A walkthrough, end to end.
- 1
Enter the first day of your most recent period and your typical cycle length and period duration.
- 2
The calculator forecasts the next 6 periods plus the ovulation day and fertile window for each cycle.
- 3
Use it to plan around travel, events, or to monitor cycle regularity.
Cycle prediction
Each cycle starts one cycle-length after the previous one. Ovulation falls ~14 days before the next period (the luteal phase is consistent). Period duration is the number of days of bleeding.
What you can do with this.
Predicting your next period
Plan around travel, events, exercise. The calculator shows up to 6 cycles ahead based on your typical cycle length.
Spotting cycle irregularity
If your actual periods drift more than 7 days from the prediction, your cycles may be irregular — worth a conversation with your provider.
Tracking PMS / PMDD timing
PMS symptoms typically start ~7 days before the period and resolve within a day or two of bleeding starting. Knowing your dates helps you plan around the worst days.
Period after stopping the pill
Most people resume normal cycles within 1–3 months of stopping hormonal birth control. The first cycle may be shorter or longer than your pre-pill baseline — give it 2–3 cycles to stabilize before basing predictions on it.
Period during perimenopause
Cycles often shorten then become unpredictable in the years before menopause (typically 45–55). The calculator's predictions become less reliable; track each period and consult your provider about transitions.
First period (menarche)
Average age is 12, with normal range 9–15. The first 1–2 years of cycles are commonly irregular as the HPO axis matures — the calculator works best after cycles stabilize around 21–35 days.
Late period — common causes
Stress, weight changes, illness, intense exercise, travel, and early pregnancy can all delay periods. A 1–2 week delay isn't unusual; persistent late or missed periods warrant a pregnancy test and provider visit.
Heavy or painful periods
Soaking through a pad/tampon every 1–2 hours, clots larger than a quarter, or pain that limits daily activity isn't 'just normal'. These can signal fibroids, endometriosis, or other conditions worth investigating.
Period calculator 2026 — what's current
Apps with millions of cycles (Flo, Clue) predict periods more accurately than calendar math by learning your individual variation. Use the calculator for the simple math; layer an app for personalized refinement.
Frequently asked.
21–35 days is considered normal for adults; 21–45 in the first few years after menarche. Cycles outside that range are worth discussing with a provider.
Irregular cycles make calendar predictions less reliable. Ovulation tracking apps that learn from BBT or OPK data tend to do better than pure calendar methods.
2–7 days. Periods longer than 7 days, very heavy bleeding, or sudden changes warrant a check-up.
Yes — significant stress can delay ovulation, which delays the period. The luteal phase stays ~14 days regardless, so the delay tracks with however long ovulation was postponed.
Variations in estrogen levels affect endometrial thickness. Lifestyle (stress, sleep, weight) and hormonal events (postpartum, perimenopause) drive most month-to-month differences. Track patterns rather than fixate on one period.
No. Calculations run entirely in your browser — no server processing, no analytics on inputs, no cookies.