HesapHanem

Rebar Lap Length Calculator

Enter the bar diameter and lap factor to size the splice.

How it is calculated

The lap length is a simple multiple of the bar diameter: lap = d × n, where d is the bar diameter in millimetres and n is the lap factor, the number of bar diameters you overlap. The result is shown in millimetres (rounded to a whole number) and in centimetres (mm ÷ 10, two decimals), so 16.5 mm × 44.5 shows 734 mm but 73.43 cm. The factor is echoed back as you typed it, with a decimal such as 37.5× kept. Both inputs must be above zero, otherwise no result is shown. A factor of 40 is a common rule of thumb for tension splices, but the factor is entirely your choice. The tool does not look up concrete strength, steel grade, bar coating, spacing, cover or splice class, and it does not follow any particular code table. Use it for quick sizing and estimating, and take the final lap length from the design drawings or the governing code.

Worked example

A 20 mm bar is to be lapped with a factor of 45 diameters. Lap length = 20 × 45 = 900 mm, which is 90.00 cm. So the two bars must overlap by 0.9 m. With a factor of 40 the same bar would lap 800 mm, so the larger factor adds 100 mm per splice. For estimating steel, add 900 mm of extra bar for every splice of this bar.

Try this example →

Common mistakes

  • Entering the diameter in centimetres or inches. The field is in millimetres, so a 20 mm bar must be typed as 20, not 2 or 0.79.
  • Using the same factor for everything. The required factor depends on the code, concrete, steel grade and bar position, and compression or top bars can need a different value.
  • Treating the result as a code-compliant value. The tool is a multiplier, not a code calculation, so the drawing or specification must govern.
  • Forgetting that a lap adds bar length. Each splice consumes about one lap length of extra steel, which matters in the quantity take-off.
  • Mixing bar sizes in one splice. When two different diameters are lapped, the design rules decide which diameter the lap is based on; entering one of them blindly may be unsafe.

About this calculator

Reinforcing bars can't be manufactured or shipped in infinite lengths, so where two bars must continue as one, they are overlapped by a calculated lap (splice) length so that force transfers from one bar to the other through the surrounding concrete's bond strength rather than relying on a mechanical connector. ACI 318, the governing US concrete design code, specifies tension lap lengths as a function of bar diameter, concrete strength, bar spacing/cover, and splice class (Class A or the more conservative Class B, used when more than half the bars are spliced at one location).

Because the full code calculation involves several factors, field practice commonly uses a simplified rule of thumb — a lap length of roughly 40 times the bar diameter (40d_b) for typical tension splices in normal-strength concrete — as a quick sizing check before a detailed engineering calculation. This calculator applies a bar diameter and a chosen lap factor (40× by default) to size the splice for planning and estimating purposes.

Was this helpful?

You can pick up to 3 reactions.

Comments (1)

  • Editorial team

    This tool multiplies whatever factor you type, so the result is only as good as the factor. A 16 mm bar at 40 diameters gives 640 mm, and a 25 mm bar at 40 gives 1,000 mm, but a required lap can change with bar size, concrete grade, coating and spacing. Take the factor from the structural drawings or general notes, not from habit. Which lap factors do your project notes specify for the larger bars?

Popular calculators

Loan Calculator

Monthly payment, total interest and full amortization schedule.

Percentage Calculator

Percent change, percent of, and ratio calculations.

Discount Calculator

Calculate final price, savings, and discount amount.

BMI Calculator

Body mass index from height and weight (metric or imperial).

Age Calculator

Compute your exact age in years, months and days from a birth date.

Compound Interest Calculator

Calculate the final amount and interest earned with compounding.

All Calculators