Predict your next period date, fertile window and ovulation day from your last period and average cycle length. Free period tracker calendar for women everywhere.
Your next period
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Period Fertile window Ovulation
Estimates only — not medical advice. See a doctor for health concerns.
Enter the first day of your last period and your average cycle length (count from the first day of one period to the first day of the next). The calculator predicts your next 3 periods, your fertile window (the 6 days when pregnancy is most likely) and your ovulation day (usually 14 days before your next period).
Why track your cycle?
Knowing your cycle helps you plan around your period, understand PMS symptoms, and identify your most fertile days whether you are trying to conceive or avoid pregnancy. Pair it with our ovulation calculator for a fuller picture.
FAQs
How does a period calculator work?
A period calculator uses the first day of your last period and your average cycle length to estimate when your next period will start, plus your fertile window and ovulation day.
How accurate is a period calculator?
It is an estimate. If your cycles are regular, predictions are usually close. Irregular cycles, stress, travel and illness can shift your period by several days.
What is a normal cycle length?
Most cycles are 21 to 35 days, with 28 days being the average. Track 3 or more cycles to find your personal average.
Is this period calculator free?
Yes. It is 100% free with no signup required, and all calculations happen in your browser.
Can I use a period calculator for birth control?
No — this is important. Calendar predictions are estimates, and ovulation can shift by days due to stress or illness. The fertile window also opens five days before ovulation. Use proper contraception; never rely on an app's 'safe days.'
What should I do if my period is late?
Wait a few days — a delay of up to a week is common with stress, travel, or illness. Take a pregnancy test if there is any chance of pregnancy, and see a doctor if periods are persistently irregular, very painful, or absent for months.
What is the luteal phase?
The roughly 14-day stretch between ovulation and your next period. It is the most consistent part of the cycle, which is why calculators count backward from the expected period to find ovulation rather than forward from the last one.
How period prediction works
The calculator needs two inputs: the first day of your last period (LMP) and your average cycle length (first day of one period to the first day of the next). Prediction one: next period ≈ LMP + cycle length. Prediction two works backward from that date, because the luteal phase is the stable part of the cycle: ovulation ≈ next period − 14 days. Prediction three: the fertile window covers the five days before ovulation plus ovulation day, since sperm survive up to 5 days while the egg lasts 12–24 hours.
Worked example: LMP October 1, 28-day cycle
Next period: October 1 + 28 days = October 29. Ovulation: October 29 − 14 = October 15. Fertile window: October 10–15. Note the fertile days sit roughly mid-cycle — the "calendar method" in a nutshell. With a 32-day cycle the same LMP gives ovulation on October 19 instead; cycle length is the lever that moves everything.
Practical tips and limits
Track at least three cycles and use the average — one unusual month skews everything. Predictions degrade with irregular cycles, stress, illness, or perimenopause. And critically: never rely on calendar predictions for contraception; ovulation can shift without warning, and the fertile window opens days before any symptom appears.
What "irregular" really means — and when to see a doctor
Cycle lengths varying by ±2–3 days are normal; that is variation, not irregularity. True irregularity — swings of 7+ days, cycles consistently under 21 or over 35 days, or missed periods for months — deserves medical attention, as it can signal thyroid issues, PCOS, or perimenopause. What to track for your doctor: cycle lengths (not just dates), flow heaviness, pain levels, and any spotting between periods. Three months of clean data turns a vague "my periods are weird" into a diagnosable pattern. The calculator predicts; your log explains — and no app replaces a clinician for persistent irregularity.
Key takeaways
Next period ≈ LMP + cycle length; ovulation ≈ next period − 14.
Track 3+ cycles and average them for the best predictions.
Calendar predictions are not contraception — see a doctor for persistent irregularity.