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The bar's development length, computed before the top-bar and coating factors are applied.
Take it from the development length calculation for the same bar, concrete and cover, with the top-bar and epoxy factors left at unity — this page applies those below, so entering an ld that already carries them would count them twice. Everything else that belongs in ld, including the lightweight factor and the confinement term, stays in the number you enter.
The bar being spliced, which fixes the diameter used by the compression formula.
Bar size does not enter the tension lap at all — that is carried entirely by the development length you entered. It matters for the compression splice, which is a direct function of bar diameter rather than of ld.
Which force the splice has to transfer at the section it sits in.
The two are computed by unrelated rules. A tension lap is a multiple of the development length and responds to class, coating and casting position; a compression lap is a direct multiple of bar diameter and ignores all three. A bar in a column that sees tension under any load combination is spliced as a tension bar.
How much surplus area the section carries at the splice location.
One of the two conditions for the shorter Class A splice. It is evaluated at the splice, not at the point of maximum moment, which is why moving a splice toward a low-moment region can turn a Class B lap into a Class A one and shorten the bar.
What share of the bars at the section have their splice inside the same lap window.
The second Class A condition, and the one a detailer controls directly. Staggering alternate bars puts half of them outside the window, which with enough surplus steel is what earns the shorter lap. Splicing everything at one line is always Class B.
Whether a deep lift of fresh concrete is placed beneath this bar.
Bleed water and air rise through a deep pour and collect under horizontal bars near the top, leaving a weaker bond there. The code answers with a flat 1.3 multiplier. It applies to the casting position of the bar, not to where the bar sits in the finished member.
Whether the bar is epoxy-coated, and if so how tightly it is covered and spaced.
Coating reduces bond, and the penalty is heavier where the concrete around the bar is thin enough that splitting governs. The two coated cases are 1.5 and 1.2; combined with the top-bar factor the product is capped at 1.7, so a coated top bar does not pay both penalties in full.
The specified yield strength of the reinforcement.
Used only by the compression branch, where the formula changes form above Grade 60. Grade 60 is the ordinary US reinforcing bar; the two compression expressions happen to meet exactly at that grade, so there is no step in the answer as you cross it.
The specified compressive strength of the concrete at the splice.
Also used only by the compression branch, where a weak mix earns a one-third increase in the lap. In tension the concrete strength is already inside the development length you entered above, which is why it does not act twice here.
Required lap splice length
31.2 in
Class B applies, at 1.3 times the development length. Either the surplus steel is under two, or more than half the bars splice within one lap window — the breakdown shows which lever is available if a shorter lap is wanted.
- Splice class multiplier applied
- 1.3 x ld
- Combined top-bar and coating factor
- 1 ld
- Minimum offset between adjacent splices
- 31.2 in
- Development length used
- 24 in
- Bar diameter
- 0.63 in
- Tension lap for this bar
- 31.2 in
- Compression lap for this bar
- 18.75 in
They open the calculator with your figures already in it
Rebar Lap Splice Length Calculator (ACI 318): 31.2 in — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- ACI 318 Building Code Requirements for Structural Concrete, tension lap splices: Class A is 1.0 x ld and Class B is 1.3 x ld, with 12 in as the floor for either. Class A applies only where the steel provided is at least twice the steel required AND no more than half the bars are spliced within the required lap length; every other case is Class B
- ACI 318 modification factors: the top-bar factor psi_t is 1.3 where more than 12 in of fresh concrete is cast below the bar, the epoxy factor psi_e is 1.5 where cover is under 3db or clear spacing under 6db and 1.2 otherwise, and their product is capped at 1.7
- ACI 318 compression lap splices: 0.0005 x fy x db for fy up to 60,000 psi and (0.0009 x fy - 24) x db above it, increased by one third where f'c is below 3,000 psi, and never less than 12 in. The splice class does not apply in compression
- The stagger reported is the lap length itself, derived from the code's own wording rather than from a published dimension: the percentage limit counts bars spliced WITHIN the required lap length, so offsetting a neighbour by one full lap puts it outside that window
Inputs used
- Tension Development Length ld (factors at 1.0)
- 24 in
- Bar Size
- #5 (5/8 in, 15.9 mm)
- Splice in Tension or Compression
- Tension — the usual case in beams, slabs and walls
- Steel Provided Divided by Steel Required at the Splice
- 1.5
- Percentage of Bars Spliced Within One Lap Length
- 100
- Top-Bar Condition
- No — 12 in (300 mm) or less of fresh concrete cast below the bar
- Epoxy Coating
- Uncoated or zinc-coated bar
- Steel Yield Strength fy
- 60000 psi
- Concrete Strength f'c
- 4000 psi
Intermediate steps
- Splice class multiplier applied
- 1.3 x ld
- Combined top-bar and coating factor
- 1 ld
- Minimum offset between adjacent splices
- 31.2 in
- Development length used
- 24 in
- Bar diameter
- 0.63 in
- Tension lap for this bar
- 31.2 in
- Compression lap for this bar
- 18.75 in
Confidence note: Class B applies, at 1.3 times the development length. Either the surplus steel is under two, or more than half the bars splice within one lap window — the breakdown shows which lever is available if a shorter lap is wanted.
What this calculation does not cover
- Mechanical and welded splices follow separate provisions and are not covered.
- Column compression laps enclosed by ties or spirals meeting the code's requirements may be reduced further; that reduction is not applied here.
- Bundled bars, and splices in tension tie members, carry additional requirements this page does not test.
- Bar spacing and cover at the splice must still satisfy the code independently of the length.
Add the equipment this sizes
This result is a specification — 31.2 in — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-08-30 · in the site-wide review of 2026-09-06 · v1.0.0
Regulatory standards & verification citations4
- ACI 318 Building Code Requirements for Structural Concrete, tension lap splices: Class A is 1.0 x ld and Class B is 1.3 x ld, with 12 in as the floor for either. Class A applies only where the steel provided is at least twice the steel required AND no more than half the bars are spliced within the required lap length; every other case is Class B
- ACI 318 modification factors: the top-bar factor psi_t is 1.3 where more than 12 in of fresh concrete is cast below the bar, the epoxy factor psi_e is 1.5 where cover is under 3db or clear spacing under 6db and 1.2 otherwise, and their product is capped at 1.7
- ACI 318 compression lap splices: 0.0005 x fy x db for fy up to 60,000 psi and (0.0009 x fy - 24) x db above it, increased by one third where f'c is below 3,000 psi, and never less than 12 in. The splice class does not apply in compression
- The stagger reported is the lap length itself, derived from the code's own wording rather than from a published dimension: the percentage limit counts bars spliced WITHIN the required lap length, so offsetting a neighbour by one full lap puts it outside that window
Which documents these citations point at
Standards referenced: ACI 318 (American Concrete Institute, United States).
Cite this page
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Tools and safety for this job
To fix reinforcement. Generic types, no brands, no prices.
Protection this work requires
- Saws, grinders and breakers run above 85 dB, where hearing damage accumulates and does not recover: defenders or plugs for every cut, not just the long ones.
- Breakers and grinders cause permanent nerve damage: limit continuous trigger time, keep hands warm, and stop if fingers tingle or blanch.
- Boards, blocks and bagged material cause most lasting back injuries on small sites: two people or a lifter for full sheets, and never a bag on one shoulder up a ladder.
- Nailing, chiselling and cutting all throw fragments: glasses to EN 166 or ANSI Z87.1, and goggles rather than glasses overhead.
Show the 4 tools this job needsHide tools
Essential
Tape measure
Recommended
Rebar cutter and bender
Rebar tying tool
Optional
Angle grinder
Also needed as materials: cutting and diamond discs, tie wire.
what each concrete tool is for, and the spec that decides which to buy where one does.
Class B applies. Getting to Class A needs BOTH a steel ratio of two or more and no more than half the bars spliced in one window — at present the ratio is 1.5 and 100% splice together. Offset neighbouring splices by at least 31.2 in (792 mm) and they fall outside one another's lap window, which is the geometry the percentage limit is written against.