Rebar calculator
How many bars of rebar a slab grid takes at the spacing on your plans, with the laps, the weight and a cut-list count of stock bars to buy.
Your slab takes
40 bars, 20 ft
- Lengthwise
- 20 × 19.5 ft
- Crosswise
- 20 × 19.5 ft
- Lap splices
- 0 × 30 in.
- Steel in place
- 780 ft, 521 lb
- Weight bought
- 534 lb
- Estimated cost
Bars stop the edge distance short of every side. Runs longer than a bar are lapped by the IRC Table R608.5.4(1) length for the size and grade, and short pieces are cut from offcuts where they fit. Bar size and spacing come from your plans; follow them, your local code and your engineer.
How much rebar do I need for a slab?
A slab grid has bars running both ways at the same spacing, stopping a few inches short of the edges. Count the runs each way, splice any run longer than a bar, then work out how many stock bars that takes.
- Runs lengthwise = (width − 2 × edge distance) ÷ spacing, rounded down, + 1; each is length − 2 × edge distance long
- Runs crosswise = (length − 2 × edge distance) ÷ spacing, rounded down, + 1
- Laps: a run longer than a bar needs (run − bar) ÷ (bar − lap), rounded up
- Bars = full bars in the long runs + the short pieces, as many cut from each bar as fit
- Weight = feet of bar × pounds per foot for the size (table below)
Worked example
A 20 × 20 ft slab with #4 bars 12 in. apart and 3 in. from the edges: each run is 20 − 2 × 0.25 = 19.5 ft, and 19.5 ÷ 1 ft, rounded down, + 1 = 20 runs each way. Every run fits on one 20 ft bar, so that's 40 bars: 780 ft of steel in place, 521 lb at 0.668 lb per foot. Make it 24 × 24 ft at 18 in. and each 23.5 ft run needs a 30 in. lap: 32 laps and 44 bars.
Rebar weight chart
Nominal sizes and weights of standard ASTM bars, from Caltrans Bridge Design Details 13.1, Reinforcement (September 2025). From #3 to #8 the bar number is the diameter in eighths of an inch.
| Bar | Diameter | Area | lb per ft | 20 ft bar |
|---|---|---|---|---|
| #3 | 0.375 in. | 0.11 sq in. | 0.376 | 7.5 lb |
| #4 | 0.500 in. | 0.20 sq in. | 0.668 | 13.4 lb |
| #5 | 0.625 in. | 0.31 sq in. | 1.043 | 20.9 lb |
| #6 | 0.750 in. | 0.44 sq in. | 1.502 | 30.0 lb |
| #7 | 0.875 in. | 0.60 sq in. | 2.044 | 40.9 lb |
| #8 | 1.000 in. | 0.79 sq in. | 2.670 | 53.4 lb |
| #9 | 1.128 in. | 1.00 sq in. | 3.400 | 68.0 lb |
| #10 | 1.270 in. | 1.27 sq in. | 4.303 | 86.1 lb |
| #11 | 1.410 in. | 1.56 sq in. | 5.313 | 106.3 lb |
| #14 | 1.693 in. | 2.25 sq in. | 7.650 | 153.0 lb |
| #18 | 2.257 in. | 4.00 sq in. | 13.600 | 272.0 lb |
Lap splices and cover
From the 2021 IRC Section R608.5.4 and Table R608.5.4(1), written for concrete walls and used for footings; check what your plans and local code require.
| Bar | Lap, Grade 40 | Lap, Grade 60 |
|---|---|---|
| #4 | 20 in. | 30 in. |
| #5 | 25 in. | 38 in. |
| #6 | 30 in. | 45 in. |
Minimum cover: 3 in. for concrete cast against the earth; 1.5 in. (#5 and smaller) or 2 in. (#6 and larger) against forms exposed to earth or weather; 0.75 in. where not exposed.
Rebar questions
How much rebar do I need for my concrete slab?
It depends on the bar size and spacing on your plans. A 20 × 20 ft slab with #4 bars 12 in. apart both ways takes 40 bars 20 ft long, about 521 lb of steel. Enter your slab above; the calculator counts the laps and cuts the short pieces from offcuts.
How much does rebar weigh?
#3 bar weighs 0.376 lb per foot, #4 0.668, #5 1.043 and #6 1.502. A 20 ft #4 bar is 13.36 lb, which matches the weight Home Depot lists for its bar. The chart above covers #3 to #18.
How much should rebar overlap?
The IRC's lap splice table gives 30 in. for #4 Grade 60 bars (20 in. for Grade 40), 38 in. for #5 and 45 in. for #6. The table starts at #4; the calculator uses the #4 length for #3, which is more than a #3 needs. Bars at a lap can touch or sit apart, but no farther than one-fifth of the lap or 6 in., whichever is smaller.
How far from the edge should rebar be?
The IRC asks for 3 in. of concrete over steel in concrete cast against the earth, and 1.5 in. for #5 and smaller bars against forms exposed to weather. The calculator starts with 3 in. at the edges; set the bars on supports so they sit in the slab, not on the ground.
Grade 40 or Grade 60 rebar?
Use the grade your plans call for; it's marked on the bar. Grade 40 needs shorter laps in the IRC table, and the store bar this page uses for its stock length is Grade 40, so check before you buy for a job that calls for Grade 60.
How much does rebar cost?
Enter the price of one bar and the calculator multiplies it by the bars to buy. Chairs, tie wire and labor are extra; get quotes and use these quantities to compare them.