Period began 1 January, 28-day cycle
Ovulation about 15 January. Fertile window 10 to 15 January. Next period around 29 January.
Ovulation calculator
Ovulation is taken as 14 days before the next period, and the fertile window as the 5 days before ovulation plus the day itself. It is an estimate for regular cycles. Cycles vary, so this cannot be used as contraception.
days
Estimated ovulation
1 Jan 2026
Fertile window 27 Dec to 1 Jan, with the best chances in the two days before ovulation. Your next period is due around 15 Jan 2026.
The next three cycles
- Cycle 1, ovulation1 Jan (window from 27 Dec)
- Cycle 2, ovulation29 Jan (window from 24 Jan)
- Cycle 3, ovulation26 Feb (window from 21 Feb)
Finding your fertile window
Pregnancy is only possible in a short stretch around ovulation, when an egg is released and lives for about a day. Sperm can survive for up to five days, so intercourse in the days before ovulation can also lead to pregnancy. That makes a window of roughly six days.
The textbook cycle is 28 days, with ovulation near day 14. But the cycle varies from person to person. The second half, from ovulation to the next period, is the steady one at around 14 days. So the easiest estimate is to count back 14 days from when your next period is due.
If your period started on 1 January and your cycle is 28 days, ovulation is around 15 January, the fertile window runs from 10 to 15 January, and your next period is due around 29 January. With a 35-day cycle, ovulation moves to about 22 January and the window to 17 to 22 January.
This is a calendar method, and it works best with regular cycles. If you are trying to conceive and have been for a year, or six months if you are over 35, see a doctor. Tracking over several months, or using ovulation test kits, gives a better picture than a single estimate.
Sources
What this calculator does not account for
- Irregular cycles.
- Real ovulation, which can shift from month to month.
- Medical conditions, medicines and stress.
How this is calculated
Formula: Counting back from the next period
ovulation =first day of last period + cycle length − 14What the symbols mean
- cycle lengthDays from the first day of one period to the first day of the next.
- 14The luteal phase, which is fairly steady at about 14 days.
Good to know
- The fertile window is the 5 days before ovulation plus the day itself, since sperm can survive up to 5 days.
- A 28-day cycle puts ovulation on day 14. A 35-day cycle puts it on day 21.
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Worked examples
Each example is worked out by the same code as the calculator above.
Same start, 35-day cycle
Ovulation about 22 January. Window 17 to 22 January.
Embed this calculator
The frame shows the tool only.
<iframe src="/embed/ovulation-calculator" title="Ovulation" width="100%" height="720" style="border:0;max-width:42rem" loading="lazy"></iframe>Questions
How is ovulation date estimated?
By counting back about 14 days from the day your next period is due. The luteal phase is steadier than the first half of the cycle.
How long is the fertile window?
About six days: the five days before ovulation and the day of ovulation. The two days just before are the most fertile.
Can I use this as birth control?
No. Cycles vary, and ovulation can move. This is an estimate, not contraception.
What if my cycles are irregular?
A calendar estimate is less reliable. Ovulation test strips, or advice from a gynaecologist, help more.