Period Calculator
Predict your next period — and the two after that — from the first day of your last period and your average cycle length. Built for regular cycles; irregular patterns need clinical tracking.
Calculate
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Result
The formula
Add your average cycle length (days from period start to next period start) to the last menstrual period (LMP) date. We repeat that interval for the following two cycles and place estimated ovulation 14 days before the next period under a fixed luteal-phase assumption.
Worked example
- LMP = 1 July 2026, average cycle = 28 days
- Next period ≈ 29 July 2026
- Following periods ≈ 26 August and 23 September 2026
Result: Next period: 29 July 2026
How next periods are predicted
This is forward projection from a single LMP and one average length — not a personalized hormone model.
Cycle length definition
Cycle length counts from day 1 of bleeding to the day before the next period starts (equivalently: LMP to next LMP in days). A “28-day cycle” does not mean you bleed for 28 days.
Enter an average from several recent cycles when possible. Using one unusually long or short cycle will skew the forecast.
Three upcoming periods
We show the next three predicted starts (LMP + 1×, 2×, and 3× cycle length). Beyond that, small timing errors compound — refresh with a new LMP after each period.
Linked ovulation estimate
For convenience we also show estimated ovulation as next period minus 14 days. If you need fertile-window detail, use the ovulation calculator.
Limits of calendar prediction
Stress, illness, travel, weight change, PCOS, perimenopause, and stopping hormonal contraception commonly change cycle length. Missed periods need clinical evaluation, especially with a positive pregnancy test or other symptoms.
Interesting facts
Average adult cycles are often ~21–35 days
Many textbooks cite a mid-20s day average, but healthy cycles commonly range from the low 20s to about 35 days. Outside that, ask a clinician.
Teens and perimenopause vary more
Cycles can be more irregular in the first years after menarche and again near menopause — averages alone are less predictive then.
Hormonal contraception resets the clock
Pill packs and similar methods schedule withdrawal bleeds that are not the same as a natural luteal-phase period. Use pack instructions, not this calendar, while on those methods.
Period length ≠ cycle length
Bleeding often lasts 3–7 days. Cycle length is the full interval between starts, not how many days you bleed.
Apps still need your average
Most period apps use the same calendar arithmetic unless you log symptoms, OPKs, or temperature — the math is only as good as the inputs.
Frequently asked questions
Add your average cycle length in days to the first day of your last period. This calculator does that and also projects the following two periods.
Many adults fall roughly between 21 and 35 days. This tool accepts 21–45 days. Persistent cycles outside your usual range deserve clinical review.
Ovulation timing shifts with stress, illness, travel, and hormonal changes. A late period can also mean pregnancy — take a test if intercourse occurred and the period is overdue.
Not for natural-cycle prediction. Scheduled bleeds on hormonal contraception follow the product, not an LMP + average-cycle model.
Under a 14-day luteal assumption, ovulation falls about (cycle length − 14) days after LMP — day 14 in a 28-day cycle, earlier in shorter cycles.
Probably not well. Log several cycles, note the range, and consider symptom or hormone-based tracking; see a clinician for persistent irregularity.
References
- Your menstrual cycle Public overview of cycle phases and common variations.
- Amenorrhea When periods are absent — clinical FAQ.
- Menstrual cycle fact sheet Global menstrual health context.