Rebar Lap Splice Length
How far two bars overlap where one ends and the next begins. The field default of 40 bar diameters as a chart, what the code's tension lap splices actually come to, and a calculator for any bar size and multiplier.
The 40-diameter rule
A lap is specified as a multiple of the bar diameter, and 40 diameters is the number most slab drawings, most inspectors, and most rodbusters reach for when nothing more specific is written down: a #4 laps 20″, a #5 laps 25″. It is also the lap RebarCalc seeds for a new slab. The chart gives the common multiples for every size that can be lap spliced, rounded to the nearest inch.
| Size | Diameter (in) | 30d | 40d | 50d | 60d |
|---|---|---|---|---|---|
| #3 | 0.375 | 11 | 15 | 19 | 23 |
| #4 | 0.500 | 15 | 20 | 25 | 30 |
| #5 | 0.625 | 19 | 25 | 31 | 38 |
| #6 | 0.750 | 23 | 30 | 38 | 45 |
| #7 | 0.875 | 26 | 35 | 44 | 53 |
| #8 | 1.000 | 30 | 40 | 50 | 60 |
| #9 | 1.128 | 34 | 45 | 56 | 68 |
| #10 | 1.270 | 38 | 51 | 64 | 76 |
| #11 | 1.410 | 42 | 56 | 71 | 85 |
#14 and #18 bars are not on the chart because the code does not allow them to be lap spliced; they are mechanically or welded spliced.
What the code actually requires
The drawings govern. A lap on a set of stamped drawings was chosen by the engineer of record for that concrete, that bar, and that location, and it is the number an inspector will check. What follows is where those numbers come from, so a 40d default can be judged against them.
ACI 318 sets a tension lap splice from the bar's development length — the embedment it needs to develop its full strength — and almost every splice in a slab or wall is a Class B splice, 1.3 times that length. The development length grows with the bar size and the steel grade and shrinks with the square root of the concrete strength. For the ordinary case — Grade 60, uncoated, normalweight concrete, bottom bars, at least a bar diameter of cover and two of clear spacing — the Class B lap works out as below, rounded up to the inch.
| Size | 3,000 psi | 4,000 psi | 5,000 psi |
|---|---|---|---|
| #3 | 22 | 19 | 17 |
| #4 | 29 | 25 | 23 |
| #5 | 36 | 31 | 28 |
| #6 | 43 | 37 | 34 |
| #7 | 63 | 54 | 49 |
| #8 | 72 | 62 | 56 |
| #9 | 81 | 70 | 63 |
| #10 | 91 | 79 | 71 |
| #11 | 101 | 87 | 78 |
Read against the 40d chart: at 4,000 psi a Class B splice is about 49 diameters for #6 and smaller and about 62 diameters for #7 and larger, so 40d is short of the code's tension splice for structural steel and roughly right only where the drawings say so or the bar is not carrying calculated tension. The things that push the number up from the table:
- Top bars — more than 12″ of fresh concrete cast below the bar — take 1.3 times the length.
- Epoxy-coated bars take 1.5 times, or 1.2 with generous cover and spacing (at least 3 diameters of cover and 6 of spacing).
- Tight spacing or cover — under a diameter of cover, or less than two of clear spacing without ties — pushes the length up by half.
- Lightweight concrete divides by 0.75; Grade 80 bars multiply by 1.15.
- A lap is never less than 12″. Compression laps are shorter — 30 diameters for Grade 60, also not under 12″.
Slab on grade
Steel in a driveway, patio or shop floor is usually there to hold cracks tight, not to carry calculated tension, and slab-on-grade specs tend to call out a fixed lap — 24″ for #4 is common — or a multiple like 30d or 40d. When the plan gives a number, that is the lap; when it gives nothing, 40 diameters is the convention an inspector will not argue with.
Laps cost steel
Every splice adds a full lap to the bar that has to be bought: a 50′ run of #4 from 20′ stock is three pieces and two 20″ laps, so 53′-4″ of bar, not 50′. Over a mat that adds up, and it is why a takeoff from the plan comes out short of the cut list.