Choose the start date
Select the first day of normal bleeding for your most recent period. If you only remember an approximate date, treat the result as approximate too.
free · browser-based · estimated
Period Calculator: Predict Your Next Period
Estimate your next period, upcoming cycle dates, and an approximate fertile window from the first day of your last period. Enter two simple details and get a clear calendar estimate without an account.
- Free to use
- No account
- Private by default
Period calculator tool
Calculate your next period
Use this period calculator by entering the first day you started bleeding and the average number of days between period starts. The example is ready to edit, so you can see the complete result before entering your own dates.
Example result
Estimated
Next expected period
Next expected period
This example uses a 28-day cycle and is ready to replace with your own dates.
Estimated ovulation
Estimated fertile window
August 24–30, 2026
Estimated period end
Upcoming period calendar
28-day cycle| Cycle | Estimated start | Estimated end |
|---|---|---|
| 1 | September 12, 2026 | September 16, 2026 |
| 2 | October 10, 2026 | October 14, 2026 |
| 3 | November 7, 2026 | November 11, 2026 |
| 4 | December 5, 2026 | December 9, 2026 |
| 5 | January 2, 2027 | January 6, 2027 |
| 6 | January 30, 2027 | February 3, 2027 |
Your actual dates may vary from cycle to cycle. Calendar timing cannot confirm ovulation or diagnose a health condition.
simple inputs, useful output
How to use this period calculator
The basic period cycle calculator uses the first day of your last period and your average cycle length. Period length changes the estimated end date, while irregular-cycle mode creates a range instead of pretending one day is certain.
Add your cycle average
Enter the number of days from one period start to the next. Use your own recent average instead of assuming every cycle is 28 days.
Review the estimate
Select Calculate period to see what the period calculator estimates: the next expected date, estimated period end, optional ovulation timing, fertile window, and upcoming cycle dates.
What does a period calculator show?
The main answer is an estimated start date for your next period. The rest of the result helps you understand how that date was produced and how much calendar timing can move.
Next expected period
This period calculator estimates the date by adding your average cycle length to the first day of your last period. It is a planning estimate, not a confirmed event. Update the tool with your actual start date after the next cycle begins.
Estimated period end
If you enter period length, the calculator adds that many bleeding days to the predicted start. It does not change the next start date; it only shows a possible end date for planning.
Estimated ovulation and fertile window
These dates use a general calendar method and are labeled estimated or approximate. Ovulation can shift, and a calendar cannot confirm that it happened. Do not use this result as contraception.
What if my periods are irregular?
Turn on My cycles vary to enter your shortest and longest typical cycle lengths. The calculator then shows a possible start range derived from those values. A range is more honest than one exact date when your cycles move from month to month. The average-date estimate remains available to help you compare the range with a typical cycle.
Range mode reflects the cycle lengths you enter; it does not diagnose the reason your cycles vary.
What if my period is late?
A period calculator can show that an estimated date has passed, but it cannot explain why. First review the last period date and check whether the average cycle length reflects your recent cycles. Stress, illness, travel, sleep changes, medication, hormonal changes, breastfeeding, perimenopause, and normal variation can all affect timing.
Contact a qualified healthcare professional if you are worried about a possible pregnancy, repeated major cycle changes, severe pain, unusually heavy bleeding, or other concerning symptoms. For general menstrual-cycle information, see the Office on Women’s Health menstrual cycle resource.
Read the Office on Women’s Health resource
use the estimate thoughtfully
Accuracy, privacy, and limitations
This free period calculator is designed for general education and simple planning. It gives a transparent calendar estimate from the details you provide; it is not a diagnostic tool and does not replace professional medical advice.
Accuracy
The calculator is more useful when your recent cycles are fairly consistent and your last-period date is correct. Even then, actual timing can shift. Recalculate after each real period start if you want the calendar to reflect newer information.
Privacy
The MVP performs date calculations locally in the browser. No account, name, email address, or health profile is needed for the basic tool. Read the Privacy Policy for information about site analytics, cookies, support messages, and advertising disclosures.
Limitations
Do not use a calendar estimate to prevent pregnancy, confirm ovulation, diagnose pregnancy, assess fertility, or decide whether symptoms are safe to ignore. Use appropriate contraception and seek qualified care for medical concerns.
quick answers
Period calculator FAQ
How does a period calculator work?
A period calculator adds your average cycle length to the first day of your last period to estimate the first day of your next period. It can also list future cycle dates and estimate a period end when you provide period length.
What date should I enter for my last period?
Enter the first day of normal menstrual bleeding, not the day your period ended. Do not substitute a spotting day unless that is how you normally define the start of your period.
Is a 28-day cycle required?
No. Twenty-eight days is only the prefilled example. Use the average from your own recent cycles when you can, and keep the estimate labeled as approximate.
Can I calculate my next period if my cycles are irregular?
Yes. Turn on My cycles vary and enter your shortest and longest typical cycle lengths. The result will show a possible date range derived from your inputs rather than one false-precision date.
Can I use this period calculator to prevent pregnancy?
No. A calendar prediction cannot confirm ovulation or reliably identify safe days. Do not use this tool as contraception; use appropriate contraception and ask a qualified healthcare professional about pregnancy-related questions.
Why is my actual period different from the prediction?
The calculator uses past timing to estimate future timing. Stress, illness, travel, sleep changes, medication, hormonal changes, and normal cycle variation can move the actual date. Record the real start date and recalculate with updated information.
Can a period calculator tell me when I ovulate?
It can provide a calendar-based estimate, but it cannot confirm ovulation. Ovulation timing can vary, so the estimated ovulation date and fertile window should not be treated as medical certainty.
Is this period calculator private?
The basic period calculator runs in your browser and does not require an account or personal profile. Site analytics, cookies, support, and advertising practices are described in the Privacy Policy.