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Shop Ultrahuman →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.
- Estimated ovulation
- Wed, September 9, 2026
- Fertile window (peak)
- Fri, September 4, 2026 – Thu, September 10, 2026
- Likely implantation
- Sep 15 – Sep 21
- Earliest accurate test
- Mon, September 21, 2026
- Next expected period
- Wed, September 23, 2026
- Due date if conception
- Wed, June 2, 2027
Your next 6 fertile windows
| Fertile window | Ovulation | Period due | Test from |
|---|---|---|---|
| Sep 4 – Sep 10 | Sep 9 | Sep 23 | Sep 21 |
| Oct 2 – Oct 8 | Oct 7 | Oct 21 | Oct 19 |
| Oct 30 – Nov 5 | Nov 4 | Nov 18 | Nov 16 |
| Nov 27 – Dec 3 | Dec 2 | Dec 16 | Dec 14 |
| Dec 25 – Dec 31 | Dec 30 | Jan 13 | Jan 11 |
| Jan 22 – Jan 28 | Jan 27 | Feb 10 | Feb 8 |
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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
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About this calculator
This calculator uses published formulas and reference values from the sources listed below. Results are estimates and depend on the accuracy of your inputs; they cannot account for your individual medical history.
Important: This tool is for general information only and is not medical advice, diagnosis, or treatment. Consult a qualified healthcare professional about your health.
- Sources
- Last reviewed