Free estimate — verify against local code before building
Rebar Calculator
Inventory a specified rebar grid; this tool does not design the reinforcement.
What this calculator includes
Turn an already-specified slab reinforcement layout into a purchase estimate. Enter the slab dimensions, bar spacing, edge offset, bar size, stock length, and lap assumption to see counts in both directions and whole stock bars.
How to use this rebar calculator
- 01
Start with the specified layout
Get bar size, spacing, cover or edge offset, and lap requirements from approved plans or the responsible design professional.
- 02
Enter slab dimensions
Measure the full slab length and width; the edge offset is deducted on both ends of every grid run.
- 03
Match available stock
Enter the stock length your supplier sells and the specified lap so long runs can be divided into purchasable pieces.
- 04
Review both directions
Check bar counts in both directions. The estimate groups repeated cut lengths that fit on the selected stock bar, then adds waste and rounds to whole stock bars.
Worked example
Example: 20 × 10 ft slab
At a maximum 12 in spacing and 3 in edge offset, the grid has eleven 19.5 ft runs and twenty-one 9.5 ft runs. Two short runs fit on one 20 ft stock bar, giving 22 stock bars before waste and 25 bars after a 10% allowance.
Practical buying and overage guidance
Confirm the bar grade, coating, size, stock length, bends, and fabrication schedule with the plans and supplier. Do not substitute bar sizes or change spacing because a different option is easier to buy.
Continue the project
Pour a Concrete Slab
Plan slab excavation, base, concrete yards or bags, forms, reinforcement allowances, delivery, labor, equipment, and contractor pricing.
Open the project workflow →Calculation sources and review
Primary references and formula assumptions are linked so you can verify them against the selected product, supplier, and adopted local requirements.
Internal formula review completed July 13, 2026. What this review covers
- CRSI — Reinforcing Basics (opens in a new tab)
Industry reference for reinforcing bar identification and placement concepts.
- ASTM A615/A615M (opens in a new tab)
Specification covering common deformed carbon-steel reinforcing bars.
Frequently asked questions
Does this calculator tell me what rebar size or spacing to use?
No. It inventories a layout you enter. Bar size, spacing, cover, laps, development, supports, and concrete design must come from approved documents or qualified professional guidance.
How are bars counted across the slab?
The calculator subtracts the edge offset from both sides, divides the usable span by spacing, and includes a bar at the starting edge.
How does the overlap affect stock-bar quantity?
Each piece after the first contributes only its stock length minus the entered overlap. Longer overlaps can therefore require more stock pieces.
Why can order length exceed installed grid length?
Stock bars create lap lengths, cutoffs, and unused ends. The estimate groups repeated cuts that fit on one stock bar, then adds the waste allowance and rounds to whole bars.
Does estimated weight include chairs, ties, or supports?
No. Weight is stock-bar length times published nominal pounds per foot for the selected size. Chairs, tie wire, dowels, hooks, and fabrication are separate.