Lap splice lengths by bar size and concrete strength — one table, all the numbers
← Back to the calculatorA rebar lap (lap splice) is two bars overlapping side by side in concrete. The overlap transfers force from one bar to the other through the concrete around them — not by welding, not by a coupler, just by the bond between steel and concrete. The length of that overlap is a multiple of the development length ld: 1.0 ld for Class A and 1.3 ld for Class B.
This page is the table. The splice length page is the algorithm — how ld is computed, when Class A applies, how to handle top bars, epoxy, different-size transitions. If you need a number right now, scroll down. If you need to know where the number came from, go there.
Inches. Grade 60, uncoated, normal-weight concrete, bottom bars. Class A = 1.0 ld, Class B = 1.3 ld. Every value rounded up to the next whole inch, floored at 12 in.
| Bar | 3,000 psi | 4,000 psi | 5,000 psi | 6,000 psi | lb / lap* | laps / ton* | ||||
|---|---|---|---|---|---|---|---|---|---|---|
| A | B | A | B | A | B | A | B | |||
| #3 | 14 | 18 | 12 | 16 | 12 | 16 | 12 | 16 | 0.5 | 3,989 |
| #4 | 18 | 23 | 16 | 20 | 14 | 18 | 13 | 17 | 1.1 | 1,796 |
| #5 | 22 | 29 | 19 | 25 | 17 | 23 | 16 | 21 | 2.2 | 920 |
| #6 | 27 | 35 | 23 | 30 | 21 | 27 | 19 | 25 | 3.8 | 533 |
| #7 | 39 | 50 | 34 | 44 | 30 | 39 | 28 | 36 | 7.5 | 267 |
| #8 | 44 | 57 | 38 | 50 | 34 | 45 | 31 | 41 | 11.1 | 180 |
| #9 | 50 | 65 | 43 | 56 | 39 | 50 | 35 | 46 | 15.9 | 126 |
| #10 | 56 | 73 | 49 | 63 | 44 | 57 | 40 | 52 | 22.6 | 89 |
| #11 | 62 | 81 | 54 | 70 | 48 | 63 | 44 | 57 | 31.0 | 65 |
Three things the table makes obvious. Bar size dominates: going from #5 to #8 at 4,000 psi doubles the Class B lap (25 → 50 in), while going from 3,000 to 6,000 psi on the same #5 only cuts it from 29 to 21 in (28 per cent). The #6–#7 jump is the largest in the table: Class B at 4,000 psi goes from 30 to 44 in, a 47 per cent step, because ψs changes from 0.8 to 1.0 at #7. #14 and #18 are not in the table because ACI 318-19 25.5.1 prohibits lap splices for those sizes in tension.
The last two columns answer the question the table on every other site does not: what does a lap cost in steel? One Class B lap at 4,000 psi weighs 1.1 lb on #4 and 31.0 lb on #11. A ton of #4 bar absorbs 1,796 laps; a ton of #11 absorbs only 65.
| Bar | db (in) | wt (lb/ft) | Class B lap @ 4k (in) | lap (ft) | lb / lap | laps / ton |
|---|---|---|---|---|---|---|
| #3 | 0.375 | 0.376 | 16 | 1.33 | 0.5 | 3,989 |
| #4 | 0.500 | 0.668 | 20 | 1.67 | 1.1 | 1,796 |
| #5 | 0.625 | 1.043 | 25 | 2.08 | 2.2 | 920 |
| #6 | 0.750 | 1.502 | 30 | 2.50 | 3.8 | 533 |
| #7 | 0.875 | 2.044 | 44 | 3.67 | 7.5 | 267 |
| #8 | 1.000 | 2.670 | 50 | 4.17 | 11.1 | 180 |
| #9 | 1.128 | 3.400 | 56 | 4.67 | 15.9 | 126 |
| #10 | 1.270 | 4.303 | 63 | 5.25 | 22.6 | 89 |
| #11 | 1.410 | 5.313 | 70 | 5.83 | 31.0 | 65 |
On a 40 ft run using 20 ft stock, each bar has one splice. A #5 bar costs 2.2 lb of extra steel per lap — about 5 per cent of the 40 ft run's 41.7 lb total. On a 100-bar pour that is 217 lb of lap steel, or about one-ninth of a ton. The same pour in #8 costs 1,112 lb of lap steel — over half a ton — and that is the number that pushes a job toward couplers.
The rule of thumb "lap = 40 times the bar diameter" is printed on half the placing drawings in the country, and it works for the bars people actually use in laps: #3 through #6. Above #6 it falls short, and the shortfall grows with every size.
| Bar | db (in) | 40 × db (in) | Class B @ 4k (in) | Difference | Verdict |
|---|---|---|---|---|---|
| #3 | 0.375 | 15 | 16 | +1 in | close |
| #4 | 0.500 | 20 | 20 | 0 in | exact |
| #5 | 0.625 | 25 | 25 | 0 in | exact |
| #6 | 0.750 | 30 | 30 | 0 in | exact |
| #7 | 0.875 | 35 | 44 | +9 in | short |
| #8 | 1.000 | 40 | 50 | +10 in | short |
| #9 | 1.128 | 45 | 56 | +11 in | short |
| #10 | 1.270 | 51 | 63 | +12 in | short |
| #11 | 1.410 | 56 | 70 | +14 in | short |
The reason is ψs, the size factor in ACI 318-19 25.4.2.3. It is 0.8 for #6 and smaller and 1.0 for #7 and larger, so ld jumps 25 per cent at #7 on top of the diameter increase. The rule of thumb was calibrated on the small bars and never adjusted. A drawing that says "lap 40 db" on a #8 is 10 in short, and 10 in of bond is the difference between a bar that develops and one that pulls.
Class A is 1.0 ld (shorter). Class B is 1.3 ld (the default). Class A is allowed only when both hold: the steel provided is at least twice the steel required by analysis, and half or fewer of the bars are spliced within the lap length. Miss either one and it is Class B. In practice almost everything is Class B, because a section with twice the steel it needs was not sized by bending. Detailers default to Class B, and a drawing that does not say which class means Class B. Full explanation →
The table is for the ordinary case: Grade 60, uncoated, normal weight, bottom bars. When any of those is not true, the lap changes:
| Condition | Factor | Effect on lap | #4 Class B @ 4k example |
|---|---|---|---|
| Bottom bar (table default) | ψt = 1.0 | baseline | 20 in |
| Top bar (>12 in concrete below) | ψt = 1.3 | × 1.3 before class factor | 26 in |
| Uncoated (table default) | ψe = 1.0 | baseline | 20 in |
| Epoxy coated, normal cover | ψe = 1.2 | × 1.2 | 24 in |
| Epoxy coated, low cover | ψe = 1.5 | × 1.5 | 30 in |
| Normal weight (table default) | λ = 1.0 | baseline | 20 in |
| Lightweight concrete | λ < 1.0 | divides √f'c by λ | longer |
| #14 or #18 | — | no laps in tension | n/a |
When multiple conditions apply, the factors multiply. A #4 top bar with epoxy coating and low cover at 4,000 psi: ld × 1.3 (top) × 1.5 (epoxy low cover) × 1.3 (Class B) = a lap that is 2.5 times the table value. That is why the splice length calculator exists — the table covers the common case, and the calculator covers everything else.
Every lap is extra bar length. On a 40 ft run using 20 ft stock, each bar has one splice, and the lap length is the extra steel per bar.
| Bar | 40 ft run on 20 ft stock | Lap (Class B, 4k) | Extra ft/bar | Extra lb/bar | Extra on 100 bars |
|---|---|---|---|---|---|
| #4 | 1 splice | 20 in | 1.67 ft | 1.1 lb | 111 lb |
| #5 | 1 splice | 25 in | 2.08 ft | 2.2 lb | 217 lb |
| #6 | 1 splice | 30 in | 2.50 ft | 3.8 lb | 376 lb |
| #8 | 1 splice | 50 in | 4.17 ft | 11.1 lb | 1,112 lb |
| #11 | 1 splice | 70 in | 5.83 ft | 31.0 lb | 3,099 lb |
On #4 the laps cost 111 lb per 100 bars — barely a rounding error. On #8 the same 100-bar pour costs 1,112 lb of lap steel, more than half a ton. On #11 it is 3,099 lb, a ton and a half of overlap. At that point the question is whether laps make sense at all, and the answer is usually couplers. The lap-vs-coupler cutoff is not a number in the code; it is a number on the invoice, and this table is what puts it there.
For the tonnage on a full pour — bars, laps, ties, chairs — the rebar tonnage calculator adds it all up. For the cost per foot of the bar itself, see rebar price per foot.
20 in for Class B in 4,000 psi concrete, Grade 60, uncoated bottom bars. Class A is 16 in. In 3,000 psi: Class A 18 in, Class B 23 in. In 5,000 psi: Class A 14 in, Class B 18 in. In 6,000 psi: Class A 13 in, Class B 17 in. Class B is the default unless the steel provided is at least twice that required and half or fewer of the bars are spliced at that section.
25 in for Class B in 4,000 psi concrete, Grade 60, uncoated bottom bars. Class A is 19 in. In 3,000 psi: Class A 22 in, Class B 29 in. In 5,000 psi: Class A 17 in, Class B 23 in. In 6,000 psi: Class A 16 in, Class B 21 in.
30 in for Class B in 4,000 psi concrete, Grade 60, uncoated bottom bars. Class A is 23 in. In 3,000 psi: Class A 27 in, Class B 35 in. In 5,000 psi: Class A 21 in, Class B 27 in. In 6,000 psi: Class A 19 in, Class B 25 in.
50 in for Class B in 4,000 psi concrete, Grade 60, uncoated bottom bars. Class A is 38 in. In 3,000 psi: Class A 44 in, Class B 57 in. In 5,000 psi: Class A 34 in, Class B 45 in. In 6,000 psi: Class A 31 in, Class B 41 in. The jump from #6 to #7 is large because the size factor ψs steps from 0.8 to 1.0 at #7.
One Class B lap at 4,000 psi weighs: #4 = 1.1 lb, #5 = 2.2 lb, #6 = 3.8 lb, #8 = 11.1 lb, #11 = 31.0 lb. The weight is the lap length in feet times the bar's unit weight in lb/ft. Laps per ton of bar: #4 = 1,796, #5 = 920, #6 = 533, #8 = 180, #11 = 65.
A lap splice is one of three splice types. The other two are mechanical couplers and welded splices. A lap is the cheapest and most common, but it only works where the overlap length fits the detail. When a lap reaches 50 in or more (#8 and larger in typical concrete), couplers become the practical choice.
40 times the bar diameter is a common rule of thumb, and it works for #3 through #6. Above #6 it falls short: a #8 Class B lap is 50 in but 40 × db is only 40 in, a 10 in shortfall. The size factor steps from 0.8 to 1.0 at #7, which makes development length grow faster than diameter. Use the table instead of the rule for #7 and larger.
Yes. The coating factor ψe is 1.2 or 1.5 for epoxy coated bars, versus 1.0 uncoated. A #4 Class B lap at 4,000 psi goes from 20 in uncoated to 30 in at ψe = 1.5. The factor depends on cover and spacing: 1.5 when cover is less than 3 db or clear spacing is less than 6 db, 1.2 otherwise.