Ovulation Calculator — fertile window for irregular cycles

Enter the first day of your last menstrual period and your typical cycle length. The calculator finds when you're likely to ovulate, the fertile window around that, and projects forward to your next period or due date if pregnancy occurs.

First day of last periodCycle length (days)

My cycles are irregular

Estimated ovulation: Tue, June 23, 2026
Fertile window (peak): Thu, June 18, 2026 – Wed, June 24, 2026
Likely implantation: Jun 29 – Jul 5
Earliest accurate test: Sun, July 5, 2026
Next expected period: Tue, July 7, 2026
Due date if conception: Tue, March 16, 2027

Your next 6 fertile windows

Fertile window	Ovulation	Period due	Test from
Jun 18 – Jun 24	Jun 23	Jul 7	Jul 5
Jul 16 – Jul 22	Jul 21	Aug 4	Aug 2
Aug 13 – Aug 19	Aug 18	Sep 1	Aug 30
Sep 10 – Sep 16	Sep 15	Sep 29	Sep 27
Oct 8 – Oct 14	Oct 13	Oct 27	Oct 25
Nov 5 – Nov 11	Nov 10	Nov 24	Nov 22

How it works

How ovulation timing works

In a typical menstrual cycle, ovulation occurs about 14 days BEFORE the next period — not 14 days after the last one. The luteal phase (between ovulation and the next period) is fairly consistent at 14 days for most people, while the follicular phase before ovulation varies more. So for a 28-day cycle, ovulation is around day 14; for a 32-day cycle, it's around day 18 (32 − 14).

The fertile window is the 6 days ending on ovulation day — sperm can survive in the reproductive tract for up to 5 days, so intercourse a few days before ovulation can still result in conception. The two days immediately before and the day of ovulation are the most fertile.

Accuracy and limitations

This calculator gives an estimate based on average physiology. Real cycles vary — stress, illness, weight changes, hormonal birth control transitions, perimenopause, and many other factors can shift ovulation timing. For more accurate tracking, use ovulation predictor kits (LH surge tests), basal body temperature charting, or cervical mucus observation in addition to this calculator.

If you have irregular cycles (typical cycle length varies by more than 7-9 days), this calculator's predictions may be off. A fertility-focused app or doctor's consultation will give better personalized estimates.

What 'cycle length' means

Cycle length is from the first day of one period to the first day of the next — not the length of the period itself. Average is 28 days but anywhere from 21 to 35 days is medically normal. If you don't know your typical cycle, track 3 cycles with a calendar before relying on predictions.

If your cycles are 21-25 days, ovulation happens earlier (day 7-11). If 32-35 days, it's later (day 18-21). Very short or very long cycles may indicate underlying conditions worth discussing with a doctor.

Frequently asked questions

›How accurate is this?

The calculation is mathematically correct given typical physiology, but biological reality varies. Stress, illness, and other factors can shift ovulation by several days. Use as a starting estimate, not a guarantee.

›Can I use this for birth control?

No. The fertile window method (rhythm method) has a high failure rate — about 24% per year with typical use. Use it only to maximize conception chances, not to prevent pregnancy.

›Why is the fertile window 6 days, not just 1?

Sperm survive 3-5 days in the reproductive tract. So intercourse a few days before ovulation can still result in conception. The egg itself is only viable for ~24 hours after ovulation.

›What if my cycles are irregular?

This calculator assumes regular cycles. If yours vary by more than 7-9 days, predictions are unreliable. Track basal body temperature or use ovulation predictor kits (LH tests) for more accuracy.

›Does the LH surge happen at ovulation?

It precedes ovulation by 24-36 hours. Ovulation predictor kits detect the surge, giving you the best 1-day window. Combine with this calculator's window estimate for tighter prediction.

›What's the best day to conceive?

The day before ovulation, statistically. Sperm should already be present when the egg is released. Day of ovulation and day before are roughly equal.

›What's the due date based on?

Standard 280 days (40 weeks) from last menstrual period. Use our due-date calculator for more options including 'add-2-weeks-to-cycle-day' or known conception date.

›Does the data leave my browser?

No. Calculation is local; nothing is sent to any server.

Sources & methodology

Disclosure

iHerb
Sponsored
World's largest store for natural supplements and health foods.
Learn more →

Related tools

This tool is for general information only and is not medical advice, diagnosis, or treatment. Consult a qualified healthcare professional about your health.