Period Calculator — predict your next period & fertile days

These are estimates, not a guarantee — and not birth control. Predictions assume a fairly regular cycle; ovulation timing shifts with stress, illness, travel and age, and sperm can survive for days, so do not rely on the fertile window to avoid pregnancy. Use a proven contraceptive method if you are.

How it works

A menstrual cycle is counted from the first day of one period to the first day of the next. The average is 28 days, but anywhere from 21 to 35 days is normal. To predict your next period, this calculator adds your cycle length to the first day of your last period; it repeats each cycle from there.

Ovulation — when an egg is released — happens about 14 days before your next period starts, because the luteal phase (the gap between ovulation and the next period) is usually about two weeks (roughly 14 to 16 days). This calculator uses 14 days, the shorter end of that range, which places ovulation at the latest likely date. It does vary from person to person, so this and the fertile window — the days sperm can survive (up to about 3 days before ovulation) through roughly a day after — are estimates, not fixed dates.

• Next period = first day of last period + cycle length
• Ovulation ≈ last period + (cycle length − 14)
• Fertile window ≈ 3 days before ovulation to 1 day after

Worked example

For a 28-day cycle starting January 1, 2026, with a 5-day period:

• Next period: Jan 1 + 28 = Jan 29 (ending around Feb 2)
• Ovulation: Jan 1 + (28 − 14) = Jan 15
• Fertile window: Jan 12 to Jan 16

Frequently asked questions

How accurate is a period prediction?

It's an estimate for a **regular** cycle. Real cycles vary from month to month with stress, illness, travel, exercise, breastfeeding and age, so treat the dates as a guide. The luteal phase alone can range from about **7 to 19 days** between people, so the ovulation date and fertile window are only approximate. Tracking several cycles gives a better average to work from.

How do I know my cycle length?

Count from the **first day of one period to the day before your next period begins**. Do this over a few cycles and average them; if they vary a lot, your cycle is irregular and predictions are rougher.

When am I most fertile?

In the days just before and during ovulation — roughly the fertile window shown here. Sperm can survive a few days, so the days leading up to ovulation matter most for conception.

Can I use this as birth control?

No. Cycle-based prediction is **not reliable contraception** on its own — ovulation timing shifts, and sperm survive for days. Use a proven method if you are avoiding pregnancy.

Why is my period late?

Common reasons include stress, weight change, intense exercise, illness, travel, hormonal changes, and pregnancy. An occasional late or missed period is usually not a concern, but see a clinician if it persists or you might be pregnant.

Is a 35-day or 21-day cycle normal?

Yes — cycle length varies between people, and 21 to 35 days is the normal adult range. What matters more is whether your own cycle is roughly consistent from month to month.