Ovulation & Cycle Calculator
Estimate your fertile window. Private by design.
The first day of your most recent period.
Days from one period's start to the next (typically 21–35).
Next period (estimated)
- Ovulation day
- August 12, 2026
- Fertile window
- August 7, 2026 – August 12, 2026
- Ovulation cycle day
- Day 14
Calendar estimate only, NOT a contraceptive method. Real cycles vary, so do not rely on this to prevent or achieve pregnancy. All calculations run in your browser; nothing you enter is sent anywhere.
How the ovulation calculator works
The calculator uses the calendar method (sometimes called the rhythm method). You give it two things: the first day of your most recent period and your typical cycle length in days. From those it works out three dates.
The model assumes ovulation occurs about 14 days before the next period. ACOG describes this as an
average-cycle relationship, not a rule that identifies ovulation in an individual cycle. The calculator
uses nextPeriod = lastPeriodStart + cycleLength, and places estimated ovulation on cycle day
cycleLength − 14 (with day 1 as the first day of menstrual bleeding). It highlights the six days
ending on that estimated date: fertileStart = ovulationDate − 5 days and
fertileEnd = ovulationDate.
Worked example
Suppose your last period began on 1 June 2026 and your cycles run 30 days.
- Ovulation cycle day = 30 − 14 = day 16
- Next period = 1 June + 30 days = 1 July 2026
- Ovulation date = 1 June + 15 days = 16 June 2026
- Fertile start = 16 June − 5 days = 11 June 2026
- Fertile end = ovulation day = 16 June 2026
| Result | Date |
|---|---|
| Fertile window opens | 11 June 2026 |
| Ovulation (most fertile) | 16 June 2026 |
| Next period | 1 July 2026 |
The classic anchor checks out too: enter a 28-day cycle and ovulation falls on day 14, with the fertile window running cycle days 9 through 14.
How to use it
- Enter the first day of bleeding, not the day spotting tapered off. Day 1 is the first day of full flow.
- Use your average cycle length across the last few months, not a single unusual cycle.
- To plan for conception, focus on the fertile window, with the highest odds in the two or three days before ovulation and on ovulation day itself.
- Cycles shorter than 15 days are rejected, because there is no room for a 14-day luteal phase plus a positive ovulation day.
- Re-run it each month with your actual start date, since real cycles drift.
Limitations and contraception warning
This is a calendar projection from averages, not a measurement of ovulation. It cannot detect hormonal changes or account for early, late, or absent ovulation; variable cycles; stress; illness; travel; breastfeeding; perimenopause; medication; or other causes of changed timing. Its highlighted window also does not include the possible day after estimated ovulation described by ACOG.
Do not use these dates alone as contraception or as a guarantee of conception. Fertility-awareness methods require more than a date prediction and have materially higher pregnancy rates with typical use than with perfect use. If avoiding pregnancy is important or pregnancy would pose a medical risk, discuss an appropriate method with a healthcare professional.
Sources and scope
- Jurisdiction
- General educational estimate; US clinical source
- Data current to
- July 30, 2026
- Last updated
- Fertility Awareness-Based Methods of Family Planning — American College of Obstetricians and Gynecologists
Frequently asked questions
Can I use this as birth control?
No. This is a calendar estimate based on averages, not a contraceptive method. Sperm can survive several days, ovulation timing shifts with stress, illness, and cycle variability, and the calculator cannot see any of that. If you are avoiding pregnancy, use a proven method.
What if my cycles are irregular?
Calendar prediction assumes your next cycle matches the length you entered. If your cycles vary by more than a few days from month to month, treat every date here as a rough midpoint rather than a fixed day. Tracking several cycles and using your average length gives a more honest estimate.
My cycle is 35 days. Does ovulation still happen on day 14?
No. The luteal phase (ovulation to next period) is the stable part at about 14 days, so ovulation moves with cycle length. For a 35-day cycle the calculator places ovulation on cycle day 21 (35 minus 14), not day 14.
Why does the fertile window end on ovulation day instead of after it?
This calculator reports a simplified six-day window from five days before its estimated ovulation date through that date. ACOG explains that pregnancy can occur from sex up to five days before ovulation through one day after. Because the calculator neither measures ovulation nor includes the possible following day in its highlighted range, do not treat its boundaries as exact.
Does this calculator store or send my dates anywhere?
No. All computation runs in your browser. Your last period date and cycle length are never uploaded, logged, or shared. Close the tab and nothing is retained unless you have chosen to save it locally.