What this calculator does
It gives a calendar estimate of ovulation and the fertile window from two numbers: the first day of your last period and your usual cycle length.
It is arithmetic, not a test. It assumes a regular cycle and that ovulation happens about 14 days before the next period. Real cycles vary, especially around stress, illness and travel.
The formula
Next period = last period + cycle length
Ovulation = next period − 14 days
Fertile window = ovulation − 5 days … ovulation + 1 dayThe 14 days is a convention, not a measurement. It describes the luteal phase, which is fairly stable for many people but not for everyone.
A worked example
Last period 15 October 2026, cycle 28 days:
- Next period: 12 November 2026
- Estimated ovulation: 29 October 2026
- Fertile window: 24–30 October 2026
A 32-day cycle moves every date four days later.
Why the window is wider than the day
Sperm can survive for up to about five days, and the egg is viable for roughly 24 hours. That is why the fertile window starts five days before ovulation and ends one day after. It is a probability window, not a guarantee.
What this calculator does not do
It does not confirm ovulation, detect pregnancy, or replace an ovulation test, basal body temperature tracking, or a clinician’s assessment. It is not contraception. If you are trying to conceive and cycles are irregular, or if nothing happens after a year (six months if you are over 35), speak to a clinician.
Signs your body gives
Some people notice cervical mucus changes, a small temperature rise after ovulation, or one-sided pain. Those signs can support the calendar estimate, but they are not proof and they vary. A test measures the hormone surge directly. None of them confirms ovulation on its own.
Frequently asked questions
How is ovulation estimated?
The calculator subtracts 14 days from the next expected period: it adds your cycle length to the first day of your last period, then counts back 14 days. The fertile window runs from five days before ovulation to one day after.
Is this accurate if my cycle is irregular?
Less so. A calendar estimate assumes a regular cycle. With irregular cycles, ovulation can move by a week or more, and the window shown may not match your body. Ovulation tests or temperature tracking are more direct.
Can I use this to avoid pregnancy?
No. This is not a contraceptive method. Calendar estimates miss early or late ovulation, and sperm can survive for several days. Use a method designed for contraception.
Why five days before ovulation?
Because sperm can survive in the reproductive tract for up to about five days, while the egg is viable for roughly 24 hours. That is why the fertile window starts before ovulation rather than on the day itself.