Most period apps are sold on predicting your next period. A period diary does a different job. It records the periods you’ve already had, and that record is what changes a conversation with a doctor. You can start one today with nothing but a notebook.
This guide covers what belongs in a period diary, what to leave out, why writing it down the same day matters more than writing a lot, and the patterns that mean you should book an appointment.
Why a diary beats a calendar
The NHS says in its guidance on irregular periods that using an app or a diary to keep track of your periods can help you see if they’re irregular. You can’t judge a pattern from memory, because memory keeps the dramatic cycles and quietly drops the ordinary ones.
Clinicians go further. The American College of Obstetricians and Gynecologists argues in its committee opinion on using the menstrual cycle as a vital sign that the cycle deserves to sit alongside blood pressure and pulse as a routine measure of health. It also encourages prospective charting of bleeding, which means a record kept as you go rather than one reconstructed afterwards. The aim is to catch an abnormal pattern early.
The one entry that matters most
If you record nothing else, record the first day of bleeding. The NICHD at the National Institutes of Health defines it as the first day of the menstrual cycle, so it’s day 1, and every other number comes from it. Your cycle length is the gap between one day 1 and the next, so the last day of a cycle is the day before your next period starts.
Two rules keep this clean. Spotting isn’t day 1, so note it separately and don’t let it move the start date. And if a period arrives overnight, pick the day you’d name if someone asked, then stick with that choice. Consistency matters more than being right by twelve hours.
What a clinician means by a normal cycle
Clinicians judge a cycle on four things, the ones ACOG borrows from the International Federation of Gynecology and Obstetrics. They’re frequency, regularity, duration, and volume of flow. A good diary lets you answer all four.
- Frequency is how often periods arrive. The NHS gives a typical range of every 21 to 35 days, with around 28 days in the middle. NICHD notes that teenagers can range from 21 to 45 days and still be normal.
- Regularity is how much that gap moves from cycle to cycle. A steady 33-day cycle is nothing like one that bounces between 24 and 40, even though neither average looks alarming on its own.
- Duration counts how many days you bleed. The NHS says between 2 and 7 days, usually about 5, heaviest in the first 2. Our guide to how long a period lasts covers what sits outside that range.
- Volume measures how heavy the bleeding is, and it’s the hardest one to write down accurately.
What to write down
Think in three tiers. Almost nobody keeps up the third tier for long, and that’s fine.
Tier one. Never skip these.
- The first day of bleeding.
- The last day of bleeding.
- Spotting on any day outside those, marked as spotting.
Those three facts per cycle answer frequency, regularity, and duration on their own, and they’re more than most people bring to an appointment.
Tier two. Add these if you can.
- Flow, in something countable. Products used per day beats light, medium, or heavy, because your sense of heavy drifts over the years while a count doesn’t.
- Pain. Note where it is and whether it stopped you doing something. Pain that changed your day is the detail a clinician reacts to. With endometriosis the pain record matters more than the dates, so we have a separate guide on tracking periods with endometriosis.
- Mood and energy. Watch the week before bleeding in particular. These changes feel obvious in the moment and are gone from memory a month later.
- Anything unusual. Bleeding between periods, bleeding after sex, or a period that arrives with a fever all count, and one line is enough.
Tier three. Only if you have a reason.
Basal body temperature, cervical mucus, ovulation tests, sleep, and exercise all belong here. They’re useful if you’re trying to conceive or looking into a specific problem. If you’re not, they’re the fields that make people give up on a diary in month two. NICHD notes that ovulation can happen anywhere from 10 to 21 days after day 1, so a calendar estimate is a poor guide and some people want a real signal. If that isn’t you, skip the tier.
Write it down the same day
A diary filled in on Sunday for the whole week comes from memory, and memory blurs the details. The habit that lasts is one quick entry at the same time each day. It takes a few seconds.
Missing days is normal and no reason to abandon the diary. Leave the gap. A guessed entry is worse than an empty one, because it looks like data and isn’t.
How much detail is too much
A diary can stop being a record and become a source of anxiety. You’ve crossed that line when you’re logging more than you’d ever read back, feeling bad about missed days, or checking a predicted date often enough that it affects your mood.
If that happens, cut fields. A diary you keep for two years at tier one is worth more than a perfect one you keep for six weeks, because the whole value is in the length of the series.
When the diary should send you to a doctor
The NHS suggests speaking to a GP if the gap between periods is consistently less than 21 days or more than 35, if your normal pattern changes, if periods last longer than 7 days, if you bleed between periods, or if irregular periods come with weight gain, tiredness, facial hair growth, or skin changes.
For volume, the NHS gives a concrete checklist for heavy periods: needing to change a pad or tampon every 1 to 2 hours, needing two products at once, passing clots larger than about 2.5cm, bleeding through to clothes or bedding, periods lasting more than 7 days, or skipping normal activities because of them. Mention feeling tired or short of breath alongside heavy bleeding too, because it can point to low iron.
Take the diary with you. Being able to say when the last six periods started, how long they lasted, and how heavy they were turns a vague appointment into a specific one. If your cycles are long or unpredictable, our guides on why periods go irregular, tracking with PCOS and tracking in perimenopause go deeper.
Period diary app or paper?
Paper is private and never nags you, but you have to do the arithmetic yourself. Counting gaps between dates across a year by hand is tedious enough that most people stop doing it. The pattern then stays hidden even though the data exists.
An app does that counting for you, and it’s in your hand at the moment you’d otherwise forget. The trade-off is that someone else may hold your data, and for a record this personal you should know who. Our checklist for choosing a private period tracker works on any app, including ours. If you want names, our roundup of the best period tracker apps names one app for each job, such as sharing with a group of friends or keeping everything on the phone.
How Plum handles the diary side
We build Plum, a period diary app for iPhone, so this is what it does.
Tier one is the default. Marking a period day takes a tap, and flow level and spotting sit next to it. Spotting stays separate from period days, so it never shifts your day 1. Tier two lives one level down and covers pain, discharge, skin, digestion, sleep, energy, mood, PMS, cravings, exercise, and medication, plus ovulation and pregnancy tests for anyone who wants tier three. You choose which of those categories appear, so the daily entry stays as short as you want it. Plum has no temperature entry and no free-text note, so if your diary needs either, keep that part somewhere else.
Plum does the arithmetic. It keeps your cycle lengths and predicts from a rolling median rather than a plain average, so one strange cycle doesn’t distort everything after it. Once your cycles vary enough that a single confident day would be dishonest, it shows a prediction window instead.
On the data question, Plum shows no ads and never sells personal data. It does require an account and syncs to our servers, encrypted in transit and at rest, so we won’t tell you it stays on your phone. You can delete your account and everything in it from inside the app whenever you like. If you share a cycle with a group, the group sees a summary of your dates, never your symptoms or moods.
