Concrete & Masonry
Rebar Calculator
Enter the slab size, the grid spacing and edge distance from your plans, the bar size and the stick length you can buy. You get bars to buy, linear feet and weight.
Worked example
- Bars run 19.5 ft and 19.5 ft after keeping 3 in from each edge.
- Along the width: 234 in / 18 in = 13 spaces, so 14 bars; along the length 14 bars.
- In place: 14 x 19.5 + 14 x 19.5 = 546 ft. Bars longer than 20 ft are spliced with a 20 in lap.
- That needs 28 bars of 20 ft, about 374 lb of #4.
Common sizes at a glance
| Size | 20 ft bars to buy | Bars in the grid |
|---|---|---|
| 10 x 10 ft, 18 in grid | 7 | 7 one way, 7 the other |
| 12 x 20 ft, 18 in grid | 22 | 8 one way, 14 the other |
| 20 x 20 ft, 16 in grid | 30 | 15 one way, 15 the other |
| 24 x 24 ft, 18 in grid | 64 | 16 one way, 16 the other |
| 30 x 40 ft, 12 in grid | 170 | 30 one way, 40 the other |
What this tool does and does not do
This calculator counts material for a grid layout you already have. It does not choose bar size or spacing for a load. Those come from your plans; the local building code and a licensed professional have the final word on reinforcement.
With the layout set, the job is simple counting, and mistakes there are what usually lead to a second trip to the supply yard.
Edge distance shortens every bar
Steel should not touch the forms, so each bar stops short of the edge. In the worked example, bars stay 3 in from each side, so on a 20 ft slab every bar is 19.5 ft long.
The edge distance also sets where the first and last bars sit. Across 234 in at an 18 in spacing there are 13 spaces, which means 14 bars, not 13. Forgetting that extra bar is a classic miscount.
Laps when bars are too short
When a run is longer than the sticks you can buy, two bars overlap and are tied together. The lap is usually stated as a number of bar diameters. At 40 diameters, a #4 bar (1/2 in) needs a 20 in lap.
Laps add up on big slabs. Stagger them so they do not all land in one line across the slab.
Stick length and hauling
Rebar is commonly sold in 10 ft and 20 ft sticks. Longer sticks mean fewer laps but are harder to carry home. If you can only haul 10 ft pieces, set that as the stick length and let the calculator add the extra laps.
The weight figure helps too. The worked example, 28 bars of 20 ft #4, comes to about 374 lb.
Small parts to add to the list
- Tie wire and a tie tool or pliers.
- Chairs or blocks to hold the grid at the right height in the slab.
- Cutting tool for trimming bars at the edges.
- Caps for exposed bar ends while you work.
Questions
How many bars for a 10 x 10 slab at 18 in?
With 3 in edge clearance, 7 bars each way, which is 7 sticks of 20 ft if you cut them in half.
Does the count include laps?
Yes. Runs longer than your stick length get a lap splice, and the extra length is added before bars are counted.
Where do I get the concrete amount?
Use the concrete slab calculator for the same slab, or the footing calculator for footings.
How this works: formulas and typical values are listed on our method page; every data source and its license is on sources.
Last reviewed November 2026.