Due Date Calculator
Calculate your estimated due date (EDD) using Naegele's rule from your last menstrual period, conception date, or IVF transfer date. View trimester timeline and key milestones.
Ovulation Calculator
Calculate your ovulation day and fertile window based on your last menstrual period and cycle length. Identify up to 6 fertile days each month for conception planning.
⚠️ Medical Disclaimer: This calculator provides estimates based on averages. Actual ovulation timing varies. Do not use this as your sole method of contraception. Confirm with OPKs, BBT, or your healthcare provider.
days
Estimated Ovulation Day
May 13, 2026
Cycle Day 14 | Fertile Window: May 8 – May 13
Ovulation Date⧉
May 13, 2026
Cycle day 14
Fertile Window⧉
May 8 – May 13
6 days
Peak Fertility⧉
May 11 – May 12
Highest conception chance
Next Period⧉
May 27, 2026
Cycle length: 28 days
Current Cycle Map
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Period Fertile Peak Ovulation
6-Cycle Projection
| Cycle | Period Start | Fertile Window | Ovulation | Next Period |
|---|---|---|---|---|
| #1 | Apr 29 | May 8 – May 13 | May 13 | May 27 |
| #2 | May 27 | Jun 5 – Jun 10 | Jun 10 | Jun 24 |
| #3 | Jun 24 | Jul 3 – Jul 8 | Jul 8 | Jul 22 |
| #4 | Jul 22 | Jul 31 – Aug 5 | Aug 5 | Aug 19 |
| #5 | Aug 19 | Aug 28 – Sep 2 | Sep 2 | Sep 16 |
| #6 | Sep 16 | Sep 25 – Sep 30 | Sep 30 | Oct 14 |
Daily Conception Probability
| Day Relative to Ovulation | Date | Chance |
|---|---|---|
| -5 | May 8 | ~5% |
| -4 | May 9 | ~10% |
| -3 | May 10 | ~15% |
| -2 | May 11 | ~28% |
| -1 | May 12 | ~30% |
| Ovulation Day | May 13 | ~12% |
| +1 | May 14 | ~0% |
Planning notes, formulas, and examples
About the Ovulation Calculator
The Ovulation Calculator estimates your ovulation day and fertile window based on your menstrual cycle data. Ovulation typically occurs about 14 days before your next expected period — not necessarily 14 days after the start. This means for longer or shorter cycles, the ovulation day shifts accordingly.
The fertile window spans approximately 6 days: the 5 days before ovulation (sperm can survive up to 5 days in the reproductive tract) plus the day of ovulation itself (the egg is viable for 12–24 hours). Timing intercourse within this window maximizes the chance of conception.
This calculator projects ovulation and fertile days for the current cycle and several upcoming cycles, helping you plan ahead whether you're trying to conceive or simply tracking your cycle.
When This Page Helps
Understanding the fertile window is useful in conception planning, especially because the highest-probability days usually cluster shortly before ovulation. This calculator gives a cycle-based estimate of that window, which is helpful for planning but still less precise than ovulation tracking methods such as OPKs or temperature charting.
How to Use the Inputs
- Enter the first day of your last menstrual period (LMP).
- Enter your average cycle length in days (default: 28).
- View your estimated ovulation date.
- See your 6-day fertile window.
- Review projections for the next 3–6 cycles.
- Confirm with ovulation predictor kits (OPKs) or basal body temperature for added accuracy.
Formula used
Ovulation Day = LMP + (Cycle Length − 14)
Fertile Window:
• Start = Ovulation Day − 5
• Peak = Ovulation Day − 1 to Ovulation Day
• End = Ovulation Day
The luteal phase (ovulation to period) is relatively constant at ~14 days, while the follicular phase (period to ovulation) varies with cycle length.
Conception Probability by Day:
• 5 days before ovulation: ~5%
• 4 days before: ~10%
• 3 days before: ~15%
• 2 days before: ~25–30%
• 1 day before: ~25–30%
• Ovulation day: ~10–12%
• Day after ovulation: ~0%Example Calculation
Result: Ovulation: February 17 | Fertile Window: February 12–17
With a 30-day cycle, ovulation occurs on day 16 (30 − 14): February 1 + 16 = February 17. The fertile window starts 5 days before (February 12) and ends on ovulation day (February 17). Peak fertility days are February 15–16 (2 days before and 1 day before ovulation).
Tips & Best Practices
- This calculator gives estimates — confirm with ovulation predictor kits (OPKs) that detect the LH surge 24–36 hours before ovulation.
- Basal body temperature rises 0.2–0.5°C after ovulation; tracking this over several months reveals your pattern.
- Cervical mucus that is clear, stretchy, and slippery (like egg whites) indicates peak fertility.
- Having intercourse every 1–2 days during the fertile window maximizes conception chances.
- Stress, illness, travel, and weight changes can shift ovulation timing even in regular cycles.
- If you have very irregular cycles (21–35 day variation), this calculator may be less accurate; consult a fertility specialist.
The Biology of Ovulation
Ovulation is triggered by a surge of luteinizing hormone (LH) from the pituitary gland, which causes the dominant follicle in the ovary to rupture and release a mature egg. The LH surge typically occurs 24–36 hours before egg release, which is why OPKs that detect LH in urine can predict ovulation in advance.
Maximizing Conception
For couples trying to conceive, recommended strategies include having intercourse every 1–2 days during the fertile window, maintaining a healthy BMI (18.5–24.9), avoiding alcohol and tobacco, and ensuring adequate folate intake (400+ mcg daily). The best position for conception is a myth — sperm reach the fallopian tubes within minutes regardless of position.
When to Seek Help
If you are under 35 and have not conceived after 12 months of timed intercourse, or over 35 and have not conceived after 6 months, consult a fertility specialist. Earlier evaluation is warranted if you have irregular periods, known endometriosis, or a history of pelvic inflammatory disease.
Sources & Methodology
Last updated:
Methodology
This worksheet estimates ovulation timing and the fertile window from the entered last menstrual period and average cycle length, using a luteal phase of roughly 14 days. It is a cycle-planning aid, not a fertility diagnosis or contraceptive method.
Sources
- The Menstrual Cycle (ACOG) — Patient-facing overview of cycle phases, ovulation timing, and conception context.
- Ovulation and the Menstrual Cycle (NICHD) — Official ovulation-timing and fertile-window reference.
Frequently Asked Questions
It is very unlikely. The egg survives only 12–24 hours after ovulation, and sperm typically survive 3–5 days in the reproductive tract. Conception outside the 6-day fertile window is extremely rare, though cycle irregularity can make it hard to pinpoint this window precisely.
Research shows that conception rates are highest when sperm are already present in the fallopian tubes when the egg is released. Sperm need 6–12 hours of capacitation in the reproductive tract before they can fertilize. Having intercourse the day before ensures a ready supply of capacitated sperm.
The luteal phase (post-ovulation) is relatively fixed at 12–16 days (average 14). Cycle length variation is almost entirely due to the follicular phase. A woman with a 35-day cycle ovulates around day 21, while a woman with a 25-day cycle ovulates around day 11.
For irregular cycles, this calculator uses the average cycle length you enter, but actual ovulation may vary significantly. In this case, OPKs and BBT charting are especially important. If cycles consistently range from less than 21 days to more than 35 days, consult a reproductive endocrinologist.
Yes. Ovulation frequency and egg quality decline with age, particularly after 35. Women over 35 may ovulate less consistently and have shorter follicular phases. After 40, anovulatory cycles (cycles without ovulation) become more common. This calculator assumes regular ovulation.
This calculator alone is not reliable for contraception. The fertility awareness-based method (FAM) requires tracking BBT, cervical mucus, and cycle length, and has a typical-use failure rate of about 12–24%. For effective natural family planning, formal training and multiple indicators are essential.
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