Period Predictor Calculator

How it is calculated

The tool counts forward from the first day of your last period. The cycle length is rounded to a whole number of days, and each upcoming period is start + cycle × i for i = 1, 2 and 3. Estimated ovulation is start + (cycle − 14), which assumes a luteal phase (ovulation to next period) of 14 days. If you leave the cycle empty or enter a value below 20 or above 45, the engine quietly uses 28 days instead. Dates are always printed in the form Apr 9, 2026. The result is a calendar projection from averages, not a medical prediction, and a cycle that varies from month to month will drift away from it.

Common mistakes

• Entering the last day of the period instead of the first. The cycle is counted from the first day of bleeding, so every predicted date will be too late by the length of the period.
• Typing a date like 10/03/2026. It is read as the American month-first form, so it becomes 3 October, not 10 March. Use the year-first form 2026-03-10.
• Entering a cycle outside 20 to 45 days and trusting the output. The tool silently replaces it with 28 days, so a 50-day cycle will not be reflected in the dates.
• Entering the length of the bleeding (for example 5 days) in the cycle field. The cycle runs from one start date to the next start date, usually near 28 days.
• Treating the ovulation date as exact. It is always the next-period date minus 14 days, so it is wrong for anyone whose luteal phase is shorter or longer, and it should not be relied on for contraception.