Find your fertile window and best days to conceive
Enter the first day of your last period and your average cycle length. We'll estimate your ovulation day, your most fertile days, and when your next period is likely — instantly, in your browser.
Don't do this math every month
MoodSwings estimates your fertile window, ovulation, and next period automatically — and gets more accurate as it learns your cycle. Mood, symptoms, and gentle reminders too.
How the fertile window works
Ovulation is when an ovary releases an egg — usually about 14 days before your next period starts, though the timing varies between people and from cycle to cycle. Your fertile window is roughly the five days before ovulation plus ovulation day, because sperm can survive several days while the egg is viable for about a day. The two to three days right before ovulation, plus ovulation day, are typically the best days to conceive.
Because ovulation timing shifts, a calendar estimate is a helpful starting point rather than a precise date — particularly if your cycles are irregular. Logging a few signs (cervical-mucus changes, a small rise in basal body temperature after ovulation, mid-cycle twinges) over several cycles gives a far more personal estimate, which is what a tracking app like MoodSwings does for you.
Questions people ask
How does the ovulation calculator work?
It estimates ovulation as roughly 14 days before your next expected period, then marks your fertile window as the five days before ovulation plus ovulation day — the span when conception is most likely. It is an estimate from your cycle, not a guarantee.
When are the best days to get pregnant?
The two to three days leading up to ovulation and ovulation day itself are usually the most fertile. The calculator highlights these as your best days to conceive.
Can I use this to avoid pregnancy?
No. Cycle estimates are not reliable contraception — ovulation timing shifts, especially with irregular cycles. If preventing pregnancy is your goal, talk to a clinician about appropriate methods. The lower-fertility days shown here are for awareness only, not birth control.
Is the calculator accurate for irregular cycles?
It is less precise when your cycle length varies a lot, because ovulation timing moves with it. Tracking several cycles in an app gives a much more personal estimate than a single calculation.
Does MoodSwings do this automatically?
Yes — MoodSwings estimates your fertile window, ovulation, and next period from your logged cycles and updates as it learns your pattern. It is free to try on iPhone.