Period Calculator: Predict Your Next Period Date
Use our free Period Calculator to estimate when your next period may start. Enter your last period date and cycle length to see upcoming periods, expected bleeding days, and a monthly calendar.
What Is a Period Calculator?
A period calculator is an online tool that estimates your next menstrual period based on your previous period date and cycle length.
It helps you understand when your period might begin, how long it may last, and what future cycles could look like.
Our calculator supports both regular and variable menstrual cycles, making it useful for everyday planning.
How to Use the Period Calculator
Follow these simple steps:
- Select the first day of your last period.
- Choose Regular Cycle or Variable Cycle.
- Enter your average cycle length or shortest and longest cycle lengths.
- Enter how many days your period usually lasts.
- View your next expected period date.
- Explore the monthly calendar and six upcoming period predictions.
- Select Copy Result to save your estimates.
You can also enter previous period start dates to calculate your average cycle length automatically.
How to Calculate Your Next Period
Your menstrual cycle begins on the first day of one period and ends the day before your next period starts.
To estimate your next period, add your average cycle length to the first day of your last period.
Next Period Date = Last Period Start Date + Cycle Length
Period Calculation Example
Suppose your last period began on October 1, 2026, and your average cycle length is 28 days.
October 1 + 28 days = October 29, 2026
If your period usually lasts five days, your estimated bleeding dates would be October 29 through November 2.
These are predicted dates, and your actual period may begin earlier or later.
What Is a Normal Menstrual Cycle Length?
Menstrual cycle length varies between individuals.
For many adults, cycles between approximately 21 and 35 days are common. Menstrual bleeding often lasts about two to seven days.
The following table shows how cycle length affects the next expected period date.
| Cycle Length | Next Period After October 1, 2026 |
|---|---|
| 21 days | October 22, 2026 |
| 25 days | October 26, 2026 |
| 28 days | October 29, 2026 |
| 30 days | October 31, 2026 |
| 35 days | November 5, 2026 |
Your cycle does not need to be exactly 28 days. What matters for prediction is your usual pattern.
Regular vs Variable Period Cycles
Our menstrual cycle calculator supports two calculation methods.
Regular Cycle
If your periods arrive at fairly consistent intervals, choose Regular Cycle.
Enter your average cycle length, such as 28 or 30 days.
The calculator uses this value to estimate your next period and future menstrual cycles.
Variable Cycle
If your cycle length changes, select Variable Cycle.
Enter your shortest and longest typical cycle lengths.
For example:
- Shortest cycle: 26 days
- Longest cycle: 31 days
- Last period started: October 1, 2026
Your estimated next period start window would be October 27 – November 1, 2026.
This range is a planning estimate based on the values you enter, not a medically validated prediction interval.
How to Calculate Your Average Cycle Length
If you know the start dates of previous periods, you can estimate your average menstrual cycle length.
For example, imagine your periods began on:
| Period Start Date | Days Until Next Start |
|---|---|
| July 10 | 28 days |
| August 7 | 30 days |
| September 6 | 27 days |
| October 3 | — |
Your average cycle length is:
(28 + 30 + 27) ÷ 3 = 28.3 days
That gives an average of approximately 28 days.
Our calculator lets you enter up to three previous period dates alongside your last period date and apply the observed cycle lengths automatically.
Track Upcoming Periods With the Calendar
The built-in period calendar helps you visualize predicted menstrual dates.
It highlights your estimated period start, expected bleeding days, and possible date windows when using Variable Cycle mode.
You can move between months and review a schedule of your next six projected periods.
This can be helpful when planning travel, work, holidays, or daily activities.
Future predictions become less certain when cycle lengths vary. The calendar does not confirm that a period occurred on a predicted date.
How Accurate Is a Period Calculator?
A period calculator can provide useful estimates, especially when your cycle follows a reasonably consistent pattern.
However, no calendar-based tool can guarantee the exact date your next period will begin.
Cycle timing may change due to stress, illness, hormonal changes, medication, and other factors.
For more useful results, update the calculator with your latest actual period start dates.
Important: Period predictions cannot reliably confirm ovulation, determine fertile days, or serve as a dependable method of contraception.
Why Use Our Period Calculator?
Our free online period calculator includes:
- Next expected period date
- Regular and variable cycle modes
- Shortest and longest cycle calculations
- Previous period date history
- Average cycle length calculation
- Estimated bleeding-date windows
- Interactive monthly period calendar
- Six upcoming period predictions
- Copy Result and Reset
- Private calculations without registration
The tool is designed for quick, convenient menstrual-cycle planning.
Frequently Asked Questions
When will my next period start?
Enter the first day of your last period and your average cycle length. The calculator will estimate your next expected period date.
How do I calculate my period with a 28-day cycle?
Add 28 days to the first day of your last period. For example, if your period started on October 1, your next estimated start would be October 29.
Can I calculate my period with an irregular cycle?
Yes. Select Variable Cycle and enter your shortest and longest typical cycle lengths to get an estimated period start window.
How many days does a normal period last?
Menstrual bleeding commonly lasts around two to seven days, but individual patterns vary.
Can I predict my next six periods?
Yes. The calculator provides six upcoming period projections based on your selected cycle pattern.
What if I don't know my cycle length?
If you have recorded previous period start dates, enter them into the optional history section. The calculator can estimate your average from those dates.
Why is my period earlier or later than predicted?
Cycle timing can vary due to hormonal changes, stress, illness, medication, and other factors. Period prediction dates are estimates rather than guarantees.
Can a period calculator tell me if I'm pregnant?
No. A period calculator cannot determine pregnancy. If your period is late and pregnancy is possible, use an appropriate pregnancy test and seek medical advice when needed.
Is the Period Calculator free?
Yes. You can estimate upcoming periods, explore your menstrual calendar, and copy the results without creating an account.
Is my menstrual information private?
The calculator processes the entered dates locally in your browser and does not require registration or save a tracking history by default.
Health note: Consult a healthcare professional if you experience persistent unusual cycle changes, very heavy bleeding, severe pain, or other concerning symptoms. If you have gone three months without a period unexpectedly, medical assessment is also advisable.