Menstrual Cycle Calculator
A menstrual cycle runs from the first day of one period to the first day of the next, and splits into a follicular phase before ovulation and a luteal phase after it.
Example: For a 28-day cycle with a last period starting 1 January 2026, ovulation falls around 15 January 2026, the fertile window runs 10 January 2026 to 15 January 2026, and the next period is due 29 January 2026.
Enter the first day of your last period to see your estimate.
These are estimates from calendar dating only. Irregular cycles make them unreliable.
Find an OB-GYN near youHow this is calculated
The cycle is divided at the estimated ovulation date, which is counted back 14 days from the next expected period. The luteal phase is treated as a fixed 14 days; the follicular phase absorbs the variation in cycle length.
This tool is for information only. It does not diagnose, treat, or replace advice from a licensed clinician. Reviewed against ACOG guidance.
Frequently Asked Questions
What are the phases of the menstrual cycle?
The cycle splits into a follicular phase before ovulation and a luteal phase after it, with your period falling at the start of the follicular phase. Almost all the variation in cycle length sits in the follicular phase. This calculator treats the luteal phase as a fixed 14 days.
What is the luteal phase?
The luteal phase is the stretch between ovulation and the start of your next period. It is the steadier half of the cycle, which is why ovulation is estimated by counting back 14 days from the next expected period rather than counting forward from the last one.
Why does the follicular phase change length but the luteal phase does not?
The follicular phase absorbs the variation because it simply runs until ovulation happens, and ovulation timing shifts with stress, illness and other factors. Once the egg is released, the countdown to the next period is comparatively fixed. A 21-day cycle and a 35-day cycle differ mostly in how long the follicular phase lasted.
Is a 28-day cycle normal?
Yes, and so is anything from 21 to 35 days. 28 is the textbook default this calculator falls back to, not a target to hit. Whether your own cycles stay roughly consistent matters more than the exact number.
How should I track my cycle?
Write down the first day of every period for a few months and work out the average gap between them â that average is the number this calculator needs. Noting cramping, cervical mucus changes and how heavy the bleeding is alongside the dates gives an OB-GYN far more to work with. A phone note is enough; you do not need an app.
When should I see a doctor about my cycle?
See an OB-GYN if your periods stop, fall well outside the 21 to 35 day range, last unusually long, or are heavy enough to disrupt your day. Pain that ordinary painkillers do not touch is also worth checking. A sudden change in a cycle that used to be steady deserves a look even if nothing else feels wrong.