Predict your next periods, ovulation day, and fertile window — based on your last menstrual period and cycle length.
Results update live as you change inputs.
Your next period starts
Days until
—
next period starts
Period ends
—
est. last day
Ovulation
—
mid-cycle
Fertile window
—
6-day span
| Cycle | Period starts | Period ends | Ovulation |
|---|
The Method
The calendar (rhythm) method assumes your cycle length is reasonably consistent. Your next period arrives cycle days after the first day of the last one. We also estimate ovulation at 14 days before the next expected period (luteal phase is stable across women), and your fertile window as the 5 days leading up to and including ovulation.
Calendar method
About This Tool
A period calculator — also called a menstrual cycle calculator or period tracker — predicts when your next several periods will arrive based on the first day of your last period and your usual cycle length. Knowing those dates makes it easier to plan travel, manage symptoms, recognise late periods, and identify your fertile window for conception or avoidance.
This calculator uses the classic calendar method: it adds your cycle length to your LMP to project the next several cycles. It also marks your ovulation day (14 days before the next expected period) and the fertile window (the 5 days leading up to and including ovulation), because both are commonly asked for.
The calendar method is most accurate when your cycle is regular — varying by less than ±3 days from month to month. If your cycles swing widely or you have a known menstrual disorder (PCOS, endometriosis, perimenopause), use the prediction as a rough guide and pair it with ovulation predictor kits or basal body temperature charting for higher accuracy.
This free online period calculator runs entirely in your browser — no data is saved or transmitted. Choose how many cycles to display (1–12) and review the upcoming-period table to plan months in advance.
Up to 12 Cycles
Project a full year of upcoming periods, end dates, and ovulation days.
Ovulation Day
Marks ovulation 14 days before each predicted next period — the most reliable point.
Cycle Visualisation
Donut chart shows period, fertile window, ovulation, and luteal phase at a glance.
Live Calculation
Type, pick a date, or change cycle length — all dates update instantly.
Fertile Window
Highlights the 6-day window around ovulation when conception is possible.
100% Private
Every date is calculated locally in your browser. No account, no storage.
A few quick inputs give you months of period and ovulation predictions.
Pick the first day of your last period — the day bleeding started, not when it ended.
A standard cycle is 28 days. If yours is consistently shorter or longer, change it (21–45 days).
Most periods last 3–7 days. Type your usual duration so end-date predictions are accurate.
Pick from 1 to 12 cycles. Six is a good default — half a year of predictions without clutter.
The big date is your next period. Below it: days until, end date, ovulation, and fertile window.
The table lists every projected period start, end, and ovulation day — plan months ahead.
Common questions about menstrual cycles, ovulation, and using a period calculator.
Accuracy depends on how regular your cycles are. With a steady cycle (varying by ±2 days), the calendar method predicts the next period within 1–2 days. With irregular cycles or hormonal changes (postpartum, perimenopause, PCOS), accuracy drops considerably. Use the prediction as a planning aid, not a guarantee — and re-enter your LMP each month for best results.
A typical adult menstrual cycle runs 21 to 35 days, with 28 days being the textbook average. Teens often have cycles ranging from 21–45 days as their hormones stabilise. Cycles shorter than 21 or longer than 35 days that occur consistently warrant a conversation with a gynaecologist.
Ovulation reliably occurs 14 days before the next period, not 14 days after the last one. The luteal phase (ovulation → next period) is stable across women; the follicular phase (period → ovulation) is what varies with cycle length. So a 28-day cycle has ovulation around day 14, a 32-day cycle around day 18, and a 25-day cycle around day 11.
Sperm can survive up to 5 days in the female reproductive tract and the egg lives 12–24 hours after release. So the fertile window covers roughly the 5 days before ovulation through ovulation day itself — a 6-day span when conception is possible. Days 2 before through 1 after ovulation carry the highest conception odds.
It is unlikely but possible, especially in women with short cycles. With a 21-day cycle, ovulation can occur around day 7. If you have sex on the last day of a 6-day period, sperm can survive until day 11. With shorter periods and shorter cycles, the overlap is real. Don't rely on "I'm on my period" as contraception.
Many things can shift a period: stress, sudden weight change, intense exercise, illness, travel across time zones, new medications, hormonal birth control, perimenopause, and of course pregnancy. A period delayed by 3–5 days is common. If you are sexually active and more than 5 days late, take a pregnancy test. If late periods become a pattern, see a doctor.
Yes, but treat the predictions as rough estimates. Track your actual cycle lengths for at least 6 months and use the average as your cycle length input. For trying to conceive with irregular cycles, supplement the calendar method with ovulation predictor kits (OPKs), basal body temperature charting, or cervical mucus tracking, which detect ovulation when it actually happens.
Calendar-method-only contraception is among the least reliable approaches, with typical-use failure rates of 13–24% per year. Modern fertility-awareness-based methods that combine calendar, temperature, and cervical mucus can achieve 1–5% failure with perfect use, but require disciplined daily tracking. If preventing pregnancy is critical, use a high-efficacy method (IUD, implant, or hormonal pill).
Yes. Cycles are often longer and more variable in teens until ~age 20. They are typically most regular between ages 20 and 40. Perimenopause (typically starting in the mid-40s) brings shorter cycles, then skipped cycles, until menopause around age 51. Hormonal contraception, pregnancy, and breastfeeding all temporarily change cycle behaviour.
Postpartum cycles take time to return — typically 6–8 weeks if not breastfeeding, and anywhere from 3–18 months if exclusively breastfeeding. Until your cycle settles into a regular pattern, calendar predictions will be unreliable. Re-enter your LMP each month and expect 6+ months of variability before predictions become useful.
See a gynaecologist if you experience: cycles shorter than 21 or longer than 35 days consistently; periods lasting more than 7 days; very heavy bleeding (changing tampon/pad hourly); severe pain that interferes with daily life; bleeding between periods; or more than 3 months without a period (when not pregnant or postpartum). These can signal treatable hormonal, structural, or other gynaecologic conditions.
It can help identify your fertile window for timing intercourse, but it's only one tool. For conception, also consider: OPKs to confirm ovulation as it happens, BBT charting to verify ovulation occurred, and seeing a fertility specialist if you've tried for over 12 months (or 6 months if you're over 35) without success.