Ovulation Calculator
Predict your fertile window and ovulation date based on cycle length.
About the Ovulation Calculator
This Ovulation Calculator estimates your fertile window — the six days each cycle when intercourse can result in pregnancy — using the well-established fact that ovulation occurs approximately 14 days before the next menstrual period. The fertile window spans five days before ovulation to one day after, reflecting sperm survival (up to 5 days in fertile cervical mucus) and the egg’s 12-24 hour viability after release.
The 14-day luteal phase is remarkably consistent across cycle lengths. A 28-day cycle ovulates around day 14; a 35-day cycle around day 21; a 24-day cycle around day 10. What varies is the follicular phase (LMP to ovulation), not the luteal phase (ovulation to next period).
The landmark study by Wilcox, Weinberg, and Baird (NEJM 1995;333:1517-1521) tracked 221 women through 696 cycles with daily urinary hormone testing. They found the fertile window lasted six days (five before ovulation plus the day of ovulation), and that intercourse on the day of ovulation had the highest pregnancy probability (~33% per cycle).
How It Works
The calculation:
Ovulation day = LMP + (cycle_length - 14) days
Fertile window = ovulation - 5 days to ovulation + 1 day
Next period = LMP + cycle_length daysFor a 28-day cycle: ovulation on day 14, fertile window days 9-15. For a 35-day cycle: ovulation on day 21, window days 16-22.
Why five days before? Sperm survive up to 5 days in fertile cervical mucus (which appears 5-6 days before ovulation, peaking at ovulation). The egg survives 12-24 hours post-release. Daily intercourse during the fertile window maximises conception probability; the highest single-day chance is intercourse 1-2 days before ovulation.
Cycle-length variation matters: cycles <21 or >35 days, or variation >7-9 days between cycles, suggest oligo- or anovulation. The American Society for Reproductive Medicine (ASRM) recommends evaluation after 12 months of unprotected intercourse (6 months if over 35).
Worked Examples
LMP = March 1, 2025, cycle = 28 days:
Ovulation = Mar 1 + 14 = March 15, 2025
Fertile window = March 10-16
Next period estimate = March 29, 2025LMP = May 5, 2025, cycle = 35 days:
Ovulation = May 5 + 21 = May 26, 2025
Fertile window = May 21-27
Next period estimate = June 9, 2025LMP = June 1, 2025, cycle = 24 days:
Ovulation = June 1 + 10 = June 11, 2025
Fertile window = June 6-12
Next period estimate = June 25, 2025Cycle length variations of 1-2 days are normal; persistent >7-day variation warrants evaluation for PCOS, thyroid dysfunction, or perimenopause. Ovulation predictor kits (OPKs) detect the LH surge 24-36 hours before ovulation — useful for confirming the calendar estimate.
When to Use This Tool
- Time intercourse for conception — daily during the 6-day fertile window maximises probability.
- Plan fertility awareness-based (FAB) natural family planning (less reliable than hormonal methods).
- Schedule intrauterine insemination (IUI) or timed intercourse during fertility treatment cycles.
- Identify when to start ovulation predictor kits (OPKs) — typically 17 days before expected period.
- Track basal body temperature (BBT) — rises 0.3-0.5°C after ovulation (progesterone effect).
- Detect irregular cycles — >7-day cycle variation warrants evaluation (ASRM).
- Plan prenatal vitamin start (folic acid 400 mcg/day at least 1 month before conception — ACOG).
Limitations & Disclaimer
Calendar-based ovulation prediction is accurate within ±2 days for women with regular cycles; irregular cycles, stress, travel, illness, and perimenopause shift timing unpredictably. The method is less reliable than hormonal monitoring (OPKs, BBT, ultrasound). This tool is not medical advice and not a contraceptive method; consult a gynaecologist or reproductive endocrinologist for fertility concerns. See our disclaimer for full terms.
Frequently Asked Questions
How accurate is calendar-based ovulation prediction?
Within ±2 days for women with regular cycles. Cycle length variation, stress, travel, illness, and perimenopause all shift ovulation. Ovulation predictor kits (OPKs) detect LH surge 24-36 h before ovulation; basal body temperature confirms retrospectively; transvaginal ultrasound is the gold standard for fertility treatment.
Can I get pregnant outside the fertile window?
Practically, no. The fertile window spans 6 days: 5 before ovulation (sperm survival) plus the day of ovulation. Intercourse 6+ days before or >24 h after ovulation has <1% conception probability per cycle. However, predicting ovulation itself has ±2-3 day uncertainty.
What if my cycle is irregular?
Cycles <21 or >35 days, or variation >7-9 days between cycles, suggest oligo- or anovulation. Common causes include PCOS (most common), thyroid dysfunction, hyperprolactinaemia, and perimenopause. ASRM recommends evaluation after 12 months of unprotected intercourse (6 months if >35).
Does ovulation always happen on day 14?
No. Day 14 is the average for a 28-day cycle. The luteal phase (ovulation to next period) is consistently ~14 days, but the follicular phase (LMP to ovulation) varies with cycle length. A 35-day cycle ovulates ~day 21; a 24-day cycle ~day 10. Stress, illness, and travel can delay ovulation.
When should I start taking prenatal vitamins?
ACOG recommends 400 mcg folic acid daily starting at least 1 month before conception and continuing through the first trimester. Higher doses (1-4 mg) are recommended for women with prior neural-tube-defect pregnancies, certain anticonvulsants, or MTHFR variants. Folic acid reduces neural tube defects by 50-70%.
How long should we try before seeing a fertility specialist?
ASRM recommends evaluation after 12 months of unprotected intercourse for women <35, and 6 months for women ≥35. Earlier evaluation is warranted with known fertility issues (PCOS, endometriosis, prior pelvic surgery, or male factor). IVF success rates decline after 35, so timing matters.
Last updated: September 9, 2026 · Author: HT99 Tools Editorial Team