How to work out your cycle length
Your cycle length is the number almost every cycle app quietly assumes, and the one most people have never actually counted. It takes two dates.
The counting rule
Day 1 is the first day of real bleeding. Not spotting the day before. Count from that day until the day before your next period starts. That total is your cycle length.
If your last period started on 3 August and the next one started on 1 September, your cycle was 29 days, not 30. The last day belongs to the next cycle.
What is normal
Most cycles fall somewhere between 21 and 35 days, and the same person’s cycles vary from month to month. A few days of movement between cycles is ordinary, not a malfunction.
Two things are worth noticing rather than worrying about:
- A cycle that is consistently shorter than 21 or longer than 35 days.
- A sudden, sustained change from your own normal.
Neither is something an app can interpret for you. They are worth a conversation with a doctor, which is a different thing from a training decision.
Why the number matters for training
Every phase estimate is scaled to your cycle length. If an app assumes 28 days and yours is 33, its phase estimate drifts a few days out by the end of the cycle, and any “train heavy now” advice built on top drifts with it.
That is one of the reasons Lunia sizes the phase ring to your logged cycle length instead of a fixed 28-day model, and why an irregular cycle is not a problem for logging. If you want a quick estimate right now without doing arithmetic, the cycle phase calculator does it from your last period and your usual length.
The better version of this
Counting once gives you an estimate. Logging each period start gives you a real average, and the average is what makes phase estimates worth anything. It takes one tap per cycle.
After a few cycles you can stop guessing entirely: rather than asking what phase you are supposed to be in, you can look at what your own training actually did in each one. That is the whole point of a cycle-aware training log.
See your own pattern, not a hormone theory.
Lunia is a free, cycle-aware training log. Log your sessions and watch how you actually perform across your cycle.