Free Period Tracker Online: Predict Your Next Period & Ovulation
Log your last period, or your last few cycles for a more personalized estimate, and this calculator predicts your next period date, ovulation day, fertile window, and current cycle phase. Everything runs in your browser, nothing you enter is uploaded or stored.
If you’re not sure, 28 days is the commonly used average, but yours may run shorter or longer.
Not medical advice. This is an estimate from average cycle data. Irregular cycles, PCOS, thyroid conditions, hormonal contraception and recent pregnancy all shift the result, sometimes by weeks. Do not use it to plan or prevent a pregnancy. See a doctor or midwife for anything that matters.
How this period tracker predicts your next cycle from real history, not just a fixed average
Most simple period calculators assume a flat 28-day cycle and never adjust. This tool does something closer to how a clinician would think about it, it lets you log actual past period start dates and computes your own average cycle length from the gaps between them, then uses that personalized number to project forward. If you don’t have history logged yet, it falls back to a manual cycle length you provide.
Every date you enter, and every prediction the tool produces, stays on your device. Nothing is uploaded, logged remotely, or shared, the source file for this tool states that explicitly in its own header comment.
Building your personal average from logged history
That standard deviation calculation uses the sample variance formula, dividing by n minus 1 rather than n, the standard correction (Bessel’s correction) for estimating a population’s spread from a limited sample rather than treating your logged cycles as the entire population.
From average cycle length to phase and prediction
Once the tool has an average cycle length, whether personalized from your history or manually entered, the same downstream math applies: a 14-day luteal phase assumption chained backward from the next projected period to estimate ovulation, and a phase label based on where today falls relative to your last logged start.
| Phase | Window | What’s Happening |
|---|---|---|
| Menstrual | Cycle day 1 through your period length | Bleeding phase |
| Follicular | After bleeding, before ovulation | Estrogen rising |
| Ovulation | Within 1 day of the estimated ovulation date | Peak fertility window |
| Luteal | After ovulation, until next period | Post-ovulation |
If today falls past your predicted next period date, the tool doesn’t reset or hide the count, it reports the period as overdue and keeps counting cycle day off your last known start, since a late period is still meaningful information rather than something to silently discard.
Two entry methods, one engine
Simple method
Enter just your last period’s start date and a manual cycle length estimate. Good for a first-time quick check, but predictions carry no personalization, they’re a straight projection off one assumed number.
History method
Log two or more past start dates and the tool derives your own average and variability from them. More data points produce a more reliable average, and the tool shows exactly how many logged periods informed the number it gave you.
- American College of Obstetricians and Gynecologists, Your Menstrual Cycle, on normal cycle length ranges and when variation warrants medical attention.
- NICHD, Menstruation and the Menstrual Cycle, background on cycle phases and hormonal patterns.
- CDC, Polycystic Ovary Syndrome, a common cause of cycle irregularity this tool cannot detect or screen for.
This tool is not medical advice and cannot replace a clinical evaluation. It uses standard cycle-math assumptions and cannot diagnose PCOS, endometriosis, thyroid conditions, perimenopause, or other causes of irregular cycles. Talk to a doctor or gynecologist about anything related to fertility, contraception, or a missed or unusual period.
What tracking actually reveals
Anticipating your next period well enough to plan travel or events around it, spotting a pattern of irregularity worth bringing to a doctor before it becomes a bigger question, cross checking a phase estimate against symptoms you’re tracking elsewhere, and exporting predicted period and ovulation dates directly to your calendar so you’re not recalculating by hand every month.
FAQ: Free Period Tracker Online
It’s a statistical estimate, not a guarantee, and we don’t claim otherwise. For someone with very regular cycles, predictions are often within a day or two of reality. For anyone with irregular cycles, or a condition like PCOS, thyroid disorders, or perimenopause, the actual date can be off by many days. Logging two or more real past cycles instead of guessing a cycle length noticeably improves accuracy, but it can never replace an actual test or a doctor’s evaluation.
No. This calculator uses simple average-based math and cannot diagnose or reliably predict cycles affected by PCOS, endometriosis, thyroid conditions, perimenopause, recent pregnancy or breastfeeding, certain medications, or significant weight or stress changes. If your logged cycles vary by more than about a week, the tool will flag them as irregular, but that flag is just a heads up, not a diagnosis. Please see a doctor or gynecologist for any of these situations.
The calculator works backward from your predicted next period using a standard assumption: the luteal phase, the days between ovulation and your next period, averages around 14 days for most people and tends to stay fairly constant even when the rest of the cycle varies. This is a widely used clinical estimate, but actual ovulation timing can only be confirmed with tools like ovulation predictor kits, basal body temperature tracking, or a fertility clinic.
A single assumed cycle length, like the commonly cited 28 days, is just an average across a large population. Your own cycle might genuinely run 24 days or 33 days and still be perfectly normal. When you log two or more of your actual past period start dates, the calculator measures your real average cycle length instead of guessing, which is a meaningfully better basis for the prediction.
A late period based on this calculator’s math can mean many things: pregnancy, a naturally longer cycle that month, stress, illness, travel, changes in exercise or weight, certain medications, or an underlying condition. This tool has no way to tell those apart. If your period is significantly late and unexpected, a pregnancy test and a conversation with a healthcare provider are the appropriate next steps, not this calculator.
No. This is a general estimate meant for planning and awareness, not a method of contraception or a fertility guarantee. Ovulation timing can shift from cycle to cycle for reasons this tool cannot see, and sperm and egg viability windows used here are population averages, not certainties for any individual. For contraception or conception planning, talk to a healthcare provider about a validated method.
This tool flags your history as irregular when the length of your logged cycles swings by more than roughly a week from one cycle to the next. Occasional variation is normal for almost everyone, cycles are rarely identical month to month. Persistent, large swings are worth mentioning to a doctor, since they can sometimes point to PCOS, thyroid issues, perimenopause, or other conditions, but they can also just be your normal pattern.
No. Every calculation happens locally in your browser with JavaScript date arithmetic. Nothing you type in, including period start dates or cycle length, is ever sent to a server, logged, or stored. The calendar files it generates download directly to your device without touching any backend.
From the blog
The research behind the numbers
Where these formulas came from and what the studies actually showed.
