Example 1 — 28-day cycle
- LMP: January 1, 2026
- Cycle length: 28 days
Ovulation: Jan 14
Window: Jan 9\u201315
A typical 28-day cycle: ovulation lands roughly midway, matching the textbook model.
Ovulation and fertile window calculator
Enter the first day of your last period and your average cycle length to estimate ovulation day and your most fertile days.
This calculator estimates ovulation day and the fertile window using a simple calendar model: ovulation \u2248 first day of last period + (cycle length \u2212 14 days), and the fertile window spans 5 days before ovulation plus the day of ovulation. It is a simplification — real cycles vary, and stress, illness, or a routine change can shift ovulation by a few days. Treat the result as planning support, not a certainty or a contraceptive method.
Result
Enter the first day of your last period.
How results are calculated
Ovulation: LMP + (cycle length \u2212 14 days).
Fertile window: from 5 days before ovulation through the day of ovulation (a 6-day range).
Next period: LMP + cycle length.
How to use the calculator
- Enter the first day of your last period.
- Enter your average cycle length — if unsure, leave it at 28.
- Click Calculate or watch the live result as you type.
- Check the cycle timeline below the form — it highlights the fertile window and ovulation day.
- If you are planning over several months, track a few cycles to improve accuracy.
Usage examples
Example 2 — Longer cycle (32 days)
- LMP: January 1, 2026
- Cycle length: 32 days
Ovulation: Jan 18
Window: Jan 13\u201319
A longer cycle shifts ovulation later — the luteal phase stays fixed, so the difference comes from the pre-ovulation phase.
Example 3 — Shorter cycle (24 days)
- LMP: January 1, 2026
- Cycle length: 24 days
Ovulation: Jan 10
Window: Jan 5\u201311
A shorter cycle means earlier ovulation — worth factoring into planning.
FAQ
How is ovulation calculated here?
Ovulation \u2248 first day of last period + (cycle length \u2212 14 days). We assume a typical luteal phase of about 14 days — the time from ovulation to the next period is relatively stable, unlike the phase before ovulation.
Why a 6-day fertile window?
It is a simplified, widely used range: 5 days before ovulation plus the day of ovulation, because sperm can survive in the reproductive tract for several days while the egg is fertilizable for about 12\u201324 hours.
Is this a contraceptive method?
No. This is a planning-oriented tool, not a contraceptive method or fertility diagnostic. Cycles can be irregular, and ovulation may shift by several days in either direction.
What if my cycles are irregular?
The more irregular your cycle, the less precise the estimate — the formula assumes a fixed cycle length and luteal phase. Consider tracking several cycles or consulting a fertility specialist.
Can conception happen outside the calculated window?
Yes, though less commonly. The fertile window marks the highest-probability range, not a hard cutoff — biology is not that precise.
How accurate is this estimate?
This is a simplified calendar model. Methods like basal body temperature tracking, LH ovulation tests, or follicle-tracking ultrasound are more accurate because they use signals from your actual cycle, not just a date.
When should I enter the last period date?
Right after it starts — the first day of bleeding is the LMP used in the formula.
Which calculators pair well with this?
Due date once pregnancy is confirmed. Pregnancy weight gain for later weeks. BMI as a general health reference point.