Period Calculator
Estimate when your next menstrual period may start based on your cycle.
Usually between 21 and 35 days.
Usually between 2 and 7 days.
Understanding your cycle
What Is a Period Calculator?
A Period Calculator helps estimate when your next menstrual period may start. It uses your last period start date, average cycle length, and usual period length to predict upcoming period dates.
The menstrual cycle is counted from the first day of one period to the first day of the next period. A 28-day cycle is common, but many normal cycles can range from about 21 to 35 days.
How to use
Key Inputs Explained
The calculator needs three inputs to estimate upcoming period dates and likely ovulation windows.
- First Day of Last Period
- The date your most recent period started. This acts as the baseline for predicting the next cycles.
- Average Cycle Length
- Number of days from one period start to the next. The default average is often 28 days.
- Period Length
- Number of days your period usually lasts. Common averages are 5 to 7 days.
The math
Period Calculator Formulas
To estimate your next period, the calculator uses straightforward math based on your cycle length.
Next period start
This is the primary formula used to project the next cycle.
| Output | Formula |
|---|---|
| Period End Date | Period Start + Period Length - 1 day |
| Future Period | Last Period + (Cycle # x Cycle Length) |
| Estimated Ovulation | Next Period Start - 14 days |
| Fertile Window Start | Ovulation Date - 5 days |
Example
Example Calculation
If your last period was on May 22, 2026, and your cycle length is 28 days, the estimated next period and ovulation dates follow this pattern.
| Step | Date |
|---|---|
| Last Period Start | May 22, 2026 |
| Next Period Start | June 19, 2026 |
| Estimated Ovulation | June 5, 2026 |
| Fertile Window | May 31 - June 6, 2026 |
Cycle changes
Why Period Dates Can Change
A period calculator gives an estimate because real cycle dates can shift. If your cycle length changes, your next period dates will also shift accordingly.
- Stress
- Stress can shift hormone patterns and affect timing.
- Illness
- Illness and recovery can temporarily change cycle timing.
- Travel and sleep changes
- Schedule disruption may affect cycle regularity.
- Weight and exercise changes
- Rapid changes in weight, nutrition, or activity can shift periods.
- Hormonal changes
- Birth control, pregnancy, breastfeeding, perimenopause, PCOS, and thyroid conditions can all affect timing.
Frequently Asked Questions
A period calculator estimates your next period start date, period end date, ovulation date, fertile window, and future cycle dates based on your last period and average cycle length.
The basic formula is: Next Period Start = Last Period Start Date + Average Cycle Length.
Cycle length is counted from the first day of one period to the first day of the next period.
The average menstrual cycle is often about 28 days, but cycles from about 21 to 35 days can also be normal for many people.
Many periods last about 2 to 7 days, but this can vary from person to person.
Yes. It can estimate ovulation by counting backward from the next expected period. A common formula is: Ovulation Date = Next Period Start - 14 days.
No. A period calculator should not be used as the only method to avoid pregnancy because ovulation and fertile days can shift from cycle to cycle.
Someone should consider speaking with a healthcare provider if periods are very heavy, very painful, missing for several months, unusually frequent, unusually far apart, or if there is bleeding between periods.
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