13th month pay calculators
A 13th month payment is a legal entitlement, not a bonus an employer chooses to give. These calculators work from the basic salary you actually earned — which is a narrower figure than your gross pay, and the reason most manual estimates come out too high.
Why the divisor is always twelve
The most common mistake in a 13th month calculation is dividing by the number of months worked instead of by twelve. Dividing by months worked hands a July joiner a full month's salary, when what they are due is roughly half of one. The proration is already built into the numerator — the salary you earned — so the denominator never changes.