Ovulation typically occurs 14 days before your next period, not 14 days after your last period starts. For a 28-day cycle, this is day 14. For a 32-day cycle, this is approximately day 18. The luteal phase (ovulation to period) is relatively fixed at 12–16 days, while the follicular phase varies between women.

How long is the fertile window?

The fertile window is typically 6 days: the 5 days before ovulation plus the day of ovulation itself. Sperm can survive in the reproductive tract for up to 5 days, while the egg survives only 12–24 hours after release. The most fertile days are the 2–3 days just before ovulation. This is consistent across NHS, ACOG, and BZgA (Germany) clinical guidance.

What is the average cycle length worldwide?

A 2019 study of 612,613 cycles from the Clue app (published in npj Digital Medicine) found the median cycle length was 29.3 days globally, with 95% of cycles between 21 and 35 days. The distribution was similar across countries. The 28-day cycle is a statistical average, not a biological constant — most women's cycles vary by 2–7 days from month to month.

🇬🇧 NHS Guidance 🇺🇸 ACOG 🇩🇪 BZgA Germany

Ovulation Calculator

Find your most fertile days by calculating your ovulation date and fertile window. The calculation is the same worldwide — the luteal phase (ovulation to period) is approximately 14 days in most women.

Quick Answer

Ovulation occurs approximately 14 days before your next period — not necessarily 14 days after your LMP. For a 28-day cycle: ovulation ~day 14. For a 32-day cycle: ~day 18. The fertile window spans 5 days before ovulation to 1 day after (6 days total). This is consistent across NHS, ACOG (US), and BZgA (Germany) clinical guidance.

First day of last period (LMP)

Average cycle length (days)

Average 28 days, range 20–45

Ovulation Guidance by Country / Authority

Country / Authority	Fertile Window Guidance	Tracking Method Recommended
🇬🇧 UK — NHS	6 fertile days (5 before + day of ovulation)	Calendar method + OPK strips; BBT for irregular cycles
🇺🇸 USA — ACOG	Ovulation ~14 days before period; fertile window 5–6 days	OPK, BBT, ultrasound for fertility treatment
🇨🇦 Canada — SOGC	Same 6-day fertile window principle	OPK + BBT combination; midwife-supported tracking
🇦🇺 Australia — Jean Hailes	Fertile window 5 days before + ovulation day	Cervical mucus monitoring + OPK strips
🇩🇪 Germany — BZgA	Luteal phase fixed ~14 days; follicular phase varies	Symptothermal method (BBT + cervical mucus)
🇯🇵 Japan — JFPA	Standard 28-day / 14-day ovulation model	Basal body temperature charting widely used in Japan
🌍 WHO	Calendar method (standard days) for 26–32 day cycles	Fertility awareness-based methods for family planning

Fertile Window by Cycle Length

Cycle Length	Ovulation Day	Fertile Window
24 days	Day 10	Days 5–11
26 days	Day 12	Days 7–13
28 days (average)	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

Days counted from first day of period. Ovulation = cycle length minus 14 days. Fertile window = 5 days before ovulation + ovulation day.

Frequently Asked Questions

What are the signs of ovulation? ▾

Common signs of ovulation include: (1) Changes in cervical mucus — becomes clear, slippery, and stretchy (like raw egg white) in the days approaching ovulation. (2) Slight increase in basal body temperature (BBT) by 0.2–0.5°C after ovulation. (3) Mild pelvic pain or cramping on one side (Mittelschmerz). (4) Increased sex drive. LH surge detected by ovulation predictor kits (OPKs) occurs 24–36 hours before ovulation — widely used in the UK, US, Japan, and Germany.

Is the ovulation calculator accurate for irregular cycles? ▾

Calendar-based methods are less reliable for irregular cycles (cycles that vary by more than 7 days month to month). If your cycles are irregular, basal body temperature (BBT) tracking, ovulation predictor kits (LH strips), or transvaginal ultrasound monitoring (available at fertility clinics in the UK via NHS, or privately in most countries) are more accurate. Germany's BZgA and the UK NHS both recommend combination methods for irregular cycles.