Under Presidential Decree 851, every rank-and-file employee who has worked at least a month during the year is entitled to 13th month pay, computed as total basic salary actually earned for the calendar year divided by 12. Only basic salary counts toward that base: overtime pay, allowances, and other bonuses don't factor into the calculation at all.
What can trip people up is the exemption. The TRAIN Law exempts the first ₱90,000 of 13th month pay and other benefits, Christmas bonus, productivity incentives, and similar bonuses, from income tax, but that ₱90,000 is one combined ceiling across all of them, not a separate allowance for each. This calculator sums your 13th month pay and any other bonus input before checking it against the cap, exactly as the BIR does at year-end annualization.
Anything above the combined cap isn't taxed as a flat rate on its own. It gets folded into your regular taxable compensation for the year and runs through the same six-bracket TRAIN Law graduated table used for ordinary income, from 0% up to 35%. This calculator isolates the tax attributable to just the excess by computing your BIR tax with and without that excess added, and taking the difference, the same reconciliation logic employers apply when they annualize withholding in December.