The same starting date, two calculators
A last menstrual period (LMP) of January 1, 2026 with a 28-day cycle gives an ovulation date of January 15 — 14 days after the LMP — from the ovulation calculator. The pregnancy calculator, given that same January 1 LMP, returns a due date of October 8, 2026, using Naegele's rule (LMP + 280 days). The two numbers agree with each other precisely because Naegele's rule assumes ovulation happens 14 days in, exactly what the 28-day-cycle ovulation calculation confirms.
Where the assumption breaks: a 35-day cycle
Feeding that same January 1 LMP into the ovulation calculator with a 35-day cycle instead pushes the real ovulation date to January 22 — 21 days in, a full week later than the 28-day-cycle case. But the pregnancy calculator's due date for that same LMP is still October 8, unchanged — because Naegele's rule has no cycle-length input at all, it always assumes the day-14 ovulation timing regardless of what a person's actual cycle looks like.
What this means in practice
For someone with a longer-than-28-day cycle, the true due date (measured from actual conception) tends to run later than Naegele's rule predicts, since ovulation itself happened later than the formula assumes. This is exactly why clinical guidance treats an early ultrasound's direct measurement as more reliable than the LMP-based estimate when the two disagree, particularly for anyone whose cycle isn't close to 28 days.
Why both calculators are still useful separately
The ovulation calculator is the more accurate tool for someone tracking their own cycle length and timing conception. The pregnancy calculator's LMP-only estimate is what's used as a standard clinical starting point precisely because cycle length often isn't known precisely at the time of a first prenatal visit — a reasonable default, not a claim of precision for every cycle length.