The full gap from one period start to the next.
Intent-focused tool page
Cycle length calculator
This page is built for cycle length calculator intent. It gives users the core cycle number first, then routes them into the broader forecasting tools.
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 keyword deserves its own tool page because the job is very specific: measure the cycle length first.
The measured cycle length is 28 days, which is ready to reuse across the homepage, 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
Date-estimate pages should show where the timing logic comes from
Next-period estimates are most useful as educational forecasts built from the first day of the last period and recent cycle length. Visible sources make the planning boundary clear.
Office on Women's Health: Your menstrual cycle
Cycle basics, first-day counting, and when irregular timing deserves extra attention.
Open official sourceNHS: Missed or late periodsPlain-language guidance on common causes of late or missed periods and when to seek care.
Open official sourceOffice on Women's Health: Period problemsPatient guidance on missing periods, irregular timing, and symptom-led escalation.
Open official source