Period calendar and cycle tracker

An online tool for keeping a cycle calendar: log period days, symptoms, mood, cervical mucus and basal body temperature. The period calendar forecasts your next menstruation and ovulation as a range rather than a single date, and it does not give up on an irregular cycle. The maths runs in your browser.

How to use it

1

Enter the first day of your last period and, if you remember it, your usual cycle length.

2

Tap any day in the calendar to log flow, symptoms, mood or temperature.

3

The "My period started today" button marks the day in one tap.

4

Read the summary: cycle day, phase, the range for your next period and the fertile window.

5

Log an ovulation test or your basal temperature — the forecast gets noticeably sharper.

6

Export CSV for a clinician, or ICS so the forecast lands in your own calendar.

Log your days and get a forecast with an honest range

This is a diary, not a medical device
The calendar helps you see the patterns in your own cycle. It does not diagnose anything and does not replace a clinician. Every prediction here is an estimate from your past cycles, not a guarantee.
Your entries stay with you
Your cycle and health entries are saved in this browser, on your device. We do not copy them to our servers — even when you are signed in. If you like, make an encrypted backup with your own password.
Period Predicted period Chance of conceiving Ovulation
Menstrual
Oestrogen and progesterone at their lowest
The uterine lining sheds. Cramps, low energy, headaches and a need for rest are common. Cycle day 1 is the first day of real flow, not spotting.
Follicular
FSH rises, oestrogen climbs behind it
The most variable phase: it runs anywhere from 10 to 30 days. This is exactly why the "ovulation on day 14" rule misses. Energy, mood and focus usually climb.
Ovulatory
Oestrogen peaks, LH surges
The egg lives about a day; sperm survive up to five. That is why the fertile window opens five days before ovulation and closes almost immediately after it.
Luteal
Progesterone dominates
The steadiest phase, usually 12–14 days. At its end hormones fall sharply, which is where PMS comes from: irritability, bloating, cravings, tender breasts.
How the forecast is computed
No "artificial intelligence" — just statistics you can check:
  • Cycle length is estimated robustly rather than by a plain average: outlying cycles are rejected using the median and median absolute deviation, and recent cycles weigh more than old ones.
  • While you have few cycles logged, the estimate is pulled toward the population value of 29.3 days with a standard deviation of 5.2, measured across 612,613 cycles by Bull et al. (2019).
  • Ovulation is counted backward from the predicted period: the luteal phase (12.4 days) is steadier than the follicular one (16.9 days), which is why counting forward is the core mistake behind the "day 14" rule.
  • The day of ovulation is a distribution, not a point. The chance of conceiving on each day comes from averaging the day-specific probabilities of Wilcox et al. (1995) over that distribution.
  • If you log a positive LH test or a basal temperature shift, ovulation is treated as confirmed and your period is predicted forward from it.
  • The forecast bounds are roughly an 80% interval. They widen for an irregular cycle: an honest window beats one exact date that nobody hits.
Your entries
Days logged: 0
Import understands our own file (plain or encrypted), Drip CSV, Apple Health export.xml and Clue measurements.json. Flo publishes no export schema, so we do not pretend to read its file.
Entries are saved in your browser's localStorage — clearing site data or using private mode wipes them. Export if the history matters to you. CSV is easy to show a clinician; ICS drops the forecast into Google Calendar.

Features

A range, not one false date
The tool measures the spread of your own cycles and shows a window such as "period between the 12th and the 16th". Even the best models miss by 2–3 days on average, so an exact date is fiction.
Ovulation counted backward from your period
The luteal phase runs 12.4 days while the follicular one runs 16.9 with a 10-to-30 day spread. That is why "day 14" misses on short and long cycles, and counting backward does not.
An irregular cycle does not break it
One forgotten entry will not move the forecast: outlying cycles are rejected by median absolute deviation yet stay visible on the chart — a long cycle is a signal, not noise.
Ovulation tests and basal temperature
A positive LH test or a "three over six" thermal shift pins ovulation, and from then on your period is predicted from that — more accurately than any calendar can.
Patterns worth raising with a clinician
Cycles under 21 or over 35 days, a spread beyond the FIGO age reference, bleeding longer than 8 days, 90 days without a period, spotting between periods.
CSV, JSON and ICS export
Take the history to a clinician as a spreadsheet, keep a backup you can import again, or drop the predicted dates straight into Google Calendar.

Frequently asked questions

Why show a range of dates instead of one day?

Because one day would be a lie. Even neural-network models predict the start of a period with a mean error of about 2.3 days, and a plain average manages roughly 2.9. The tool shows an interval that covers about 80% of cases and narrows it as your own cycles accumulate.

Why is ovulation not on day 14?

Only about 13% of real cycles are exactly 28 days long. The variability sits in the first half of the cycle: the follicular phase runs 10 to 30 days while the luteal one holds near 12.4. So ovulation is counted backward from the expected period rather than forward from the last one.

Can I use this calendar instead of contraception?

No. With typical use of calendar-based methods, roughly one woman in four becomes pregnant within a year. This tool measures no hormones and cannot know when ovulation actually happened, so it never marks a day as safe. Talk to a clinician about contraception.

How many cycles do I need before the forecast is accurate?

An estimate appears after a single cycle, but the range will be wide because the maths leans on the population value of 29.3 days. After three cycles confidence is moderate; after six it is high, provided your cycle is regular.

Where are my entries kept, and what happens if I clear my browser?

Entries are saved in your browser's localStorage, so clearing site data, using private mode, or switching device means an empty calendar. Export JSON — the same file imports back. You can also set a code: entries are then stored encrypted (AES-GCM with a key derived from the code via PBKDF2), but if you lose the code they cannot be recovered.

Does the tool diagnose anything?

No. It highlights what clinicians check first: cycles shorter than 21 or longer than 35 days, a length spread beyond the FIGO age reference, bleeding for more than 8 days, no period for 90 days or more, and bleeding between periods. It is an invitation to a conversation, not a conclusion.

You May Also Like

🎯 ADHD Test 📊 Anxiety Tracker 🧩 Autism Spectrum Test 🫁 Breathing Exercise 💭 Cognitive Test 👁️ Color Blind Test 📝 Color Name Identifier 📷 Colorblind Camera Filter

How do you rate this tool?

Thank you for your rating!
Want to share more? Leave a comment!
Thank you! Your comment will appear after moderation.
Who is this tool for?
🧘 Wellness & Meditation
Published Updated Authors: