Period Calculator
Knowing roughly when your next period is due helps with travel plans, training blocks and just not being caught off guard. This calculator takes the first day of your last period, your typical cycle length in days, and how long bleeding usually lasts, and projects the start and end dates of your next six cycles. It is an estimate based on calendar math, not a medical prediction.
How to estimate your next periods
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1
Enter last period start date
Day 1 of your most recent period.
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2
Set cycle length
Typical cycle length in days - usually 21 to 35, with 28 as the average.
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3
Set period duration
How many days bleeding usually lasts (typically 3-7).
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4
View projected cycles
Start and end dates for the next six projected periods appear in a table.
The simple math
next_start = last_start + cycle_length days
next_end = next_start + (duration - 1) days
Each subsequent cycle is the previous start plus the cycle length. This is a naive projection - a real cycle varies by a few days most months even for people with fairly regular cycles.
Typical ranges
| Phase | Range | Notes |
|---|---|---|
| Cycle length | 21-35 days | Average 28; varies by person and life stage |
| Period duration (bleeding) | 3-7 days | Average 5 |
| Ovulation (from next period) | ~14 days back | Can shift, especially with stress/illness |
| Fertile window | 5 days before + day of ovulation | Sperm can live up to 5 days |
What this tool is not
- Not a medical device. Irregular cycles, very heavy flow, severe pain or sudden changes deserve a proper conversation with a clinician.
- Not a contraceptive method. Calendar-based methods (rhythm, Standard Days) have failure rates typically in the 12-24% range with typical use. If you are avoiding pregnancy, use a reliable method in addition.
- Not an ovulation test. Tracking cervical mucus, basal body temperature or LH strips gives far better ovulation signals than pure calendar projection.
Making the prediction better
Keep a log of actual start dates for a few months. If your cycles are 26-30 days, do not assume 28; use your own average. If they are wildly irregular (more than 7 days of variation), calendar projection will be unreliable and a period-tracking app with adaptive models does better.
Frequently Asked Questions
As accurate as your cycle length is stable. For people with fairly regular 26-30 day cycles, projections are usually within 2-3 days. For highly irregular cycles, treat any calendar projection as a rough heads-up, not a date to plan around.
Day 1 is the first day of full bleeding, not spotting. If you are ever unsure, use the first day of noticeable flow - small errors here propagate through all six projected cycles.
This simple version projects cycle starts and ends only. Ovulation is typically 14 days before the next period, with a fertile window of 5 days before ovulation plus the day itself. Use a dedicated ovulation calculator for those dates.
No. Everything runs locally in your browser and nothing is transmitted or saved server-side.