Your fertile days, by your real cycle
Estimate your next ovulation and fertile window in seconds from your last period date and average cycle length. We use the calendar method: ovulation happens about 14 days BEFORE the next period, and the fertile window is the 5 prior days plus ovulation day. An educational estimate for regular cycles — not a contraceptive method.
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Calendar method for regular cycles: an educational ESTIMATE. NOT a contraceptive method and not a substitute for medical advice. With irregular cycles, prediction reliability drops.
Your cycle, month after month
The app keeps your calendar and adjusts calories and cravings to the phase you are in.
On the first day of your next period, open it and log it: from there the calendar adjusts itself.
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How are ovulation and fertile days calculated?
The calendar method estimates ovulation by counting backwards: the luteal phase — from ovulation to your next period — is the most consistent part of the cycle, at around 14 days for most people. So we take the first day of your last period, add your cycle length, and subtract 14 to place the likely ovulation day. The fertile window is that day plus the five before it, because sperm can survive several days in the reproductive tract while the egg lasts about 24 hours. This is an educational estimate built on averages, and cycles vary — including from one month to the next in the same person.
Fertile days by cycle length
The key that almost no site explains: what stays constant is NOT day 14, but the 14 days from ovulation to the next period. That is why a 32-day cycle ovulates around day 18, not day 14. The fertile window includes the five days before because sperm survive that long.
| Cycle length | Estimated ovulation | Fertile window |
|---|---|---|
| 21 days | Day 7 | Day 3 – 8 |
| 24 days | Day 10 | Day 6 – 11 |
| 28 days | Day 14 | Day 10 – 15 |
| 30 days | Day 16 | Day 12 – 17 |
| 32 days | Day 18 | Day 14 – 19 |
| 35 days | Day 21 | Day 17 – 22 |
Estimated from a 14-day luteal phase and sperm survival of up to 5 days. Real cycles vary from month to month; this is an estimate, not a contraceptive method. These are reference ranges for healthy adults, not a diagnosis or individual advice.
FAQ
When do I ovulate if my cycle is 28 days?
Around day 14, counting day 1 as the first day of bleeding, with the fertile window roughly from day 9 to day 15. On longer cycles ovulation happens later — on a 32-day cycle, closer to day 18 — because it is the first half of the cycle that stretches, not the second.
Which days have the highest chance of pregnancy?
The two days before ovulation and the day itself. The window opens about five days earlier because sperm survive, but probability is far from evenly spread across it — it concentrates sharply at the end.
Can this calculator be used as contraception?
No. It is an educational estimate based on averages, and it cannot account for an early or late ovulation, which is exactly when unplanned pregnancies happen. If you are avoiding pregnancy, use a contraceptive method and talk to a health professional.
What if my cycles are irregular?
The calendar method loses accuracy as variability grows, and with very irregular cycles it says little of use. Basal body temperature, cervical mucus changes and ovulation tests track what is actually happening rather than what an average predicts. Persistent irregularity is worth raising with a doctor.