Predict your next three period dates from your last period and cycle length, plus the estimated ovulation day and fertile window using the 14-day luteal convention.
| Input | Result |
|---|---|
| Last period Mar 1, 2026 · 28-day cycle | next period Mar 29 · ovulation ≈ Mar 15 · fertile Mar 10–16 |
| Last period Mar 1, 2026 · 35-day cycle | next period Apr 5 · ovulation ≈ Mar 22 |
| Last period Mar 1, 2026 · 24-day cycle | next period Mar 25 · ovulation ≈ Mar 11 |
| Same as row 1 with a 6-day period | bleed days Mar 29 – Apr 3 |
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.
| 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 |
The second half of the cycle — from ovulation to the next period — is the luteal phase, and it is the more stable half, averaging about 14 days. The first half (period to ovulation) is what stretches or shrinks when a cycle runs long or short. That is why the estimate counts backwards from the predicted next period rather than forwards from the last one: in a 35-day cycle ovulation lands near day 21, not day 14.
The estimate is still an average. Stress, illness, travel and disrupted sleep can each move ovulation by days, which is exactly why calendar methods alone are unreliable for contraception.
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. A textbook cycle is 28 days, but ACOG considers anything from 24 to 38 days normal for adults.
The 14-day figure is an average luteal phase. Real luteal phases vary roughly 10 to 16 days between women and between cycles, so ovulation can land several days either side of the estimate. For precise timing, ovulation predictor kits or basal body temperature tracking are more reliable.
A single average is a poor predictor when cycle lengths swing widely. Use your shortest and longest recent cycle to build a window instead — the irregular period calculator on this site does exactly that.
Sperm can survive up to about five days in the reproductive tract, while the egg lives roughly 12 to 24 hours after ovulation. Fertility references therefore put the fertile window at the five days before ovulation plus about a day after.
No. Calendar-based estimates are not reliable birth control: ovulation timing shifts with stress, illness, travel and sleep, and sperm survival makes the window hard to bound. Talk to a clinician about effective options.