Period & Ovulation
Calculator

Tracking your menstrual cycle is one of the most practical things a person with a uterus can do for their health. Beyond predicting periods, cycle tracking reveals patterns that can indicate hormonal shifts, stress responses, thyroid changes, and other health signals well before they become symptoms that warrant a doctor's visit. It is not just a fertility tool — it is a health monitoring practice.

How to use this calculator

1. Enter the first day of your last period. Use the date picker to select the date your most recent period started — not when it ended.
2. Enter your average cycle length. Most cycles are 21–35 days. If you're unsure, 28 days is the default. Tracking 3–6 previous cycles gives a more accurate average.
3. Enter your average period length. How many days your period typically lasts — usually 3–7 days.
4. Read your predictions. The calculator shows your next expected period date, estimated ovulation date, and fertile window. Predictions are estimates — cycle length varies naturally.

Understanding the menstrual cycle

A "normal" menstrual cycle ranges from 21 to 35 days, with the average sitting around 28 days. The first day of your period is Day 1. The cycle has four phases: menstruation (Days 1–5 on average), the follicular phase (Days 1–13), ovulation (around Day 14 in a 28-day cycle), and the luteal phase (Days 15–28). These phases are averages. Your cycle may be consistently shorter, longer, or variable — all of which can be normal for you specifically.

The luteal phase — from ovulation to the start of the next period — is the most consistent part of the cycle for most people. It typically lasts 12–16 days regardless of cycle length. This means that if you have a 32-day cycle, ovulation likely occurs around Day 16–20, not Day 14. Using a fixed "Day 14 = ovulation" assumption for longer cycles is a common source of miscalculation.

The fertile window

Sperm can survive in the reproductive tract for up to 5 days. An egg is viable for 12–24 hours after ovulation. This means the fertile window extends from roughly 5 days before ovulation to 1 day after — a window of approximately 6 days per cycle. The two days immediately before ovulation and the day of ovulation itself carry the highest probability of conception.

Our calculator estimates this window based on your cycle length and last period start date. It uses the average luteal phase assumption of 14 days to back-calculate ovulation from expected next period date. For cycles that vary by more than 3–4 days month to month, tracking basal body temperature (BBT) or using LH surge tests gives more reliable individual ovulation timing.

What affects cycle regularity?

Cycles are sensitive to many external factors. Stress is one of the most common disruptors — cortisol can delay or suppress ovulation, extending the follicular phase. Significant weight changes (either direction), intense exercise, illness, travel across time zones, and starting or stopping hormonal contraception all commonly cause cycle irregularity. A single irregular cycle is rarely a cause for concern. Persistent irregularity — cycles consistently outside the 21–35 day range, or periods that stop for 3 or more months without pregnancy — warrants a conversation with a healthcare provider.

A note on using this tool for contraception

This calculator is an educational planning tool. Calendar-based period and ovulation prediction has a failure rate of approximately 24% per year when used as the sole method of contraception — meaning roughly 1 in 4 people relying on it as their only contraceptive method will experience an unintended pregnancy within a year. It should not be used as a contraceptive method on its own. Consult a healthcare provider for reliable contraceptive options appropriate for your circumstances.

This period and ovulation calculator estimates your next period date, ovulation day, and fertile window based on the first day of your last period and your average cycle length. The fertile window calculation uses the standard assumption that ovulation occurs approximately 14 days before the next period — meaning for a 32-day cycle, ovulation falls around day 18, not day 14.

Common cycle questions: when will my next period start if my last period was 10 February with a 28-day cycle? (approximately 10 March), when do I ovulate with a 30-day cycle? (around day 16, approximately 14 days before the next period), what is the fertile window? (5 days before ovulation plus the day of ovulation itself — 6 days total). Enter your dates above for personalised estimates.