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Rebar Lap Length Calculator

Work out rebar lap length instantly with clear inputs, formula shown and shareable results.

About Rebar Lap Length Calculator

A lap splice transfers force from one bar to the next through bond with the concrete, so the length needed scales with bar diameter and falls as concrete strength rises. Tension laps are longer than compression laps because the bar end cannot bear directly on the concrete. Codes also impose an absolute floor of 24 diameters or 200 mm.

Formulas

Lap length

Lap = factor x bar diameter

factor
Tension 50/45/40 and compression 40/36/32 for M20/M25/M30

Minimum

Lap not less than 24 x diameter or 200 mm

Worked example

With the default values below, Rebar Lap Length Calculator returns Lap length required of 720. Change any input above and every figure here is recalculated in your browser.

Inputs used

Bar diameterminimum 4 mm
16 mm
Concrete gradeone of M20, M25, M30 and above
M25
Bar inone of Tension, Compression
Tension

Results

Lap length required
720 mm
Lap in bar diameters
45 x diameter
Absolute minimum lap
384 mm
Lap to detail
720 mm

Step by step

  1. 1Values used
  2. 2Lap length
  3. 3Minimum
  4. 4Lap length required= 720 mm
  5. 5Lap in bar diameters= 45 x diameter
  6. 6Absolute minimum lap= 384 mm
  7. 7Lap to detail= 720 mm

Assumptions and limits

  • The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to whole numbers, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
  • Preliminary estimate only. Lap and anchorage lengths must be taken from the governing code and confirmed by a qualified structural engineer.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Preliminary estimate only. Lap and anchorage lengths must be taken from the governing code and confirmed by a qualified structural engineer.

Frequently Asked Questions

How is the Rebar Lap Length figure worked out?

Lap = factor x bar diameter. A lap splice transfers force from one bar to the next through bond with the concrete, so the length needed scales with bar diameter and falls as concrete strength rises.

Can laps be placed anywhere?

No. They should be staggered and located away from points of maximum stress. Many codes also increase the lap where more than half the bars are spliced at one section.

What values do I need to enter?

This calculator takes 3 inputs: Bar diameter, Concrete grade, Bar in. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.

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