Period Calculator
Knowing roughly when your next period may start can help with travel, training and everyday planning. This calculator takes the first day of your last period, your usual cycle length and how long bleeding normally lasts, then estimates the start and end dates of the next six periods. It is calendar math, not a medical prediction.
How the period estimate works
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1
Enter the last start date
Use day 1 of your most recent full period, not spotting.
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2
Add your cycle length
Use your usual number of days from one period start to the next.
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3
Add bleeding duration
Enter how many days bleeding usually lasts for you.
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4
Review the calendar estimate
The tool lists the estimated start and end dates for the next six periods.
The simple math
next_start = last_start + cycle_length days
next_end = next_start + (duration - 1) days
Each later cycle uses the previous estimated start date plus the cycle length. Real menstrual cycles can move by a few days from month to month, even when they are usually regular.
Typical ranges
| Item | Typical range | Notes |
|---|---|---|
| Cycle length | 21-35 days | 28 days is common, but your own average matters more |
| Period bleeding | 2-7 days | Many people bleed for about 5 days |
| Ovulation timing | Often about 10-16 days before the next period | Stress, illness, contraception and life stage can shift timing |
| Fertile days | Around the days before and including ovulation | This calculator does not identify fertile days or confirm ovulation |
What this tool is not
- Not a diagnosis or medical device. Irregular cycles, very heavy bleeding, severe pain, bleeding between periods or sudden changes are worth discussing with a qualified clinician.
- Not a contraceptive method. Calendar estimates can be wrong. If you are avoiding pregnancy, use a reliable contraception method and professional advice where needed.
- Not an ovulation test. Cervical mucus, basal body temperature and LH tests can give better ovulation clues than calendar projection alone.
Making the prediction better
Track your actual start dates for at least a few cycles. If your recent cycles were 26, 29 and 28 days, use that personal pattern instead of assuming 28 every time. If your cycle length often varies by more than about a week, treat any calendar projection as a broad reminder rather than a date to rely on.
Frequently Asked Questions
It depends on how stable your cycle length is. If your cycles are usually close together, the estimate is often within a few days. If your cycles are irregular, use it as a broad heads-up rather than a date to plan around.
Day 1 is the first day of normal period flow, not light spotting. If you are unsure, use the first day bleeding was clearly noticeable. A one-day difference at the start affects every estimated cycle after it.
No. This version estimates period start and end dates only. Ovulation often happens roughly 10-16 days before the next period, but calendar math alone cannot confirm ovulation or fertile days.
The tool uses your entries to calculate the result for the current page, but it does not create a user record or save your period dates as part of the page content. Avoid entering sensitive data on shared devices.
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