Tracking a period on a calendar means marking the first day of bleeding as day 1, marking every bleeding day, and counting the days from one day 1 to the next to get the cycle length. The NHS says most cycles run around 28 days and that 21 to 35 days is normal. After three to six cycles the calendar shows your pattern, predicts the next period roughly and flags changes a doctor should see; on its own it cannot reliably find the fertile window.
What counts as day 1, and what is a normal cycle length?
Day 1 is the first day of menstrual bleeding, and the cycle runs from that day to the day before the next period starts. The US NICHD states that "the first day of bleeding is considered the first day of the menstrual cycle", and MedlinePlus says the cycle is counted from the first day of one period to the first day of the next.
Normal ranges differ slightly by authority.
| Authority | Normal cycle length | Normal bleeding | Notes |
|---|---|---|---|
| NHS | 21 to 35 days, around 28 on average | 2 to 7 days, usually about 5 | Heaviest in the first 2 days; 20 to 90 ml lost |
| NICHD (US) | 21 to 35 days for most women; 21 to 45 in teens | About 5 days | Average cycle 28 days, varies month to month |
| Cleveland Clinic | 21 to 35 days | 3 to 7 days, most 3 to 5 | 2 to 3 tablespoons of blood |
| FIGO 2018 system (gynaecologists) | 24 to 38 days | Up to 8 days including spotting | Variation up to 7 days (ages 26 to 41) or 9 days (18 to 25 and 42 to 45) |
The FIGO system, described in a 2023 review by Jain, Munro and Critchley, is what a gynaecologist uses to label bleeding as frequent (under 24 days), infrequent (over 38 days) or prolonged (over 8 days). A cycle inside either range that stays roughly the same each month is normal.
How do you mark your period on a calendar?
Mark day 1 with a clear symbol, mark every day of bleeding, then write the cycle length next to the next day 1 when it arrives. Cleveland Clinic describes the same four steps: put an X on the first day of bleeding, put an X on each bleeding day, count the days between first-day marks to get cycle length, and count the X's to get bleeding duration.
A table keeps the record readable for a doctor:
| Cycle | Day 1 (date) | Last bleeding day | Bleeding days | Cycle length (days) | Flow | Notes |
|---|---|---|---|---|---|---|
| 1 | 3 Jan | 7 Jan | 5 | - | normal | cramps day 1 to 2 |
| 2 | 31 Jan | 5 Feb | 6 | 28 | heavy day 2 | spotting 20 Jan |
| 3 | 1 Mar | 5 Mar | 5 | 29 | normal | - |
So 3 January to 31 January is a 28-day cycle. Record flow consistently (light, normal, heavy) and note anything the NHS flags: soaking a pad or tampon every 1 to 2 hours, clots larger than about 2.5 cm, bleeding between periods or after sex.
Keep the record for at least six cycles before drawing conclusions; MedlinePlus says it can take a few years after the first period for cycles to become regular, so a teenager's calendar often looks irregular without anything being wrong.
How do you predict your next period?
Add your average cycle length to the date of your last day 1. If your last three cycles were 28, 29 and 30 days, the average is 29, and the next period is expected 29 days after the last day 1, give or take a few days. The FIGO system treats variation of up to 7 days between your shortest and longest cycle as normal at ages 26 to 41, and up to 9 days at 18 to 25 and 42 to 45, so a prediction is a window, not a date.
Use the last three to six cycles rather than one, and drop any cycle that was obviously disrupted by illness, travel or a missed pill. The NHS lists stress, significant weight change, excessive exercise, hormonal contraception, thyroid problems and polycystic ovary syndrome (PMOS) among the things that shift cycle length, and says periods become less frequent over months or years before they stop at menopause, which usually happens in the mid-40s to mid-50s.
Practical points:
- Count from the first day of real flow, not from spotting.
- Note the predicted date in pencil and correct it when the period arrives.
- If you are sexually active and the period is more than 5 days late, Cleveland Clinic advises a pregnancy test; the NHS says see a GP if a test is negative and you have missed 3 consecutive periods.
- A calendar predicts the period, not ovulation; the next section explains why.
Can a calendar tell you your fertile window?
Only roughly, and not reliably enough for contraception or for timing a pregnancy attempt in an irregular cycle. The NHS says ovulation happens about 12 to 16 days before the next period starts, and its trying-to-get-pregnant page puts it at "usually about 14 days before the start of your next period", which depends on cycle length. NICHD gives a wider range: ovulation "can occur anywhere between 10 and 21 days after the first day" of the cycle. Counting back from a period that has not happened yet is the weak link; you only know the true ovulation day in hindsight.
For couples trying to conceive, the NHS advice avoids the problem: sex without contraception every 2 to 3 days makes pregnancy likely within a year for most people aged 39 or under.
For contraception the numbers are sobering. The NHS says fertility awareness methods, which combine the calendar with daily temperature and discharge observations, are 91% to 99% effective with perfect use but 76% effective with typical use, meaning 24 in 100 women get pregnant in a year. It takes 2 to 3 cycles to learn, and the NHS says the method may not suit people with irregular periods. The US CDC puts the typical-use failure range at 2% to 23% a year. A 2004 Cochrane review found only three weak randomised trials and concluded that the comparative efficacy of these methods "remains unknown". Calendar-only counting, without temperature or mucus signs, is the least reliable version. The NHS does not officially recommend any app or fertility monitor.
Which period patterns need a doctor?
Cycles shorter than 21 days or longer than 35 days, bleeding that lasts more than 7 days, and bleeding between periods or after sex are the patterns the NHS and Cleveland Clinic say to report. Your calendar is the evidence; take it to the appointment.
See a gynaecologist or a general physician if the record shows:
- cycles under 21 days or over 35 days (NHS, Cleveland Clinic)
- bleeding longer than 7 days (NHS), or more than 8 days including spotting (FIGO)
- no period for 3 months or longer, with a negative pregnancy test (Cleveland Clinic; NHS)
- soaking a pad or tampon every 1 to 2 hours, clots larger than about 2.5 cm, needing two products at once, or feeling tired and breathless, which the NHS lists as signs of heavy periods and possible anaemia
- bleeding or spotting between periods, or after sex
- irregular periods together with weight gain, facial hair, acne or tiredness, which the NHS links to PMOS and thyroid problems
- irregular periods when you are trying to conceive
- no period by age 16 (Cleveland Clinic)
See a doctor the same day if you are bleeding through a pad every hour for several hours, feel faint or breathless, or have severe pain with a possible pregnancy. In an emergency in India call 112.
A doctor often starts with an examination, a pelvic ultrasound and blood tests; the calendar tells them where to look.
Are period-tracking apps safe, and what about privacy?
Apps do the counting for you, but they hold intimate data, and several share it. Cleveland Clinic says tracking apps can replace the paper calendar, and the NHS notes that no app or fertility monitor is officially recommended. A 2025 analysis in the Journal of Medical Internet Research by Zadushlivy and colleagues reviewed four widely used apps (Clue, Flo, Period Tracker by GP Apps and Stardust) and found that three of the four state that some data are shared with third parties, including for marketing. On a 5-point privacy and security scale the apps scored 2 to 4.5. The authors flagged collection of IP addresses and device identifiers, past sharing with Facebook analytics, and the possibility of data being handed to authorities.
Their advice to users, which applies in India too:
- choose an app that collects less data and lets you use it without an account or with an anonymous one
- turn on a passcode
- read what the app says it shares before entering symptoms, sex or pregnancy details
- export or copy your records so the data is yours if you delete the app
A paper calendar or a plain phone note shares nothing. If you use an app, enter only what you need: day 1, bleeding days and flow. The prediction an app shows is the same arithmetic described above, with the same limits; it does not know your ovulation day either.
This guide is general information from the sources listed below, not medical advice; a registered doctor makes decisions with you.
