Period Calculator
Predict your next periods, fertile window and ovulation day from your last period, with a 12-cycle calendar. Free, no sign-up, and it runs in your browser.
Dedicated page: /en/period-calculator
Estimate only, not medical advice. Dates are calendar estimates for regular cycles. Do not use this tool as birth control, and see a doctor if cycles are very irregular or bleeding is heavy.
Adult cycles of 21 to 35 days are normal. 28 is the average.
Quick answer
A period calculator predicts your next period, your fertile window and your ovulation day from the first day of your last period and your average cycle length. It shows your cycle day today, whether you are on your period now, and a calendar for the next 3, 6 or 12 cycles, with an irregular-cycle warning.
About this Period Calculator
Predict your next periods, fertile window and ovulation day from your last period, with a 12-cycle calendar. It is one of the Health and Family on ToolUstad, and like every tool here it runs entirely in your browser: nothing you type, paste or open is uploaded to a server, and there is no sign-up or usage limit.
What is a period calculator?
A period calculator is a calendar tool for the menstrual cycle. It takes the first day of your last period, your average cycle length, and your period length, and predicts when the next periods will start, when ovulation will happen, and when the fertile window falls. It is the fastest way to answer the three questions most people ask: when is my next period, what cycle day am I on today, and when am I fertile. It works best for regular cycles, and it is an estimate, not a medical device.
How the dates are worked out
A cycle runs from the first day of one period to the first day of the next. Add the cycle length to the last period start and you have the next period, then keep adding for the year. The period end date is the start plus the period length minus one day.
What counts as a normal cycle
For adults, a normal cycle is 21 to 35 days, with 28 the average, and bleeding lasts 2 to 7 days, averaging 5. For adolescents, 21 to 45 days is normal. The American College of Obstetricians and Gynecologists calls the menstrual cycle a vital sign, because irregularities can signal thyroid problems or polycystic ovary syndrome. Cycles shorter than 21 days or longer than 38 days in adults are potentially abnormal. This calculator flags both cases.
Current cycle day and period status
The calculator counts the days from your last period to today, which is your current cycle day, and tells you whether you are on your period now. PMS symptoms often start about 10 days before the next period, so knowing the cycle day helps you plan around heavy days and give your doctor accurate dates.
The fertile window, explained plainly
The fertile window is six days: the five days before ovulation plus ovulation day. The best days to try to conceive are the two to three days before ovulation. The reverse matters just as much: this window is not a safe period, and calendar methods must never be used as birth control.
Best practices
- Use your real average cycle length, not the standard 28 days: a wrong length shifts every prediction by several days.
- Update the calculator with each real period start date, every cycle.
- Track your shortest and longest cycles: a spread over 9 days means the dates are unreliable.
Common mistakes
- Using period tracking as contraception: it is not reliable, and the risk is real.
- Counting ovulation forward 14 days from the period: it is counted back 14 days from the next period.
- Forgetting to update the tool after a late or early period, so every later date drifts.
- Assuming all cycles are the same: stress, illness, travel and exercise all shift timing.
Who this tool is for
It is for anyone tracking a menstrual cycle: planning around periods, trying to conceive, watching for irregularities, or keeping a record for a doctor. It needs no account and stores nothing: the dates stay in your browser. For detailed fertility tracking, pair it with the ovulation calculator on this site, and for pregnancy planning, with the due date calculator.
Learn: how to use the Period Calculator
- Enter the first day of your last period.
- Enter your average cycle length and period length.
- Read your next period, your cycle day today, ovulation and the fertile window.
- Open the calendar for the next 3, 6 or 12 cycles, and download it as a CSV.
Features
- Next 3, 6 or 12 periods with start and end dates
- Current cycle day and whether you are on your period now
- Ovulation day counted back 14 days from the next period
- Six-day fertile window for every cycle
- Irregular-cycle warning outside the 21 to 35 day adult range
- Cycle variation check from your shortest and longest cycles
- Periods per year and bleeding days per year
- Downloadable calendar as a CSV file
Worked example
Last period started 1 November 2026, with a 28 day cycle and a 5 day period: the period ends 5 November, ovulation falls on 15 November, the fertile window runs 10 to 16 November, and the next period is expected on 29 November 2026. The periods after that fall on 27 December 2026 and 24 January 2027. With a 28 day cycle you have about 13 periods a year and 65 bleeding days a year.
Frequently asked questions
How is my next period predicted?
By adding your average cycle length to the first day of your last period. A 28 day cycle starting 1 November gives a next period on 29 November. Each later period adds one more cycle length.
What is a normal cycle length?
For adults, 21 to 35 days, with 28 the average. For adolescents, 21 to 45 days. A period usually lasts 2 to 7 days, averaging 5. If your cycles stay outside 21 to 35 days, talk to a doctor.
How is ovulation worked out?
Ovulation usually happens 14 days before the next period starts, whatever the cycle length. The tool adds your cycle length to the last period, then counts back 14 days. That is more accurate than counting forward 14 days from the period.
What is the fertile window?
The six days ending on ovulation day: the five days before it plus ovulation day itself. Sperm can live up to five days, while the egg lives about 24 hours, so the best days to try are the two to three days before ovulation.
Can I use this as birth control?
No. Never use a calendar calculation to avoid pregnancy. Ovulation shifts with stress, illness and travel, and sperm can live five days, so the safe window is far narrower than this tool can promise. Only FDA-cleared digital contraceptives should be used that way.
How accurate are the predictions?
For regular cycles, usually within 1 to 2 days. Predictions tighten after 3 to 6 cycles of consistent logging. Stress, illness, travel, weight changes and intense exercise can all shift a cycle by several days.
My cycles are irregular. What should I do?
This tool assumes a regular cycle. If your shortest and longest cycles differ by more than 7 to 9 days, the dates are unreliable. Track your periods for several months, then talk to your doctor, who can check for causes such as thyroid problems or PCOS.
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