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Period Calculator
Your cycle
Estimates based on averages, not medical advice. Cycles vary from month to month, and this calculator cannot diagnose pregnancy, irregular cycles or any health condition.
Example: last period started Oct 26, 2026, with a 28-day cycle and a 5-day period — ovulation falls on Nov 9, the fertile window runs Nov 4–10, and the next period starts Nov 23, 2026.
Behind the numbers
How your next period is predicted
Your next period is your last period plus one full cycle. Ovulation usually happens about 14 days before the next period starts, so it is the last period plus your cycle length minus 14 days. The fertile window covers the five days before ovulation plus ovulation day, because sperm can survive up to five days while an egg lives for about a day.
Worked example: Oct 26, 2026 + 28 days = Nov 23, 2026 for the next period, and Oct 26 + (28 − 14) = Oct 26 + 14 days = Nov 9, 2026 for ovulation. The fertile window is five days before ovulation through the day after it: Nov 4–10, 2026.
A few comparisons
Example cycles (last period Oct 26, 2026)
| Cycle length | Ovulation day | Fertile window | Next period |
|---|---|---|---|
| 26 days | Nov 7, 2026 | Nov 2–8, 2026 | Nov 21, 2026 |
| 28 days | Nov 9, 2026 | Nov 4–10, 2026 | Nov 23, 2026 |
| 30 days | Nov 11, 2026 | Nov 6–12, 2026 | Nov 25, 2026 |
| 32 days | Nov 13, 2026 | Nov 8–14, 2026 | Nov 27, 2026 |
Make the prediction more useful.
Work out your average cycle over the last three to six cycles instead of using one month — a single short or long cycle can throw the prediction off by days.
Count day 1 as the first day of full flow, not spotting. Counting from the wrong day shifts every date the calculator gives you.
Remember the fertile window is an estimate. If you are trying to conceive or avoid pregnancy, use ovulation tests or basal body temperature as well — do not rely on dates alone.
See a doctor if your cycles are often shorter than 21 days or longer than 35 days, if periods stop for three months, or if bleeding is very heavy or painful.
Good to know
Your period questions, answered.
How accurate is this period calculator?
It is an estimate based on averages. If your cycles are regular, the next period date is usually right within a few days, but stress, illness, travel and hormonal changes can shift ovulation and delay or bring forward a period. Track at least three cycles to learn your own pattern.
Can I get pregnant outside the fertile window?
Pregnancy is most likely during the fertile window, the five days before ovulation plus ovulation day itself, because sperm can survive up to five days. Pregnancy outside that window is unlikely but not impossible, especially if your cycle is irregular and ovulation happens earlier or later than expected.
What if my cycle length changes from month to month?
Enter your average cycle length, worked out over the last three to six cycles. If your cycles regularly vary by more than seven days, or you often go longer than 35 days between periods, talk to a doctor rather than relying on a calculator.
Does a longer period change my ovulation day?
No. Ovulation depends on your cycle length, not on how many days you bleed. Ovulation usually happens about 14 days before your next period starts, so a 28-day cycle gives ovulation around day 14, whatever the period length.