What does it mean?
Lap length is the length over which reinforcing bars overlap to transfer force through the surrounding concrete. Required length depends on the design conditions, including bond, bar stress, concrete strength, cover, confinement and the proportion of bars lapped. A fixed multiplier such as 40 times diameter is only an illustrative assumption unless justified by the applicable design rules.
In practice
For a 12 mm bar, 40d equals 480 mm. That multiplication is easy to check, but it does not establish the required design lap. Use the engineer-approved schedule when measuring and ordering reinforcement.
Keep in mind
When preparing an estimate, record the specification reference, units and source of each assumption. Separate the net measured quantity from purchase allowances so changes remain easy to trace. Confirm product dimensions, yields and installation requirements with the supplier. Quantities do not establish structural adequacy or material suitability: use the project’s approved design and details, and seek the responsible designer’s clarification where the specification is incomplete.