What the calculator does with your dates
It sorts the dates, measures the gap between each consecutive pair, and reports the average, the shortest and longest gaps, and the spread. The regularity label is based on that spread: 4 days or fewer is regular, 5 to 8 is somewhat irregular, 9 or more is irregular. This is the same interval-between-starts method the Femora app uses for its predictions.
The next period is predicted from your most recent date plus the rounded average. The range beside it shows how far your longest cycle could push it.
Using the result
Plug the rounded average into the ovulation calculator, period calculator, or due date calculator. If the label says irregular, use the ovulation calculator for irregular periods with your shortest and longest cycles, and see the irregular periods guide for causes worth ruling out.
Frequently asked questions
How do I calculate my cycle length?
Count from the first day of one period to the day before the next period starts. That is one cycle. Day one is the first day of full bleeding, not spotting. Do this for several periods and average them; the calculator does exactly that from the dates you enter.
What is a normal cycle length?
Anything from 21 to 35 days is considered normal for adults, and 21 to 45 days for teens. The average is about 28 days but most people are not average. What matters more than the number is consistency from cycle to cycle.
How many periods do I need to enter?
Two dates give you one cycle, which is a starting point. Three to six dates give a meaningful average and reveal how much your cycles vary. Apps typically use the last six cycles, weighted towards recent ones, which is what Femora does.
Why does my cycle length change every month?
The follicular phase, from your period to ovulation, responds to stress, illness, travel, sleep, and weight change, so it stretches and shrinks. The luteal phase after ovulation is more stable at 12 to 14 days. A variation of up to 7 to 9 days between cycles is normal; more than that, or cycles outside 21 to 35 days, is worth discussing with a clinician.
What should I enter in an ovulation calculator if my cycles vary?
Use the rounded average from this page for a standard ovulation or period calculator. If your cycles vary by more than a few days, enter your shortest and longest cycle into the ovulation calculator for irregular periods instead, which gives a fertile range rather than a single day.
These calculators give estimates based on cycle averages and standard formulas. They are for general information only and are not medical advice. For anything concerning your health or pregnancy, talk to a qualified healthcare provider.