Free Period & Menstrual Cycle Calculator Online - No Signup
Estimate your next period, current cycle day, menstrual phase, likely date range, and upcoming cycles using your own average pattern. Your entries stay in your browser.
Next-period estimate
Cycle phase overview
Future projections
Private local calculation
Menstrual Cycle Calculator
Enter the first day of your most recent period and your typical cycle pattern. Dates are estimates-not medical findings or confirmation of ovulation.
Runs privately in your browser
Your cycle estimate
Based on the average pattern you entered.
Estimated next period
-
Estimated start range: -
-
days until the modeled start date
Timing may vary from cycle to cycle.
Estimated cycle phase
Cycle phases are calendar-based estimates.
Period
Follicular
Ovulation
Luteal
Projected cycle calendar
Future dates assume the average pattern repeats. Update the calculator after each actual period.
Period estimate
Ovulation estimate
Important health note:
This calculator provides calendar estimates only.
It does not diagnose a condition, confirm ovulation
or pregnancy, or provide a reliable method of
contraception. Seek qualified medical advice for
concerning symptoms, pregnancy questions, or
significant cycle changes.
Private by design
Entries stay in your browser
Flexible ranges
Accounts for cycle variation
Future planning
Projects six upcoming cycles
Calendar ready
Export period estimates as ICS
How a menstrual cycle calculator works
A menstrual cycle begins on the first day of menstrual bleeding and ends the day before the next period begins. The cycle length is the number of days between those two starting dates.
This calculator adds your average cycle length to your last period start date. It then uses your entered variation to display a possible date range rather than presenting one date as certain.
If the first modeled date has already passed, the calendar continues the average pattern to display the next projected start. This does not mean an unrecorded period actually occurred-it is simply a mathematical projection.
Why your actual date may be different
Cycle timing can change with stress, travel, illness, medication, sleep disruption, exercise, nutritional changes, breastfeeding, perimenopause, adolescence, and hormonal or reproductive health conditions.
The menstrual cycle
Four phases in a calendar-based cycle model
Hormonal timing differs between individuals and cycles. These phases provide a simplified educational overview.
01
Menstrual phase
The cycle begins with menstrual bleeding. Cycle day one is generally the first day of full flow.
Cycle begins02
Follicular phase
This phase extends from menstruation toward ovulation. Its length can vary considerably between cycles.
Variable timing03
Ovulation estimate
Calendar methods often estimate ovulation approximately 14 days before the next period, but actual timing varies.
Not confirmation04
Luteal phase
The luteal phase follows ovulation and continues until the next menstrual cycle begins.
Before next period
Useful planning tools
More than a single next-period date
View a practical range, modeled cycle phase, future periods, and calendar-ready estimates in one responsive interface.
Period date range
Cycle variation creates an earlier-to-later range around the central estimated date.
Cycle phase timeline
See your modeled cycle day and approximate position across menstrual, follicular, ovulatory, and luteal phases.
Six future projections
Review upcoming period, fertile-window, and ovulation estimates for planning purposes.
Private processing
The calculation runs locally without requiring an account or sending your entered dates to a calculation server.
Calendar export
Download estimated periods as an ICS calendar file for compatible calendar applications.
Copy and print
Copy a plain-text summary or print a clean version of your estimated cycle calendar.
Responsible cycle tracking
How to use menstrual estimates safely
Calendar projections are most useful as planning aids when combined with real observations and appropriate healthcare.
1
Record actual start dates
Replace predictions with observed dates each month. An estimate does not confirm that a cycle began.
2
Track patterns, not one cycle
Several completed cycles provide more context than a single month, especially when timing varies.
3
Do not treat ovulation as confirmed
Calendar arithmetic cannot determine whether ovulation happened or identify its exact date.
4
Consider relevant health changes
Illness, stress, medication, postpartum changes, and life stage can make previous averages less useful.
When professional advice may be appropriate
Consider contacting a qualified healthcare professional if you have very heavy bleeding, severe or sudden pain, dizziness or fainting, bleeding during pregnancy, periods that repeatedly last longer than expected, a major unexplained change in your usual pattern, or concerns about a missed period or possible pregnancy. Seek urgent care for severe symptoms.
Common questions
Menstrual cycle calculator FAQ
How is my next period date calculated?
The calculator adds your average cycle length to the start date of your last period. If that modeled date has passed, it continues the same average pattern to show the next modeled period date.
How accurate is a menstrual cycle calculator?
The result is an estimate based on the information entered. It may be more useful when several recent cycles are regular, but stress, illness, medication, hormonal changes, life stage, and other factors can change cycle timing.
What counts as cycle day one?
Cycle day one is generally the first day of menstrual bleeding rather than light spotting before the period begins.
Can this calculator confirm ovulation?
No. It only estimates ovulation using a calendar method. It cannot confirm whether or when ovulation occurred.
Can I use these dates to prevent pregnancy?
No. Calendar estimates should not be used as contraception because ovulation and fertile timing can vary, even in people with generally regular cycles.
Are my menstrual cycle details stored or uploaded?
No. Calculations run locally in the browser. This page does not need to upload the dates entered in the calculator.
What should I enter for cycle variation?
Enter the typical number of days your recent cycle lengths differ from your average. For example, if cycles are usually 27 to 31 days around a 29-day average, a variation of about 2 days may be appropriate.
Related health tools
Continue your health planning
Explore related educational calculators while remembering that online estimates are not a substitute for healthcare.
Plan your next cycle with a clearer estimate
Enter your latest period date and average cycle pattern to view a private, responsive cycle timeline with future period projections.
Calculate My Cycle