A rebar grid is two sets of parallel bars. The count depends on spacing and on how far in from the edge the grid starts.
Rebar calculator
Enter the slab size, bar size and spacing. Add a lap length if bars must be spliced.
Enter your numbers to see the result.
How it works
Bars in each direction = (span / spacing) + 1, after taking the edge distance off both sides. Total length is the bars in each direction times their length. If a bar is longer than the stock length you buy, the calculator adds the lap you enter at each splice.
Bar weights (ASTM A615 nominal)
| Bar | Diameter | lb per ft | 20 ft bar |
|---|---|---|---|
| #3 | 3/8 in | 0.376 | 7.5 lb |
| #4 | 1/2 in | 0.668 | 13.4 lb |
| #5 | 5/8 in | 1.043 | 20.9 lb |
| #6 | 3/4 in | 1.502 | 30.0 lb |
What to check
Spacing, bar size, edge cover and lap length all come from your plans or local code. The calculator does the counting; it does not decide how much steel a slab needs.
Sources
Sources last checked 1 November 2026.
Frequently asked questions
How much does #4 rebar weigh?
0.668 lb per foot, so a 20 ft bar weighs about 13.4 lb.
How many 20 ft bars do I need for a 24 x 24 slab at 18 in?
Run the calculator: with 3 in edge distance it is 17 bars each way of about 23.5 ft, so splices are needed with 20 ft stock.
What size is #4 rebar?
1/2 in. The bar number is its diameter in eighths of an inch.