The full gap from one period start to the next.
Intent-focused tool page
Cycle length calculator
Use this page when you want the core cycle number first, then reuse it for next-period, ovulation, and fertile-window estimates.
Measure cycle length first
Use two period start dates to calculate cycle length fast
Enter the most recent and previous period start dates. This page measures the gap and reuses it for next-period and ovulation estimates.
Most recent period start
Previous period start
Reuse that number for the next forecast.
The key timing anchor built from this cycle length.
A practical planning window around ovulation.
This page starts with the most specific job: measure the cycle length from one period start date to the next.
The measured cycle length is 28 days, which is ready to reuse across the monthly forecast, ovulation page, and irregular-cycle page.
Why this page matters
Measure the cycle gap once, then feed the rest of the site with a stronger timing input.
Start with the core number
One clean cycle-length number gives the rest of the site a stronger timing anchor.
Measure from date to date
This page works well for people who know two start dates and want the fastest way to measure the gap.
Carry it into the next tools
After the gap is measured, you can reuse it for next-period forecasts, fertile timing, and cycle tracking.
FAQ
The practical answers most people need before they move into other tools.
How do I count cycle length correctly?
Count from the first day of one period to the first day of the next period. That full gap is the cycle length.
What is the difference between cycle length and period length?
Period length only counts the bleeding days. Cycle length covers the whole month between two period starts.
What if my cycle length changes every month?
If the number changes a lot each month, track your shortest and longest cycles and move into the irregular period calculator.
Why does this number matter so much?
Once you know the cycle length, the main calculator and ovulation calculator become much more useful.
Reviewed guidance
Cycle and fertility pages should stay range-based and source-backed
Ovulation dates and fertile windows are best handled as planning ranges built from cycle timing. Clear sources help the page stay practical, careful, and medically grounded.
ACOG: Fertility Awareness-Based Methods of Family Planning
Cycle-day counting, fertile timing, and why fertility awareness works as a planning range.
Open official sourceNHS: Natural family planningRange-based fertility guidance and the importance of irregular-cycle caution.
Open official sourcePlanned Parenthood: Fertility awarenessPatient-friendly explanation of fertile days, tracking methods, and limits of timing-only planning.
Open official source