Enter the start dates of your last two to four periods to calculate your average cycle length, the shortest and longest cycles, and how regular you are.
| Input | Result |
|---|---|
| Starts Jan 5, Feb 2, Mar 2, 2026 | average 28.0 days · spread 0 — very regular |
| Starts Jan 10, Feb 8, Mar 6, Apr 1, 2026 | average 27.0 days · spread 3 — regular |
| Starts Jan 5, Feb 10, Mar 4, 2026 | average 29.0 days · spread 14 — irregular |
| Two dates only: Jan 5 and Feb 9, 2026 | one interval: 35 days |
Day 1 is the first day of full bleeding — not spotting. Count from that day up to and including the day before your next period starts, and that count is your cycle length. Do this for three or four cycles and average the counts; one cycle alone tells you very little because normal cycles vary by several days on their own.
If your cycle lengths swing by more than about a week between cycles, a single average is a weak predictor. In that case use the shortest and longest recent cycle to build a prediction window instead — the irregular period calculator does exactly that.
Two dates produce a single interval — a fact, not an average. Three dates give two intervals and a first hint of variation; four to six dates is the practical sweet spot, because one odd cycle (a stressful month, an illness) stops dominating the result. Beyond about six cycles, older data starts describing a different phase of life more than your current pattern.
When a cycle is clearly anomalous — a postpartum cycle, one disrupted by travel across time zones or a known illness — leave it out and measure the cycles either side of it. The average you want describes your typical cycle, because that is what the prediction calculators on this site consume.
| Measure | Typical adult range | Source basis |
|---|---|---|
| Cycle length (day 1 to day 1) | 24–38 days | ACOG guidance for adult cycles; adolescents run wider (roughly 21–45 days) in the first years |
| Cycle-to-cycle variation | Up to about 7–9 days | FIGO/ACOG regularity definitions, age-dependent |
| Bleeding length | Up to 8 days | ACOG normal menstruation parameters |
| Luteal phase (ovulation to next period) | About 14 days on average | The convention this calculator uses; real cycles vary roughly 10–16 days |
Two dates give one interval, which is a start but not an average. Three to six dates (two to five intervals) gives a meaningful average and a spread you can trust.
The first day of full bleeding, not spotting. Spotting in the days before full flow belongs to the previous cycle.
FIGO and ACOG definitions treat variation of up to about 7 to 9 days (depending on age) as regular. More than that, consistently, is the working definition of irregular cycles.
Round to the nearest whole day for the period cycle and period date calculators. The decimal itself is informative: 27.8 says your cycles cluster tightly, while an average built from a 24 and a 33 says very little.
If you know why it was odd — postpartum, illness, major travel — excluding it gives a more typical average. If odd cycles happen often and without explanation, include them and treat the spread, not the average, as the headline.