Period Calculator
Predict your next period from three inputs — last period start, how long it lasts, and your average cycle. You get the next start date, the bleeding days, a countdown, and a six-cycle calendar with estimated fertile days for each cycle.
How your cycle compares
How period prediction works
A menstrual cycle restarts each time bleeding begins: that first day of flow is day 1, and your cycle length is the gap between one day 1 and the next. Prediction is honest arithmetic — add your average cycle length to your last start date, repeat for as many cycles as you like. The wobble comes from the front half of the cycle: stress, illness, and travel delay ovulation, and the period follows ~14 days behind it. That is also why the calendar marks estimated fertile days in each cycle — the same math that predicts your period predicts the ovulation that precedes it.
How it’s calculated
Next period = last period start + cycle length, then + one cycle length per row for six future cycles; bleeding days run from each start date through (period length − 1) days later. Ovulation for each cycle ≈ cycle start + (cycle length − 14), from the ~14-day luteal phase, with the fertile window covering the 5 days before ovulation plus ovulation day. If your last period was a while ago, the calendar fast-forwards to the first upcoming cycle.
Informational only, not medical advice — predictions assume regular cycles and are not a contraceptive method; confirm concerns with your provider.
Your next six periods
Assumes every cycle matches your average; even regular cycles drift a few days.
Worked example
With a last period starting June 20, 2026, a 28-day cycle, and 5 days of flow, the next period is expected July 18–22, 2026 — 16 days away as of July 2. The current cycle’s estimated ovulation is July 4 (fertile June 29–July 4), and the two periods after that begin August 15 and September 12.
Common mistakes
- Counting the cycle from the last day of bleeding — day 1 is the first day of flow.
- Averaging only one or two cycles; track at least 3–6 to get a usable average.
- Assuming a textbook 28 days — anywhere from 21 to 35 is typical for adults.
- Treating predicted “safe days” as contraception — calendar math is not birth control.
Where it is used
- Planning around trips, events, races, and big days at work.
- Spotting irregularity early — a log of predicted vs. actual starts is exactly what an OB-GYN asks for.
- Flagging a late period so you know the earliest sensible day to take a pregnancy test.
- Pairing with the ovulation calculator when you switch from avoiding to trying.
Frequently asked questions
What counts as day 1 of my cycle?
The first day of actual bleeding — not spotting, and not the day the period ends. Cycle length is measured from one day 1 to the next day 1. Getting this anchor right is the single biggest factor in the accuracy of any period or ovulation prediction.
What is a normal cycle length?
For adults, cycles typically run 21–35 days (ACOG); for teens the normal range is wider, about 21–45 days. Cycles are usually considered regular when the difference between your longest and shortest cycle is less than 7–9 days. Consistently outside those ranges is worth a conversation with your OB-GYN.
How long should a period last?
Bleeding typically lasts 2–7 days, with 4–5 days most common. Periods that regularly run longer than 7–8 days, are heavy enough to soak through protection hourly, or stop for 3+ months without pregnancy warrant a medical checkup.
Why does my period come early or late?
The pre-ovulation phase of the cycle is sensitive to stress, illness, travel, weight change, breastfeeding, and conditions like PCOS or thyroid disorders, so ovulation — and the period ~14 days after it — shifts with it. A single odd cycle is rarely meaningful; a persistent change in pattern is what merits attention.
Can I use this calendar to avoid pregnancy?
No. The fertile days shown are calendar estimates, and real ovulation can move by several days in either direction. Fertility-awareness contraception requires daily observations (temperature, cervical mucus, LH tests) and training; a prediction calendar alone is not reliable birth control.