Free tool
Ovulation & Fertile Window Calculator
Enter the first day of your last period and your average cycle length to see when you are most likely to ovulate — and the six-day window when pregnancy is most likely.
Enter the first day of your last period to see your estimated fertile window.
This is an estimate, not contraception. Calendar predictions assume regular cycles and cannot detect when you actually ovulate. Do not rely on them to avoid pregnancy — see how reliable fertility awareness methods really are. Nothing you enter here leaves your browser.
How this calculator works
Most people are taught that ovulation happens on day 14. That is only true on a textbook 28-day cycle, and most cycles are not 28 days. The more useful rule is that ovulation happens roughly one luteal phase before your next period — and the luteal phase is the most consistent part of the cycle, usually 12 to 16 days and typically around 14.
So the calculation works backwards from your next period, not forwards from your last one. Counting the first day of bleeding as cycle day 1:
- On a 28-day cycle, ovulation lands around day 14 or 15.
- On a 24-day cycle, around day 10 or 11.
- On a 34-day cycle, around day 20 or 21.
The one-day spread is not vagueness — it is the difference between counting forward from your last period and counting back from your next one. It makes almost no practical difference, because the fertile window is six days wide regardless.
What does make a difference is cycle length. Calculators that assume ovulation on day 14 for everyone are wrong for the majority of people using them, because the luteal phase stays roughly constant while the first half of the cycle is what varies.
Why the fertile window is six days, not one
An egg survives only about 12 to 24 hours after release. If that were the whole story, the fertile window would be a single day. It is longer because sperm can survive in the reproductive tract for up to about five days when cervical mucus is favourable.
That means intercourse several days before ovulation can still result in pregnancy — sperm are already present and waiting when the egg is released. The practical consequence is that the highest- probability days are the two or three days leading up to ovulation and the day itself, not the days after.
How accurate is this?
Honestly: it is a reasonable starting point and nothing more. Calendar prediction has three real limitations.
- It assumes your next cycle matches your average. Cycles vary. Stress, illness, travel, sleep disruption, and weight change can all shift ovulation by days.
- It cannot confirm ovulation happened. Some cycles are anovulatory — a period arrives without an egg having been released.
- It degrades sharply with irregular cycles. If your cycle length varies by more than a few days, the predicted window becomes wide enough to be of limited use.
If you want a more accurate picture, real-time signs beat calendar maths: cervical mucus changes, basal body temperature (which confirms ovulation after the fact), and ovulation predictor kits that detect the luteinising hormone surge. Our guide to ovulation symptoms covers what to look for, and how to track your menstrual cycle explains how to combine them.
Why this is not a contraceptive tool
It is worth being blunt about this, because calculators like this one are frequently misused. Predicting a fertile window is not the same as avoiding one. Fertility awareness methods — done properly, with training, daily observation of multiple signs, and consistent rules about when to abstain or use barriers — are a legitimate approach. A calendar estimate on its own is not that.
In typical use, fertility awareness methods are among the least reliable forms of contraception. If avoiding pregnancy is the goal, read how reliable fertility awareness methods really are and compare it against the alternatives in our birth control methods comparison.
When to speak to a healthcare provider
- Your cycles are consistently shorter than 24 days or longer than 38 days, or vary widely month to month — see causes of irregular periods.
- You have been trying to conceive for 12 months (or 6 months if you are over 35) without success.
- Your periods have stopped, or you are bleeding between periods.
- You have severe period pain, very heavy bleeding, or symptoms that interfere with daily life.
Your privacy
Everything above runs in your browser. The dates you enter are never sent to a server, never stored, and never shared — there is no submission step at all. You can confirm this by disconnecting from the internet after the page loads; the calculator still works.
Frequently asked questions
How do you calculate your ovulation date?
Ovulation happens roughly one luteal phase — usually about 14 days — before your next period starts, not 14 days after your last one began. So you count back 14 days from the day your next period is due. On a 28-day cycle that lands around cycle day 14 or 15; on a 34-day cycle it lands around day 20 or 21. The exact day matters less than it sounds, because the fertile window spans six days either way.
How many days is the fertile window?
About six days: the five days before ovulation plus the day of ovulation itself. It is that long because sperm can survive up to five days in the reproductive tract, so intercourse several days before an egg is released can still result in pregnancy. The egg itself only survives about 12 to 24 hours.
Can I use an ovulation calculator as birth control?
No. Calendar predictions assume your cycles are regular and cannot detect when you actually ovulate, which shifts with stress, illness, and travel. Fertility awareness methods used properly are far more involved than a calendar, and even then are less reliable than most other contraception in typical use.
How accurate is an ovulation calculator if my periods are irregular?
Much less accurate. The calculation depends entirely on knowing your cycle length in advance, so if your cycles vary by more than a few days the predicted window can be off substantially. Tracking real-time signs such as basal body temperature or cervical mucus, or using ovulation predictor kits, gives a better picture.
What are the signs that you are ovulating?
Common signs include a change in cervical mucus to a clearer, stretchier consistency, a small rise in basal body temperature after ovulation has happened, mild one-sided pelvic twinges, and a rise in luteinising hormone that ovulation predictor kits detect. Not everyone notices them.
References
Part of our Reproductive & Menstrual Health topic. See all free tools.