Calculate your ovulation day, fertile window, and daily conception probability based on your menstrual cycle. Enter your last period date, cycle length, and luteal phase to see a visual calendar, key dates, and the best days to conceive. Fast, private, no signup.
Quick answer: Ovulation = LMP + (cycle length − luteal phase). Fertile window is 6 days ending on ovulation day. See How It Works, Fertile Window and Conception Probability — or jump to BMI Calculator and Calorie Calculator.
Enter your last period date and cycle details then tap Calculate Ovulation. Results include ovulation day, fertile window, conception probabilities, and a visual calendar. How to use →
Find Your Fertile Window & Conception Probability
Most women have a 12-16 day luteal phase. If unknown, 14 days is standard.
Tip: Most women have a 28-day cycle with a 14-day luteal phase. If your cycles are irregular, track at least 3 months before using this tool.
Ovulation is the release of an egg from the ovary, typically occurring once per menstrual cycle. The timing can be predicted using two relatively reliable markers: your cycle length and your luteal phase.
Ovulation Day = LMP + (Cycle Length − Luteal Phase)
With a 28-day cycle and 14-day luteal phase: 28 − 14 = day 14. Your ovulation falls on day 14 of your cycle.
The luteal phase (after ovulation) is relatively constant at 12-16 days. The follicular phase (before ovulation) is what varies between women and cycles.
For irregular cycles, you can estimate a range by using your shortest and longest cycle lengths. The calculator provides your best estimate based on your input.
The fertile window spans approximately 6 days: the 5 days before ovulation and ovulation day itself. This window exists because sperm can survive up to 5 days in the reproductive tract, while the egg is viable for only 12-24 hours after release.
| Day Relative to Ovulation | What Happens | Conception Chance |
|---|---|---|
| O-5 | Sperm can reach egg if released early | ~5% |
| O-4 | Sperm survival improving | ~9% |
| O-3 | Good fertility window | ~15% |
| O-2 | High fertility | ~25% |
| O-1 (Peak) | Highest fertility — day before ovulation | ~33% |
| O (Ovulation) | Egg released — viable 12-24 hours | ~30% |
| O+1 | Egg declining — limited viability | ~18% |
| O+2 | Egg no longer viable | ~5% |
Source: Wilcox AJ et al. "Timing of sexual intercourse in relation to ovulation." NEJM 1995.
Per-cycle conception probability varies dramatically by day. The highest chance is the day before ovulation (~33%), not ovulation day itself. This is because sperm need time to capacitate and the egg is freshest when sperm arrive.
For healthy couples under 35 with regular intercourse, the cumulative probability of conception within 6 months is about 75-80%, and within 12 months is about 85-90%.
The uterine lining sheds. Hormone levels are at their lowest. Some women experience cramps, fatigue, or mood changes.
FSH stimulates follicle growth. Estrogen rises, thickening the uterine lining. Energy and mood typically improve.
LH surge triggers egg release. Cervical mucus becomes clear and stretchy (egg-white consistency). Mild cramping may occur on one side.
Progesterone rises, maintaining the uterine lining. PMS symptoms may appear. If no implantation occurs, hormones drop and the cycle restarts.
This ovulation calculator provides estimates based on standard cycle patterns and should be used as an educational tool, not a medical diagnostic.
Cycles naturally vary by 1-7 days. Stress, illness, travel, and hormonal changes can shift ovulation unpredictably.
This calculator is for educational purposes. For fertility concerns, irregular periods, or suspected ovulation disorders, consult a healthcare professional.
Women with PCOS, thyroid disorders, or other hormonal conditions may have highly irregular ovulation that this calculator cannot accurately predict.
For best results, combine this calculator with OPKs, cervical mucus tracking, and basal body temperature measurements.
Peer-reviewed sources behind the ovulation timing, conception probability data, and cycle phase information on this page.
This page summarizes peer-reviewed fertility research for educational purposes. Individual interpretation should include clinical context and professional advice.
This ovulation calculator provides estimates based on standard menstrual cycle formulas and is for educational and informational purposes only.
Results are approximations and should not be used for medical diagnosis or as a substitute for professional fertility treatment. Individual cycles vary based on health, age, hormones, stress, and medical conditions.
For fertility concerns, irregular periods, difficulty conceiving, or hormonal disorders, consult a qualified healthcare professional or reproductive endocrinologist.
An ovulation calculator estimates your most fertile days based on your menstrual cycle. It uses the first day of your last period (LMP) and your cycle length to predict when ovulation occurs, helping you identify the best time to conceive.
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This ovulation calculator is for informational purposes only and does not replace medical advice. Always consult a healthcare professional for personalized fertility guidance.
Content Updated: 01/09/2026 • 2,200 words • Reviewed • Evidence-based
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