and waste
- Installed length separated from purchase length
Common Use Cases
Compare grid spacing scenarios
See how a different maximum spacing changes the number and length of bars.
Limits and Important Notes
Result type: Planning estimate
Calculation Details
- Subtract twice the edge cover from slab length and width.
- Divide each clear perpendicular dimension by maximum spacing, round intervals up, and add one bar.
- Multiply each directional count by its clear run length and number of layers.
- Add waste and round purchase length up to the next whole foot.
Formula
Bars across a direction = ceil(clear perpendicular dimension ÷ maximum spacing) + 1; total length = directional bars × clear run length × layers
Rounding
Bar counts and purchase feet round up; installed length displays to one decimal foot.
Default Assumptions
- Bars form an evenly spaced orthogonal grid within the entered edge cover.
- The entered spacing is a maximum, so the number of spaces rounds up.
- Laps, hooks, chairs, supports, openings, thickened edges, and localized reinforcement are excluded.
Verified Example
Inputs: 20 ft by 10 ft slab, 18 in maximum spacing, 3 in cover, one layer, 10% waste.
Calculation: 8 lengthwise bars plus 14 widthwise bars require 289 installed ft; 289 × 1.10 = 317.9 ft.
Result: Purchase estimate = 318 linear ft of rebar.
Sources and Version
Version: Rectangular orthogonal grid planning model
Last verified: 2026-08-11
Update responsibility: OnlineTool.me
- Placing Reinforcing Bars, 10th Edition — Concrete Reinforcing Steel Institute (accessed 2026-08-11)
- Frequently Asked Questions: Bar Spacing — American Concrete Institute (accessed 2026-08-11)
- This is a material takeoff estimate, not structural design advice. An engineer or approved drawing must specify bar size, grade, spacing, cover, layers, development, and laps.
- The estimate does not convert length into bar count by supplier stock size or calculate weight and cost.
- Openings, irregular boundaries, footings, beams, dowels, supports, and local code requirements need separate treatment.
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