How this site is built
Review the calculation methodology to see which figures are exact geometry, which depend on material assumptions, and which sources have been verified. The source registry records every cited figure and its review status.
How the rebar calculator works
The calculator lays out two perpendicular directions. It divides each slab dimension by the entered maximum spacing, rounds up, and adds one run so bars occur at both sides of the grid. Each run is multiplied by the dimension it crosses.
Base linear feet are exact for that simplified edge-to-edge grid. The entered lap percentage is then added to give an adjusted planning quantity. A percentage does not replace a splice schedule; it is only a way to include a project-specific allowance supplied by the user.
This tool does not select bar size, grade, spacing, concrete cover, development length or splice length. Those are structural design and detailing decisions that must come from the drawings and specifications.
Rebar grid formula
Bar counts are rounded up so the entered maximum spacing is not exceeded.
- Bars running length
ceil(slab width ÷ spacing) + 1 - Bars running width
ceil(slab length ÷ spacing) + 1 - Base linear feet
length bars × slab length + width bars × slab width - Adjusted linear feet
base linear feet × (1 + lap allowance ÷ 100)
Rebar takeoff inputs
| Input | Use | Check |
|---|---|---|
| Slab length and width | Defines the rectangular grid | Use drawing dimensions |
| Bar spacing | Sets the maximum grid interval | Copy the structural schedule |
| Lap allowance | Adds entered splice length | Derive from the bar layout |
| Cover and edge offsets | Not calculated | Apply from the project details |
Bar count means continuous grid runs, not stock-length pieces. Convert runs to purchasable lengths from the cutting schedule.
Measure and order rebar
Use structural drawing dimensions and spacing rather than choosing values in the calculator. If concrete cover shortens the bar runs, use the bar schedule or subtract the specified edge offsets before preparing the final cutting list.
Stock-bar count depends on available lengths, permitted splice locations, lap lengths, hooks and offcut reuse. Prepare that count from the approved bar bending or cutting schedule after checking this grid quantity.
Rebar calculator questions
What does bar count mean here?
It is the number of continuous grid runs in both slab directions. It is not the number of stock-length pieces to purchase.
How should I choose bar spacing?
Do not choose it from this tool. Enter the spacing specified on the structural drawings or reinforcement schedule.
How much lap allowance should I add?
Use the approved splice details and bar layout. Lap length depends on the design, bar size, concrete and splice conditions, so there is no universal percentage.
Variables and units
For the default “Rectangular slab grid” mode. Other modes label their own fields in the calculator.
length- Slab length: ft, m.
width- Slab width: ft, m.
spacing- Bar spacing: in, cm.
lap- Lap allowance: %; optional.
Worked example: a 20 ft by 12 ft slab at 18 in spacing
A rectangular slab measures 20 ft by 12 ft. Bars run in both directions at no more than 18 in spacing, with 10% added for lap length.
| Field | Value |
|---|---|
| Slab length | 20 ft |
| Slab width | 12 ft |
| Bar spacing | 18 in |
| Lap allowance | 10 % |
| Result | Value |
|---|---|
| Rebar including lap allowance | 396 linear ft |
| Grid bar runs | 24 runs |
| Grid length before laps | 360 linear ft |
These figures are generated by the same calculation the calculator above uses, so they update automatically if the method changes.
Where these numbers come from
This calculator uses geometry and values shown in the form. Editable product dimensions, coverage figures, prices and allowances are inputs, not universal recommendations; confirm them against the product label, drawing or supplier quote.
Last updated:
Published by Material Totals. Review the methodology and source assumptions.
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