Predict your next period and track your cycle
This period calculator predicts when your next period will start, counts down the days, and lists your next six periods with start and end dates. A colour calendar shows logged periods, predicted periods and your estimated fertile days, so you can plan travel, exams, sport or a holiday around your cycle at a glance.
Most predictors assume a 28-day cycle, but many people’s cycles are shorter or longer. The built-in period tracker records each period’s start and end date, flow and symptoms, works out your real average cycle and period length, and flags cycles outside the usual ranges. The log stays in your own browser — no account, no network — and you can export it as JSON or CSV, import it on another device, or clear it.
How to use the period calculator
- Enter the first day of your last period — the first day of proper bleeding.
- Set your average cycle length (28 days if you don’t know it yet) and how many days your period usually lasts.
- Read the next expected period date and countdown, then check the table for the following six periods.
- Browse the calendar with the arrow buttons to see period and fertile days month by month.
- Each time a period starts, add its date — and, if you like, the end date, flow and symptoms — to Your period log. After two or more entries, press “Use my average” to base predictions on your own cycles. Export a backup now and then.
Period Calculator features
Six-period forecast
Start date, estimated end date and a live countdown for each of your next six periods.
Calendar view
Logged and predicted periods plus estimated fertile days, colour-coded with a legend, one month at a time.
Private period tracker
Start and end dates, flow and symptoms on your device, with each cycle length, your averages, shortest and longest cycle.
Export, import, clear
Back up your history as JSON or CSV, load it elsewhere, or delete it after a confirmation.
Late-period check
If the expected date has passed, the calculator tells you how many days late you are and what to consider next.
Sensible validation
Clear messages for future dates, impossible cycle lengths and duplicate log entries.
How your next period is predicted
Your cycle starts on the first day of bleeding (day 1) and ends the day before the next period begins. The calculator adds your average cycle length to the start of your last period to find the next one, then keeps adding it for the following five. The end date is the start date plus your usual period length, minus one day.
Example: a period that started on 20 February 2024 with a 30-day cycle and a 5-day period predicts the next period from 21 to 25 March 2024, and the sixth from 18 August 2024.
The fertile days shown on the calendar assume ovulation about 14 days before each predicted period. For a closer look at ovulation, the ovulation calculator lets you set your own luteal phase.
What counts as a normal cycle?
For adults, ACOG (FAQ “Abnormal Uterine Bleeding”, reviewed August 2025) gives a normal cycle as 21 to 35 days with bleeding for up to 7 days, and the NHS (“Irregular periods”, reviewed 26 June 2026) calls a gap under 21 or over 35 days irregular. In the first years after periods begin, ACOG says most teens’ cycles last 21 to 45 days. Your own cycle can vary by a few days — logging several cycles gives a better average than guessing.
| Measure | Typical range |
|---|---|
| Cycle length (adults) | 21–35 days |
| Cycle length (teenagers) | 21–45 days |
| Period length | 2–7 days |
| Month-to-month variation | Up to about 7–9 days |
Why a period can be late, and when to see a doctor
A late period is not always a sign of pregnancy. Common reasons include:
- Pregnancy — take a home test from the day your period was due.
- Stress, illness, travel across time zones or poor sleep.
- Significant weight loss or gain, under-eating, or a sudden increase in hard exercise.
- Polycystic ovary syndrome (PCOS) or thyroid problems.
- Breastfeeding, starting or stopping hormonal contraception, or approaching menopause.
Signs worth getting checked
See a GP or gynaecologist if your periods are irregular (under 21 or over 35 days apart), last longer than 7 days, or vary in length by more than 7 to 9 days (ACOG); also for bleeding between periods or after sex, very heavy bleeding, or three missed periods in a row with a negative pregnancy test (NHS). The tracker flags the first three in your log. It gives estimates, not a diagnosis. If a positive test changes your plans, the pregnancy due date calculator is the next step.
Period Calculator FAQ
How does a period calculator work out my next period?
It adds your average cycle length to the first day of your last period. With a 28-day cycle and a last period on 1 May, your next period is expected on 29 May. Logging several periods gives a more accurate average than the default 28 days.
How do I calculate my cycle length?
Count from the first day of one period up to the day before the next period starts. Do this for a few months and average the results — or add the start dates to the period log and the calculator does it for you.
Is my period log private?
Yes. Dates, flow and symptoms are stored in your browser’s local storage on this device only and never sent over the network. Remove an entry with its × button, export a backup, or press “Clear all data” and confirm to delete everything.
How many days late can a period be before I should worry?
A few days’ variation is normal. If you are more than a week late and could be pregnant, take a home test. If you are not pregnant and miss three periods in a row, or your cycles suddenly change, see a doctor.
Can I use the predicted safe days as contraception?
No. Ovulation can shift unpredictably, so calendar predictions are not reliable birth control. Talk to a doctor or family-planning clinic about contraception options.
Why are my predictions wrong some months?
Cycles naturally vary, and stress, illness, travel and changes in weight or exercise can delay ovulation and therefore your period. Keep logging — the average becomes more reliable the more cycles you record.