🇬🇧 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.
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.