Short answer. It is built on the holiday or rest day rate, not the ordinary one. Article 87 requires additional compensation equivalent to the rate of the first eight hours on a holiday or rest day plus at least thirty percent of that rate. The premium compounds rather than replaces.
What the law says
Work performed beyond eight hours on a holiday or rest day shall be paid an additional compensation equivalent to the rate of the first eight hours on a holiday or rest day plus at least thirty percent (30%) thereof.
Labor Code, Article 87 — Overtime Work. Read the full provision →
Two things change, not one
The sentence is: Work performed beyond eight hours on a holiday or rest day shall be paid an additional compensation equivalent to the rate of the first eight hours on a holiday or rest day plus at least thirty percent (30%) thereof. Both halves of the ordinary-day formula are altered. The percentage rises from twenty-five to at least thirty, and — more significantly — the base changes. It is no longer the regular wage but the rate already payable for the first eight hours of that holiday or rest day. Miss the second change and the figure comes out badly short.
Working from the right base
The computation runs in sequence. Establish what the first eight hours on that day are paid at, because that rate is the base the article points to. Then add at least thirty percent of that base for each hour worked beyond the eighth. Applying thirty percent to the ordinary hourly wage instead is the standard error, and it produces an overtime hour worth less than the eight that preceded it — an outcome plainly at odds with a provision written to make holiday and rest day overtime more costly, not less.
What this article leaves to other provisions
Article 87 tells you how to build on the first-eight-hours rate, but it does not itself fix what that rate is. The premium for working a rest day or a holiday at all comes from elsewhere in the Code, and it differs according to the kind of day involved — a rest day, a regular holiday and a special day are not the same question. So the article gives you the method and the multiplier while leaving the base to be established. Anyone quoting a single all-in percentage on the strength of this article alone has skipped that step.
Checking a payslip against it
Three things need to be visible before the arithmetic can be tested: which calendar days were rest days or holidays for you specifically, how many hours were worked on each, and what rate the payslip applied to the first eight of them. A payslip showing overtime hours at a flat premium regardless of the day is the pattern to look for. Bring the schedule, the timekeeping record and the payslips for the whole period rather than a single month — these shortfalls are usually systematic and repeat.