Rebar Lap Length Calculator
This rebar lap length calculator works out how far two reinforcing bars must overlap to pass their full force from one to the other. Choose Eurocode 2 or ACI 318, enter the bar diameter, the concrete strength, the steel grade, the bond condition, and the share of bars lapped at one place, and the rebar lap length calculator returns the lap length in millimeters, inches, and bar diameters, together with the anchorage or development length.
Results
The Rebar Lap Length Calculator is part of our Rebar & Steel collection. Browse more rebar & steel to size, convert, and check your work alongside this tool.
What the rebar lap length calculator does
Reinforcing bars come in fixed lengths, so long members need bars spliced end to end. In a lap splice the two bars sit side by side, and the concrete between them transfers the force through bond along the overlap. The lap must be long enough for the bond stress to carry the full bar force. The rebar lap length calculator applies the basic bond equations of Eurocode 2 and the simplified development length of ACI 318 so you can check a drawing or plan a cut list.
Lap length formula
Eurocode 2 (EN 1992-1-1, 8.4 and 8.7) starts from the design bond strength:
η1 is 1.0 for good bond and 0.7 for poor bond, η2 is 1.0 up to 32 mm bars. The lap length multiplies by α6: 1.0 with up to 25% of bars lapped at one section, 1.15 at 33%, 1.4 at 50%, and 1.5 above that, with a minimum of 15φ and 200 mm. ACI 318 (25.4.2.3) gives the development length in SI units as ld = fy ψt ÷ (2.1 √f'c) × db for bars up to 19 mm and with 1.7 instead of 2.1 for larger bars, where ψt = 1.3 for top bars; a Class B lap is 1.3 ld, at least 300 mm.
Rebar lap length calculator example
A 16 mm B500 bar in C25/30 concrete, good bond, 50% of bars lapped at one section, to Eurocode 2:
The same size bar, a #5 Grade 60 in 30 MPa concrete, needs a 585 mm development length and a 760 mm Class B lap under ACI 318.
How to use the lap length calculator
Choose the design code your project uses. Enter the bar diameter in millimeters (a #5 bar is 16 mm), pick the concrete class and steel grade, and choose good bond for bottom bars and thin slabs or poor bond for top bars in members deeper than about 250 mm. For Eurocode 2, pick how many bars are lapped at the same section; staggering laps shortens them. Press Calculate.
Reading the lap length of rebar
The result assumes the bar is fully stressed, straight, and without credit for confinement from cover or links, which is on the safe side. A detailed Eurocode check with α1 to α5 can shorten the lap, while poor bond, high strength steel, or epoxy coating lengthen it. Always follow the lap lengths on the structural drawings when they are given; this calculator is for checking and planning, and the engineer of record decides. Stagger laps where you can, and never lap all bars at the same section of a heavily loaded member.
When to use a development length calculator
- Checking lap lengths on reinforcement drawings
- Planning a bar bending schedule and cut lengths
- Estimating extra steel for laps in a quote
- Comparing Eurocode 2 and ACI 318 requirements
- Training site crews on bond and splice rules
Common rebar lap length calculator mistakes
- Using good bond for top bars. Top bars in deep members have poor bond.
- Lapping all bars at one section. It raises the lap factor.
- Forgetting the minimum. Laps are never shorter than 15φ or 200 mm.
- Lapping very large bars. Use couplers where codes limit laps.
Tips for lap length of rebar on site
- Stagger laps by at least 1.3 times the lap length.
- Tie lapped bars together so they stay in contact.
- Mark lap zones on the bar bending schedule.
Rebar overlap length and the bar schedule
Every lap adds a length of steel. In a 20 m slab run with 12 m stock bars, each bar line needs at least one lap, which for 16 mm bars adds about 0.9 m per line. Over a large slab that can add 5 to 10% to the steel, so include laps when you count bars with the rebar calculator and convert the total length to weight with the rebar weight calculator. Mechanical couplers avoid laps on large bars and in congested columns, at a higher cost per joint.
Lap length Eurocode 2 vs ACI lap splice length
The two codes reach similar results for small bars but differ in detail. Eurocode 2 links bond strength to the concrete tensile strength and rewards staggered laps with a lower α6, while ACI 318 uses the square root of the compressive strength and splits laps into Class A and Class B. Both lengthen top bars, because water and air collect under them as fresh concrete settles. Concrete strength matters in both: moving from C20/25 to C30/37 shortens Eurocode laps by about a quarter, so check the class you order with the concrete calculator.
Lap length chart: Eurocode 2, B500, good bond, 50% lapped
| Bar | C20/25 | C25/30 | C30/37 | C40/50 |
|---|---|---|---|---|
| 8 mm | 545 mm | 455 mm | 410 mm | 325 mm |
| 10 mm | 680 mm | 565 mm | 510 mm | 410 mm |
| 12 mm | 815 mm | 680 mm | 610 mm | 490 mm |
| 16 mm | 1085 mm | 905 mm | 815 mm | 650 mm |
| 20 mm | 1355 mm | 1130 mm | 1015 mm | 815 mm |
| 25 mm | 1695 mm | 1410 mm | 1270 mm | 1015 mm |
| 32 mm | 2165 mm | 1805 mm | 1625 mm | 1300 mm |
ACI lap splice length: Grade 60, 30 MPa (4,350 psi)
| Bar size | Development length | Class B lap, bottom bars | Class B lap, top bars |
|---|---|---|---|
| #3 (10 mm) | 370 mm | 475 mm (18.7 in) | 620 mm |
| #4 (13 mm) | 475 mm | 620 mm (24.3 in) | 805 mm |
| #5 (16 mm) | 585 mm | 760 mm (29.9 in) | 990 mm |
| #6 (19 mm) | 695 mm | 905 mm (35.5 in) | 1175 mm |
| #7 (22 mm) | 995 mm | 1295 mm (50.8 in) | 1680 mm |
| #8 (25 mm) | 1130 mm | 1470 mm (57.7 in) | 1910 mm |
Rebar Lap Length Calculator FAQ
How do you calculate rebar lap length?
Under Eurocode 2, divide the bar force per unit perimeter by the design bond strength and multiply by the lap factor α6. A 16 mm bar in C25/30 with 50% lapped needs about 905 mm.
Is lap length 40 times the bar diameter?
That is a common rule for anchorage in C25/30. Laps are often 50 to 60 diameters once the lap factor and bond condition are applied.
What is the difference between lap length and development length?
Development length anchors one bar into concrete; lap length is the overlap that passes force between two bars and is usually 1.0 to 1.5 times longer.
What is a Class B lap splice?
In ACI 318, a lap of 1.3 times the development length, used unless at least twice the needed steel is provided and no more than half the bars are spliced at once.