🩸 Period (Menstrual Cycle) Calculator
By ToolNimba Health Team · Reviewed by ToolNimba Editorial Review, health content review · Updated 2026-06-19
These dates are statistical estimates that assume a regular cycle. They are not a method of contraception and cannot confirm, predict or rule out pregnancy. Real cycles vary from month to month, so the fertile window shown is approximate. This tool is not medical advice. If you are trying to conceive, avoid pregnancy, or notice irregular, missed or unusually painful periods, speak with a doctor or qualified clinician.
These dates are statistical estimates based on a regular cycle. They are not a method of contraception and cannot confirm or rule out pregnancy. Cycles vary naturally, so treat the fertile window as approximate and speak to a clinician for medical decisions.
This period calculator predicts when your next few periods are likely to start, plus an estimated ovulation day and fertile window. Enter the first day of your last period, your average cycle length (the default is 28 days) and how many days your period usually lasts. You will instantly see the projected start dates for the coming cycles and the days you are most likely to be fertile. Everything runs in your browser, nothing you type is sent anywhere.
What is the Period Calculator?
The menstrual cycle is counted from the first day of one period to the first day of the next. A common figure is 28 days, but anything from about 21 to 35 days in adults is considered normal, and cycle length often varies by a few days from one month to the next. This calculator works by adding your average cycle length to the start of your last period to estimate the next start date, then repeating that step to project several cycles ahead.
Ovulation, when an ovary releases an egg, usually happens about 14 days before the next period begins, not 14 days after the last one. That distinction matters: the second half of the cycle (the luteal phase) is fairly stable at around 12 to 14 days, while the first half can stretch or shorten. So the tool estimates ovulation by counting back roughly 14 days from the predicted next period, which is more reliable than counting forward, especially for longer or shorter cycles.
The fertile window is the span of days when intercourse can lead to pregnancy. Because sperm can survive in the body for up to about 5 days and the egg lives for roughly 24 hours, the window is estimated as the 5 days leading up to ovulation plus the day of ovulation itself. This is a calendar estimate only. Stress, illness, travel, breastfeeding, hormonal contraception and conditions such as PCOS can all shift ovulation, so a calendar method should never be relied on as birth control or as proof of a safe time.
When to use it
- Anticipating when your next period is due so you can plan travel, events or packing supplies.
- Getting a rough idea of your fertile window when you are trying to conceive.
- Spotting whether your cycles are roughly regular by comparing predicted and actual start dates.
- Sharing an estimated cycle timeline with a doctor or fertility clinic as a starting point.
How to use the Period Calculator
- Enter the first day of your last period (day one of bleeding, not the day it ended).
- Enter your average cycle length in days. If you are unsure, leave the 28-day default.
- Enter how many days your period normally lasts.
- Read off the next period date, the estimated ovulation day, the fertile window and the upcoming cycle start dates.
Formula & method
Worked examples
Last period started June 1, average cycle 28 days, period lasts 5 days.
- Next period = June 1 + 28 days = June 29
- Ovulation ≈ June 29 − 14 days = June 15
- Fertile window ≈ June 10 (June 15 − 5) through June 15
- Period would run June 29 through July 3 (June 29 + 4 days)
Result: Next period June 29, ovulation around June 15, fertile June 10 to June 15.
Last period started June 1, longer cycle of 32 days, period lasts 4 days.
- Next period = June 1 + 32 days = July 3
- Ovulation ≈ July 3 − 14 days = June 19
- Fertile window ≈ June 14 (June 19 − 5) through June 19
- Period would run July 3 through July 6 (July 3 + 3 days)
Result: Next period July 3, ovulation around June 19, fertile June 14 to June 19.
Estimated ovulation day and fertile window by cycle length (counting back 14 days)
| Cycle length | Ovulation (cycle day) | Fertile window (cycle days) |
|---|---|---|
| 21 days | Day 7 | Days 2 to 7 |
| 24 days | Day 10 | Days 5 to 10 |
| 28 days | Day 14 | Days 9 to 14 |
| 30 days | Day 16 | Days 11 to 16 |
| 32 days | Day 18 | Days 13 to 18 |
| 35 days | Day 21 | Days 16 to 21 |
Common mistakes to avoid
- Counting ovulation as day 14 of every cycle. Ovulation is roughly 14 days before the next period, not 14 days after the last one. On a 32-day cycle that puts ovulation near day 18, not day 14. Counting back from the next period is more accurate for cycles that are not 28 days.
- Using the calendar as contraception. Calendar predictions assume a perfectly regular cycle. Ovulation can shift with stress, illness or travel, so this is not a reliable form of birth control. Use a proven contraceptive method if you want to avoid pregnancy.
- Entering the wrong start day. Day one is the first day of full bleeding, not light spotting beforehand and not the day your period ended. Using the end date instead of the start date throws off every prediction.
- Expecting exact dates from an irregular cycle. If your cycle length swings widely from month to month, a single average will not predict well. Track several cycles and treat the output as a rough guide, and see a clinician about persistent irregularity.
Glossary
- Menstrual cycle
- The time from the first day of one period to the first day of the next, commonly about 28 days but normally 21 to 35.
- Cycle length
- The number of days in your cycle, measured from the first day of bleeding to the day before your next period starts.
- Ovulation
- The release of an egg from an ovary, usually about 14 days before the next period begins.
- Fertile window
- The days when intercourse is most likely to lead to pregnancy, roughly the 5 days before ovulation plus ovulation day.
- Luteal phase
- The phase from ovulation to the next period, usually a fairly steady 12 to 14 days.
- Period length
- How many days bleeding lasts in a single period, commonly 3 to 7 days.
Frequently asked questions
How does the period calculator predict my next period?
It adds your average cycle length to the first day of your last period to estimate the next start date, then repeats that to project several cycles ahead. The default cycle length is 28 days, but you can enter your own. Accuracy depends on how regular your cycles are.
How is the ovulation day estimated?
Ovulation is estimated as about 14 days before the predicted next period, not 14 days after your last one. That is because the luteal phase (ovulation to next period) is fairly stable, while the first half of the cycle varies more. Counting back is more accurate for cycles that are not 28 days.
What is the fertile window?
It is the span of days when sex is most likely to lead to pregnancy. Because sperm can survive up to about 5 days and the egg lives roughly 24 hours, the window is estimated as the 5 days before ovulation plus the ovulation day itself. It is a calendar estimate, not a guarantee.
Can I use this as birth control?
No. These are statistical estimates that assume a regular cycle, and ovulation can shift with stress, illness, travel or hormonal changes. The calculator cannot confirm a safe day. If you want to avoid pregnancy, use a proven contraceptive method and talk to a healthcare provider.
My cycle is irregular. Will the predictions be accurate?
Less so. A single average cannot capture a cycle that swings widely from month to month, so treat the output as a rough guide. Tracking several cycles gives a better average. Persistent irregularity, missed periods or unusual symptoms are worth discussing with a clinician.
What counts as the first day of my period?
Day one is the first day of full menstrual bleeding. Light spotting before the flow begins does not count, and you should not use the day your period ended. Entering the correct start day is essential, since every prediction is built from it.