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Estimates assume a regular cycle with ovulation about 14 days before the next period; real cycles vary month to month, so this is a planning guide, not contraception. Cycles persistently shorter than 21 days, longer than 35, or suddenly changed are worth a GP conversation.
What The Period Calculator Works Out
- Next period due: cycle arithmetic runs from the first day of one period to the first day of the next, so the calculator adds the average cycle length to the last start date. A 1 September start on a 28-day cycle points to 29 September, then 27 October.
- Estimated ovulation: the one fixed span in the cycle is the luteal phase, so ovulation sits about 14 days before the next period, not 14 days after the last. That is why the fertile window moves with cycle length and why the calendar method misleads people with short or long cycles.
- Approximate fertile window: five days before ovulation to one day after, because sperm survive up to five days and the egg about one. Useful for conception planning and emphatically not reliable as contraception.
- Period length (advanced): how many days the bleed usually lasts, 2 to 7 being typical, so the calculator can show when the next period should end.
- Cycle variation (advanced): anything from 21 to 35 days is normal in adults and most cycles wobble a few days month to month. Entering the variation turns the single due date into a likely range, and the average of the last three cycles predicts better than any single one.
- Luteal phase length (advanced): 14 days is the average; 10 to 16 is normal. If you know yours from tracking, it shifts the ovulation and fertile window estimates.
- Where you are today (advanced): the current cycle day and phase, and how overdue a period is if the predicted date has passed. A missed period on a regular cycle earns a pregnancy test from the first missed day.
- Check a date (advanced): enter a holiday, wedding or exam date to see which cycle day it falls on and whether a period or the fertile window is likely to coincide, allowing for your cycle variation.
- Cycles to list (advanced): a table of the next several cycles with start, end, fertile window and ovulation dates for planning ahead.
- When to see a GP: cycles persistently under 21 days or over 35, absent for three months, suddenly changed, heavy enough to soak through protection hourly, or bleeding between periods or after sex. Stress, illness, travel, weight change, perimenopause and coming off hormonal contraception all legitimately shift timing, and periods stopping under heavy training or restricted eating is a health warning, not an achievement.
Source and further reading: NHS: periods.