Ovulation Calculator
Find your ovulation date and fertile window to maximize your chances of conception — or to understand your cycle better.
How to use this calculator
Enter the first day of your last period and your average cycle length. Your ovulation date, fertile window, and daily conception probability appear instantly. The calculator also shows the next 3 months of fertile windows for planning ahead.
Understanding ovulation
Ovulation — when a mature egg is released from the ovary — typically occurs 14 days before your next period, regardless of cycle length. For a 28-day cycle this is day 14; for a 35-day cycle it's day 21. The egg survives 12–24 hours after release, but sperm can survive up to 5 days. This means the 6-day window ending on ovulation day is your fertile window. The highest pregnancy rates occur from 1–2 days before ovulation.
Frequently asked questions
Conception probability by day of cycle
Research from the New England Journal of Medicine (Wilcox et al.) mapped pregnancy probability relative to ovulation day. Sperm can survive 3–5 days; the egg survives only 12–24 hours. This creates an asymmetric fertile window that opens before ovulation and closes quickly after.
| Days relative to ovulation | Estimated pregnancy probability |
|---|---|
| 5 days before ovulation | ~5% |
| 4 days before ovulation | ~12% |
| 3 days before ovulation | ~15% |
| 2 days before ovulation | ~25% |
| 1 day before ovulation | ~31% |
| Day of ovulation | ~33% |
| 1 day after ovulation | ~0–5% |
| 2+ days after ovulation | Essentially 0% |
Signs of ovulation
The body produces several observable signs around ovulation that can help confirm your fertile window, especially if cycles are irregular.
- ·LH surge on ovulation predictor kit (OPK) — the most reliable home method; LH peaks 24–36 hours before ovulation
- ·Basal body temperature (BBT) rise of 0.2–0.5°C — occurs after ovulation and confirms it has happened
- ·Egg-white cervical mucus (EWCM) — clear, stretchy, slippery discharge; signals approaching ovulation
- ·Mittelschmerz — mild one-sided pelvic pain or cramping; occurs in ~20% of women at ovulation
- ·Breast tenderness — estrogen peak before ovulation can cause mild breast sensitivity
- ·Increased libido — many women notice heightened interest in sex around ovulation (likely evolutionary adaptation)
How to improve chances of conception
Timing intercourse within the fertile window is the most important factor, but several other variables also influence conception rates.
- ·Have sex every 1–2 days during the fertile window (especially the 2 days before ovulation) — the most evidence-based approach
- ·Use OPKs to detect your actual LH surge, especially if cycles vary by more than 3–4 days
- ·Take prenatal vitamins with folic acid (400–800 mcg) before conception to reduce neural tube defect risk
- ·Maintain a healthy BMI — both underweight (BMI <18.5) and obese (BMI >30) significantly reduce fertility
- ·Limit alcohol, avoid smoking, and minimize caffeine (under 200mg/day)
- ·Sperm health matters: male partner should avoid hot tubs, keep laptop off lap, and avoid smoking
- ·Stress reduction: high cortisol can suppress the LH surge needed for ovulation
- ·If no conception after 12 months of unprotected sex (6 months if over 35), consult a reproductive endocrinologist
Ovulation timing by cycle length
Ovulation consistently occurs approximately 14 days before the next period — not on a fixed day of the cycle. This table shows expected ovulation day for different cycle lengths.
| Cycle length | Typical ovulation day | Fertile window (days) |
|---|---|---|
| 21 days | Day 7 | Days 2–8 |
| 24 days | Day 10 | Days 5–11 |
| 26 days | Day 12 | Days 7–13 |
| 28 days | Day 14 | Days 9–15 |
| 30 days | Day 16 | Days 11–17 |
| 32 days | Day 18 | Days 13–19 |
| 35 days | Day 21 | Days 16–22 |