Estimate your upcoming periods. This free menstrual cycle calculator uses your last period and average cycle length to project your next dates — a simple general tracker, not a contraceptive or medical tool.
A typical cycle is around 21-35 days. Enter your own average for a closer estimate.
These dates are rough estimates based on a regular cycle. Real cycles shift with stress, health and many other factors. Not for contraception or medical decisions.
How the Menstrual Cycle Calculator Works
This tool projects your upcoming period dates, ovulation date, and fertile window from a single anchor point: the first day of your last period, combined with your average cycle length. It’s a general cycle-tracking estimate based on a regular, average-length cycle — not a contraceptive or diagnostic method.
Formula: Next period = last period start + cycle length (in days). Period after that = last period start + (2 × cycle length). Estimated ovulation = next period date − 14 days. Fertile window = ovulation date − 5 days through ovulation date + 1 day. Cycle day today = ((days since last period) mod cycle length) + 1.
- Last period start — the first day of your most recent period, used as the starting point for every projection.
- Average cycle length — the number of days from the start of one period to the start of the next (defaults to 28 days if left blank; a typical cycle runs about 21–35 days).
Example Scenarios
| Last period start | Cycle length | Estimated next period | Estimated ovulation |
|---|---|---|---|
| Jan 1, 2026 | 28 days | Jan 29, 2026 | Jan 15, 2026 |
| Jan 1, 2026 | 30 days | Jan 31, 2026 | Jan 17, 2026 |
| Mar 1, 2026 | 21 days | Mar 22, 2026 | Mar 8, 2026 |
| Jun 15, 2026 | 35 days | Jul 20, 2026 | Jul 6, 2026 |
| Oct 10, 2026 | 26 days | Nov 5, 2026 | Oct 22, 2026 |
Improving Estimates from the Menstrual Cycle Calculator
The menstrual cycle calculator assumes every cycle is the same length, so its accuracy depends on how well your average reflects reality.
- Count cycle length from day one of one period to day one of the next, not from the end of bleeding.
- Log three to six cycles and average them before entering a number into the menstrual cycle calculator.
- Stress, illness, travel, weight change and breastfeeding can all shift a cycle, so one late period is common.
- Ovulation dates are the least certain output, because the first half of the cycle varies the most.
If you are trying to conceive, ovulation test strips or tracking basal body temperature add real data on top of the menstrual cycle calculator. It is not reliable contraception.
Speak to a doctor if cycles are regularly shorter than 21 days or longer than 35, if periods stop and you are not pregnant, or if bleeding is very heavy or painful. A menstrual cycle calculator can highlight these patterns but cannot explain them. The menstrual cycle article on Wikipedia describes each phase, so you can read the menstrual cycle calculator dates in context.
Menstrual Cycle Calculator FAQ
How does this calculator work out my next period date?
It takes the first day of your last period and adds your average cycle length in days. That projected date is only as reliable as your cycle is regular — stress, illness, travel, and many other factors can shift the actual date earlier or later.What counts as a “normal” cycle length?
Cycle lengths commonly fall somewhere between about 21 and 35 days, and can vary from cycle to cycle for the same person. Enter your own recent average rather than the default of 28 for a closer personal estimate.How is the fertile window calculated from my cycle?
The tool estimates ovulation as 14 days before your next projected period, then marks the fertile window as the 5 days before that ovulation date through 1 day after it, since sperm can survive several days while an egg survives about a day.What does “cycle day today” tell me?
It shows where you are in your current cycle, counting the first day of your last period as day 1 and cycling back to day 1 again once you pass your average cycle length. It’s a quick reference point, not a prediction of symptoms.Can I use this calculator if my cycles are irregular?
You can, but the projected dates will be less reliable, since the math assumes a consistent cycle length. If your cycles vary a lot month to month, consider tracking several cycles yourself or speaking with a healthcare professional about what’s typical for you.Is this calculator a reliable way to avoid or plan pregnancy?
No. This is a general estimate based on average cycle length, not a contraceptive or fertility treatment, and cycles vary naturally from month to month. For contraception or help trying to conceive, talk to a doctor or other qualified healthcare professional.Related Calculators
If you’re trying to pinpoint your most fertile days rather than just your next period, the ovulation calculator for estimating your fertile window builds on the same cycle data. If a period is late and you suspect you may be pregnant, the due date calculator based on your last period can estimate how far along you’d be, and the conception date calculator can help estimate when conception likely occurred.