Work out the exact calendar date of your next period from your last period start date and cycle length, with a day countdown and the following two dates.
| Input | Result |
|---|---|
| Last period Mar 1, 2026 · 28-day cycle | next period Mar 29, 2026 |
| Last period Mar 1, 2026 · 30-day cycle | next period Mar 31, 2026 |
| Last period Mar 1, 2026 · 26-day cycle | next period Mar 27, 2026 |
| Last period Mar 1, 2026 · 35-day cycle | next period Apr 5, 2026 |
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.
One cycle ahead is the most reliable prediction, because it only assumes the current cycle behaves like recent ones. Two or three cycles ahead compounds the error: if your cycle length varies by ±3 days cycle to cycle, the third predicted date already carries roughly a week of uncertainty. This page rolls dates forward automatically, so a date from many cycles ago still yields the next upcoming one.
| 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 |
Add your cycle length to the first day of your last period. For example, March 1 plus 28 days gives March 29. If that date is already in the past, the calculator keeps adding one cycle at a time until it reaches a future date.
ACOG considers 24 to 38 days normal for adults. Teenagers often run wider, roughly 21 to 45 days, in the first few years after the first period.
The calculator assumes regular cycles, and real ones slip. Stress, illness, travel and weight change each delay ovulation, and a late ovulation means a late period. If you are more than about a week past the predicted date and pregnancy is possible, take a test; if cycles stay unpredictable for three months, see a clinician.
No. The count is in calendar days, so a 28-day cycle always lands on the same weekday, while shorter or longer cycles drift through the week.
Treat the next one as solid, the second as approximate and anything further as a rough guide. Re-enter your actual start date each cycle to keep the prediction honest.