When will my next period start?
Add your usual cycle length to the first day of your most recent period. The result is an estimate; timing can vary.
YOUR CYCLE, AT A GLANCE
Period calculator
Estimate your next period, ovulation date and fertile window. Free, private and easy to use.
Estimates, not certainties. Ovulation can vary even with regular cycles. This calculator cannot diagnose a condition or reliably prevent pregnancy.
Continue tracking in Selin
Your web calculation gives you an estimate. Selin helps you track cycles over time, log symptoms, set reminders and see future predictions. These web dates are not transferred to the app.
- Log actual period starts
- Record your symptoms
- Set reminders
Quick answers about your cycle
When am I likely to ovulate?
This tool models ovulation 14 days before the next estimated period. It cannot confirm the exact day.
What are fertile days?
Days around ovulation when conception may be possible. The marked dates are approximate and do not identify safe days.
Can a period calculator be exact?
No. It projects dates from the values you enter; it does not measure future menstruation or ovulation.
How the Selin period calculator works
Enter your cycle information
Choose your most recent start and your usual cycle and period lengths.
See your estimated dates
Explore your next period, approximate ovulation and a 3, 6 or 12-month calendar.
Continue tracking with Selin
Use the app for an ongoing history, symptoms and reminders. Web entries are not transferred.
How we calculate your dates
Next period
Most recent period start + average cycle length = estimated next start.
Period duration
Estimated start + period length − 1 day = estimated final bleeding day.
Ovulation
Estimated next start − 14 days = modeled ovulation date.
Fertile window
Modeled ovulation − 5 days through + 1 day. These are estimates, not measurements or a diagnosis.
NHS — Menstrual cycles and fertility
Estimates, not certainties. Ovulation can vary even with regular cycles. This calculator cannot diagnose a condition or reliably prevent pregnancy.
Understand your menstrual cycle
What is a menstrual cycle?
A cycle starts on the first day of a period and ends the day before the next one. Hormones influence ovulation and changes in the womb lining. The bleeding days are only one part of the cycle; cycle length counts the whole interval between period starts.
What is a normal cycle length?
NHS guidance describes regular adult cycles of 21–35 days and bleeding of about 2–7 days. These are general patterns, not a diagnosis. Your age, contraception and life stage matter. This calculator accepts a wider input range so it can flag unusual values without deciding what is healthy for you.
Why isn’t every cycle exactly 28 days?
28 days is a common reference, not a target. Timing can differ between people and between cycles. Replace the example input with your usual length. Longer forecasts repeat that same length; they cannot anticipate a future change in your pattern.
Why can a period arrive earlier or later?
Pregnancy, stress, weight changes, hormonal contraception and life-stage changes can affect timing. A calendar cannot identify the cause of a late period. If pregnancy is possible, use a pregnancy test rather than treating a prediction as an answer.
How is ovulation estimated?
The tool subtracts 14 days from the estimated next period. This is a modeling assumption, not evidence that an egg was released. Ovulation can occur at a different time even in regular cycles. Calendar dates do not measure hormones or other signs of ovulation.
What is the fertile window?
It describes the days around ovulation when conception may be possible. The shaded window uses five days before the modeled ovulation date through one day afterward. It is an illustration, not a boundary: actual fertile days may fall outside it, and unmarked days are not “safe” days.
What if my periods are irregular?
A single typical cycle length may describe an irregular pattern poorly. The tool can still illustrate dates, but those dates may be less useful for planning. Keep a record of actual starts in your own diary or app and discuss changes with a healthcare professional rather than repeatedly adjusting a prediction.
How accurate is a period calculator?
It is a planning aid, with no validated individual accuracy percentage. It uses the three values you enter and repeats the same cycle length. It cannot know whether your next cycle will be shorter or longer, and uncertainty grows further into the future.
Can I use it for contraception?
No. Do not rely on this calendar to avoid pregnancy. Fertility-awareness methods require more than this calculation and appropriate instruction; they may not suit everyone. Ask a qualified healthcare professional about contraceptive options that fit your needs.
When should I talk to a healthcare professional?
Seek advice if your usual pattern changes, periods are irregular, bleeding lasts more than seven days, or symptoms concern you. Bleeding between periods or after sex also needs assessment. A calculator cannot explain these changes or replace a clinical consultation.
NHS — Periods and cycle patterns · Irregular periods and when to seek advice
Period calculator: quick facts
- What information do I need?
- Your last period start, average cycle length and average period length.
- Can it estimate my next period?
- Yes, using the values you enter. It cannot guarantee the date.
- Can it determine ovulation exactly?
- No. It uses a calendar assumption, not a measurement.
- Is it useful for irregular cycles?
- Only as a rough illustration; a changing pattern makes forecasts less reliable.
- Can I use it as birth control?
- No. Unmarked days must not be treated as safe days.
- Is the calculator free?
- Yes. No account or app installation is required to calculate.
- Does Selin save these web entries?
- No. They stay in the isolated browser tool, are not sent to analytics, and clear on reload. The app has its own privacy policy.
Your questions, answered
Practical answers about your dates, cycle changes and privacy. Open a question to learn more.
Using your calendar
How do I calculate my next period?
Enter the first day of your most recent period, then your usual cycle and period lengths. Select Calculate my cycle to see your estimate.
What if I don’t know my average cycle length?
Count the days between actual period starts over several cycles and work out your usual interval. The default 28 is only an example, not a personalized estimate. For example, intervals of 27, 29 and 28 days average 28 days. Count from the first day of one period to the first day of the next, not the number of bleeding days.
Understanding your cycle
Why is my period late?
There are several possible reasons, including pregnancy and changes in stress or hormones. A calendar cannot determine the cause. Consider a pregnancy test if pregnancy is possible.
NHS — Missed or late periodsCan my cycle length change each month?
Yes. That is why repeating one average into the future gives estimates rather than appointments your body must keep.
NHS — Irregular periods and when to seek adviceWhat if my periods are irregular?
Treat the calendar cautiously. Record actual starts and speak with a healthcare professional about irregularity or a change in your usual pattern.
NHS — Irregular periods and when to seek adviceAccuracy & privacy
How accurate is an online period calculator?
Its usefulness depends on how closely future cycles follow the entered length. This calculator has no established individual accuracy percentage. The 6- and 12-month views repeat the same cycle length; they are not more certain forecasts. Update your last start date when your next period begins.
Can I use this calculator to avoid pregnancy?
No. The calendar does not identify reliably safe days and is not a contraceptive method.
NHS — Fertility awareness and contraceptionDoes Selin store the dates I enter?
The web tool keeps them only in isolated browser memory and clears them on reload. App-store buttons do not save or transfer the dates.
Medically reviewed by
Elena VasilievaGynecologist and sexologist, Selin
Reviewed:
Sources and references
Your cycle doesn’t stop at one calculation.
Build your ongoing cycle history with Selin. Keep periods, symptoms and reminders together, at your own pace.
- Log actual period starts
- Record your symptoms
- Set reminders
