Period & cycle tracker
Enter the first day of your last period and your usual cycle length to predict your next period date, estimated ovulation day and fertile window. It helps with planning, understanding your cycle, and timing for or against conception — as an estimate, not a medical or contraceptive tool.
How it works
The tracker works forward from your last period using simple date arithmetic and the standard luteal-phase assumption:
next period = last period + cycle length
ovulation = next period − 14 days (luteal phase ≈ 14 days)
fertile = ovulation − 5 days … ovulation dayCycle length is clamped to 20–45 days and period length to 1–10 days. Because ovulation is anchored 14 days before the next period, a longer cycle pushes ovulation later.
Example
Last period started on a given date, 28-day cycle, 5-day period:
- Next period: last period + 28 days
- Ovulation: next period − 14 days (so day 14 of the cycle)
- Fertile window: days 9 to 14 of the cycle
| Cycle length | Ovulation (cycle day) | Fertile window |
|---|---|---|
| 28 days | day 14 | days 9–14 |
| 30 days | day 16 | days 11–16 |
| 35 days | day 21 | days 16–21 |
Everything is calculated privately in your browser — nothing is stored or uploaded.
Why cycles vary and how to get a better average
The 28-day cycle is a population average, not a standard. Normal cycle lengths range from roughly 21 to 35 days and can shift month to month in the same person. A single cycle length produces a single-point prediction; if your cycles vary, the window of uncertainty widens proportionally.
To get a more useful average, track the start date of your last three to six periods and calculate the mean:
average cycle = total days across all cycles ÷ number of cycles measuredFor example, if your last four cycle lengths were 27, 29, 28, and 30 days, the average is (27 + 29 + 28 + 30) ÷ 4 = 28.5 days. Enter 28 or 29 here (the tool uses whole days) for a prediction that reflects your personal pattern rather than the textbook default.
What the fertile window really means
The fertile window reflects the biology of sperm and egg viability. A released egg survives roughly 12–24 hours. Sperm, when conditions in the reproductive tract are favorable, can survive 3–5 days. The six-day window (five days before ovulation plus the day of ovulation itself) captures the span during which intercourse can result in fertilization.
The first day of the fertile window has the lowest probability of conception, rising each day toward ovulation and peaking on the day before ovulation and ovulation day itself. Calendar tracking alone cannot account for early or late ovulation caused by stress, illness, or hormonal shifts, which is why this estimate carries meaningful uncertainty.
Limitations and when to see a doctor
Calendar-based prediction is a planning aid, not a clinical tool. It does not account for anovulatory cycles (cycles where no egg is released), luteal phase defects, or the effects of hormonal contraception recently stopped. If you are actively trying to conceive and not seeing results after several months, or if your cycles are highly irregular or very short or long, a conversation with a healthcare provider is the appropriate next step — they can order targeted tests or recommend monitoring methods such as ovulation predictor kits or basal body temperature charting.