Rebar Calculator
Bar count, total length, and sticks for slab grids.
What it does
Slab steel forms a grid: bars running the length spaced across the width, plus bars running the width spaced across the length. Each direction needs spans divided by spacing plus one edge bar, and total length converts to stock sticks with laps extra.
The tool above takes slab size and spacing in imperial or metric. It returns bars each way, total bar length, stock sticks, and cost at your stick price.
Spacing follows loads and codes, with eighteen inches common for residential slabs and tighter grids for heavy or structural work. Bar size, usually #4 half inch for homes, is a separate structural choice this count does not make.
Cover protects steel from rust that cracks concrete from inside. Two to three inches from soil faces keeps water and air off the bar for decades.
How to use
- Switch the unit system first: imperial for feet, metric for meters.
- Enter slab length and width.
- Enter bar spacing from the plan.
- Buy sticks plus laps, chairs, and a few spares.
Formula
- span / spacing
- Intervals across each direction.
- + 1
- Edge bar closing each direction.
- 20 ft / 6 m
- Stock stick lengths per system.
Lap splices overlap bars by diameters per code, adding length the grid math misses. Chairs hold the grid at mid slab height during the pour.
Worked example
Example: 40 x 30 ft slab, 18 in grid, $8 sticks
Slab 40 by 30, bars every 18 inches.
- Length way: 360 / 18 = 20, plus 1 = 21 bars of 40 ft. That is 840 ft.
- Width way: 480 / 18 = 26.67, round up to 27, plus 1 = 28 bars of 30 ft. That is 840 ft.
- Total: 1,680 ft of bar.
- Sticks: 1680 / 20 = 84 sticks.
- Cost: 84 x $8 = $672.00 before laps and chairs.
Reference table
| Spacing | Bars | Sticks (20 ft) |
|---|---|---|
| 12 in | 72 bars | 124 sticks |
| 18 in | 49 bars | 84 sticks |
| 24 in | 37 bars | 64 sticks |
Limits
- Counts only. Bar size is structural, not computed.
- Laps, hooks, and chairs excluded from length.
- Openings need trimming bars separately.
- Codes vary. Plans beat calculators for structure.
Common mistakes
- Forgetting the plus one edge bar each way.
- Skipping lap lengths in the order.
- Laying steel on the ground with no chairs.
- Wrong bar size for the loads involved.
- Ignoring cover that prevents rust jacking.
Try it yourself
Quick checks. Answers sit tucked under each question.
20 x 20 ft slab at 12 in spacing. Bars?
Each way ceil(240/12)+1 = 21. So 21 + 21 bars.
Total length for those?
21 x 20 + 21 x 20 = 840 ft.
Questions
What rebar spacing for a slab?
Eighteen inches suits many homes. Heavy loads and codes tighten it.
What size rebar for driveways?
Commonly #4 half inch. Engineers size structural work individually.
How long are rebar sticks?
Twenty feet in the US, six meters metric commonly.
Do I need chairs?
Yes. Steel belongs mid slab, not on the ground.
What is lap splice?
Overlapping bars to continue length, sized in bar diameters per code.
Mesh instead of rebar?
Works for light slabs. Heavy or structural slabs want bars.