Lap splice lengths by bar size and concrete strength — both rows of the code 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. Every value here is the row of Table 25.4.2.3 that applies where the clear spacing is at least db and the cover is at least db with minimum ties along ld, or where the clear spacing is at least 2db and the cover is at least db. Outside those limits every number below is 1.5 times longer — that is the Other cases table at the end of this section. Class A = 1.0 ld, Class B = 1.3 ld, each taken as the greater of that value and 12 in, then rounded up once. The ld in those two multipliers is the expression value, before ld’s own 12 in floor: two separate gates, and the floor is never multiplied by 1.3.
| 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 | 17 | 22 | 15 | 19 | 13 | 17 | 12 | 16 | 0.6 | 3,360 |
| #4 | 22 | 29 | 19 | 25 | 17 | 23 | 16 | 21 | 1.4 | 1,437 |
| #5 | 28 | 36 | 24 | 31 | 22 | 28 | 20 | 26 | 2.7 | 742 |
| #6 | 33 | 43 | 29 | 37 | 26 | 34 | 24 | 31 | 4.6 | 432 |
| #7 | 48 | 63 | 42 | 54 | 38 | 49 | 34 | 45 | 9.2 | 217 |
| #8 | 55 | 72 | 48 | 62 | 43 | 56 | 39 | 51 | 13.8 | 145 |
| #9 | 62 | 81 | 54 | 70 | 48 | 63 | 44 | 57 | 19.8 | 101 |
| #10 | 70 | 91 | 61 | 79 | 54 | 71 | 50 | 64 | 28.3 | 71 |
| #11 | 78 | 101 | 67 | 87 | 60 | 78 | 55 | 71 | 38.5 | 52 |
Three things the table makes obvious. Bar size dominates: going from #5 to #8 at 4,000 psi doubles the Class B lap (31 → 62 in), while going from 3,000 to 6,000 psi on the same #5 only cuts it from 36 to 26 in (28 per cent). The #6–#7 jump is the largest in the table: Class B at 4,000 psi goes from 37 to 54 in, a 46 per cent step, because the tabulated divisor changes from 25 to 20 at #7. #14 and #18 are not in the table because ACI 318-19 prohibits lap splices for those sizes in tension.
The second row of Table 25.4.2.3, and the reason the first row is a row and not a formula. Same bar, same concrete, 1.5 times the length, because the concrete around the bar can no longer be counted on to confine it. The coefficient becomes 3/50 for #3 to #6 and 3/40 for #7 and up, which is exactly 1.5 times the 1/25 and 1/20 above. Nothing else changes: Class A is still 1.0 times the expression and Class B still 1.3, each floored at 12 in and rounded up once.
| 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 | 25 | 33 | 22 | 28 | 20 | 25 | 18 | 23 | 0.9 | 2,280 |
| #4 | 33 | 43 | 29 | 37 | 26 | 34 | 24 | 31 | 2.1 | 971 |
| #5 | 42 | 54 | 36 | 47 | 32 | 42 | 30 | 38 | 4.1 | 490 |
| #6 | 50 | 65 | 43 | 56 | 39 | 50 | 35 | 46 | 7.0 | 285 |
| #7 | 72 | 94 | 63 | 81 | 56 | 73 | 51 | 67 | 13.8 | 145 |
| #8 | 83 | 107 | 72 | 93 | 64 | 83 | 59 | 76 | 20.7 | 97 |
| #9 | 93 | 121 | 81 | 105 | 72 | 94 | 66 | 86 | 29.8 | 67 |
| #10 | 105 | 136 | 91 | 118 | 81 | 106 | 74 | 96 | 42.3 | 47 |
| #11 | 116 | 151 | 101 | 131 | 90 | 117 | 82 | 107 | 58.0 | 34 |
The coefficient is exactly 1.5, but the printed inches are not. Each column rounds up on its own, so the step from the table above reads anywhere from 1.47 to 1.52 depending on the bar — #5 at 4,000 psi goes 31 to 47 in (1.52), #7 goes 54 to 81 in (1.50). Use the number in the table, not 1.5 times the number in the table above.
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 1.4 lb on #4 and 38.5 lb on #11. A ton of #4 bar absorbs 1,437 laps; a ton of #11 absorbs only 52.
| Bar | db (in) | wt (lb/ft) | Class B lap @ 4k (in) | lap (ft) | lb / lap | laps / ton |
|---|---|---|---|---|---|---|
| #3 | 0.375 | 0.376 | 19 | 1.58 | 0.6 | 3,360 |
| #4 | 0.500 | 0.668 | 25 | 2.08 | 1.4 | 1,437 |
| #5 | 0.625 | 1.043 | 31 | 2.58 | 2.7 | 742 |
| #6 | 0.750 | 1.502 | 37 | 3.08 | 4.6 | 432 |
| #7 | 0.875 | 2.044 | 54 | 4.50 | 9.2 | 217 |
| #8 | 1.000 | 2.670 | 62 | 5.17 | 13.8 | 145 |
| #9 | 1.128 | 3.400 | 70 | 5.83 | 19.8 | 101 |
| #10 | 1.270 | 4.303 | 79 | 6.58 | 28.3 | 71 |
| #11 | 1.410 | 5.313 | 87 | 7.25 | 38.5 | 52 |
On a 40 ft run using 20 ft stock, each bar has one splice. A #5 bar costs 2.7 lb of extra steel per lap — about 6 per cent of the 40 ft run's 41.7 lb total. On a 100-bar pour that is 269 lb of lap steel, or about one-eighth of a ton. The same pour in #8 costs 1,380 lb of lap steel — nearly seven-tenths of 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 is short on every bar size in the table. Through #6 the shortfall is 4 to 7 in, which is why the rule has survived; from #7 it is 19 to 31 in, which is why it fails.
| Bar | db (in) | 40 × db (in) | Class B @ 4k (in) | Difference | Verdict |
|---|---|---|---|---|---|
| #3 | 0.375 | 15 | 19 | +4 in | short |
| #4 | 0.500 | 20 | 25 | +5 in | short |
| #5 | 0.625 | 25 | 31 | +6 in | short |
| #6 | 0.750 | 30 | 37 | +7 in | short |
| #7 | 0.875 | 35 | 54 | +19 in | short |
| #8 | 1.000 | 40 | 62 | +22 in | short |
| #9 | 1.128 | 45 | 70 | +25 in | short |
| #10 | 1.270 | 51 | 79 | +28 in | short |
| #11 | 1.410 | 56 | 87 | +31 in | short |
The reason is the divisor in ACI 318-19 Table 25.4.2.3. It is 25 for a #6 and smaller and 20 for a #7 and larger, so the tabulated length 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 22 in short, and 22 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, with clear spacing and cover over the limits in Table 25.4.2.3. When any of those is not true, the lap changes. Most of the rows below scale the answer by a factor; the clear-spacing-and-cover row does not scale it at all, it switches the divisor:
| Condition | Factor | Effect on lap | #4 Class B @ 4k example |
|---|---|---|---|
| Bottom bar (table default) | ψt = 1.0 | baseline | 25 in |
| Top bar (>12 in concrete below) | ψt = 1.3 | × 1.3 before class factor | 33 in |
| Uncoated (table default) | ψe = 1.0 | baseline | 25 in |
| Epoxy coated, normal cover | ψe = 1.2 | × 1.2 | 30 in |
| Epoxy coated, low cover | ψe = 1.5 | × 1.5 | 37 in |
| Normal weight (table default) | λ = 1.0 | baseline | 25 in |
| Lightweight concrete | λ < 1.0 | divides √f'c by λ | longer |
| Clear spacing below db, or cover below db | C = 3/50 or 3/40 | × 1.5 on the whole lap | 37 in |
| #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 | 25 in | 2.08 ft | 1.4 lb | 139 lb |
| #5 | 1 splice | 31 in | 2.58 ft | 2.7 lb | 269 lb |
| #6 | 1 splice | 37 in | 3.08 ft | 4.6 lb | 463 lb |
| #8 | 1 splice | 62 in | 5.17 ft | 13.8 lb | 1,380 lb |
| #11 | 1 splice | 87 in | 7.25 ft | 38.5 lb | 3,852 lb |
On #4 the laps cost 139 lb per 100 bars — barely a rounding error. On #8 the same 100-bar pour costs 1,380 lb of lap steel, nearly seven-tenths of a ton. On #11 it is 3,852 lb, close to two tons 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.
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. 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.
31 in for Class B in 4,000 psi concrete, Grade 60, uncoated bottom bars. Class A is 24 in. In 3,000 psi: Class A 28 in, Class B 36 in. In 5,000 psi: Class A 22 in, Class B 28 in. In 6,000 psi: Class A 20 in, Class B 26 in.
37 in for Class B in 4,000 psi concrete, Grade 60, uncoated bottom bars. Class A is 29 in. In 3,000 psi: Class A 33 in, Class B 43 in. In 5,000 psi: Class A 26 in, Class B 34 in. In 6,000 psi: Class A 24 in, Class B 31 in.
62 in for Class B in 4,000 psi concrete, Grade 60, uncoated bottom bars. Class A is 48 in. In 3,000 psi: Class A 55 in, Class B 72 in. In 5,000 psi: Class A 43 in, Class B 56 in. In 6,000 psi: Class A 39 in, Class B 51 in. The jump from #6 to #7 is large because the tabulated divisor changes from 25 to 20 at #7.
One Class B lap at 4,000 psi weighs: #4 = 1.4 lb, #5 = 2.7 lb, #6 = 4.6 lb, #8 = 13.8 lb, #11 = 38.5 lb. The weight is the lap length in feet times the bar's unit weight in lb/ft. Laps per ton of bar: #4 = 1,437, #5 = 742, #6 = 432, #8 = 145, #11 = 52.
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 60 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 is short on every bar size. Through #6 the shortfall is 4 to 7 in, which is why the rule has survived; from #7 it is 19 to 31 in. A #8 Class B lap is 62 in but 40 × db is only 40 in, a 22 in shortfall. The tabulated divisor changes from 25 to 20 at #7, which makes development length grow faster than diameter. Use the table instead of the rule.
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 25 in uncoated to 37 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.
No — but how the splice is detailed decides the number, and one dome builder's field rule shows it plainly: “With #4 rebar, if the bar splice will touch we do a 44″ overlap. If it does not touch, we do an 18″ overlap” (monolithic.org). For a #4 those are 88×db and 36×db — both far from the tabulated 25 in Class B on this page, because shotcrete dome work has its own detailing conditions. The table above assumes the standard case: bars held in the same plane with normal cover and spacing. If your splices float apart from the bars they partner with, or the bars are placed on a curved or formed surface, follow the project's detailer — the tabulated number is for the standard detail, not every detail.