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Period Calculator: Predict Your Next Period Dates
Enter the first day of your last period, your average cycle length and how many days your period lasts. The calculator lists your next periods with end dates and estimated ovulation.
How the prediction works
Menstrual cycles are counted from the first day of bleeding to the day before the next bleed begins. The calculator adds your average cycle length to the last start date repeatedly: next start = last start + n × cycle length. If the last period was several cycles ago, it jumps forward by whole cycles to the first date that is still ahead of the day you choose.
The end date is the start plus the period length minus one, because the first day of bleeding counts as day 1.
What counts as a typical cycle
Cycle length varies. Adults commonly have cycles of 21 to 35 days, and teenagers in the first years after menarche often have longer or less regular ones. A cycle can differ from the previous one by several days without anything being wrong. The calculator accepts 21 to 45 days so that longer cycles can be modelled.
Frequent cycles shorter than 21 days, longer than 35 days, very heavy bleeding or bleeding between periods are reasons to discuss things with a clinician.
Late or missed periods
A late period has many possible causes besides pregnancy, including stress, illness, a change in exercise, weight change, travel, thyroid problems, polycystic ovary syndrome and perimenopause. A pregnancy test taken from the first day of a missed period is dependable, and a clinician can advise if tests are negative but periods stay absent for three months.
For fertile-window timing, see the ovulation calculator, which accepts the luteal phase length as an input.
Limits of date prediction
Any calendar prediction assumes next month will resemble past months. Hormonal contraception, breastfeeding, postpartum recovery and age all change the pattern, and some methods stop periods altogether. Keep a record of actual start dates and update the average from the last three to six cycles. The page gives general information only and cannot assess your health.
When to see a clinician
Cycle length naturally varies by a few days from month to month, and a predicted date is an average, not a promise. Speak to a clinician if periods stop for three months or more without pregnancy, if cycles are consistently shorter than 21 days or longer than 35 days, if bleeding is very heavy or lasts more than seven days, or if you have severe pain. This calculator cannot diagnose a cause and is not a pregnancy test.
Worked example
A last period began on 1 March with a usual cycle of 30 days. The date counted from is 5 April.
Inputs
| First day of last period | 2026-03-01 |
|---|---|
| Average cycle length | 30 days |
| Period length | 5 days |
| Periods to list | 3 periods |
| Count days from | 2026-04-05 |
Result
Expected to start on Thursday 30 April 2026 and last about 5 days.
| Next period starts | 30 April 2026 |
|---|---|
| Next period ends (approx.) | 4 May 2026 |
| Ovulation before that period (approx.) | 16 April 2026 |
Predicted periods
| Starts | Ends (approx.) | Ovulation before next period (approx.) |
|---|---|---|
| 30 April 2026 | 4 May 2026 | 16 May 2026 |
| 30 May 2026 | 3 June 2026 | 15 June 2026 |
| 29 June 2026 | 3 July 2026 | 15 July 2026 |
- Ovulation is placed 14 days before each period, the usual luteal phase assumption. Predictions assume a perfectly regular cycle.
Medical disclaimer: This tool is for education and general planning. It is not medical advice and does not diagnose any condition. If you are pregnant, under 18, have an eating-disorder history or a medical condition, speak to a doctor or registered dietitian before changing your diet or exercise.
Frequently asked questions
How do I calculate when my next period is due?
Add your average cycle length to the first day of your last period. With a 28-day cycle starting on 1 March, the next period is expected on 29 March.
How is cycle length counted?
From the first day of one period to the day before the first day of the next. The first day of bleeding is day 1.
What if my cycle is irregular?
Predictions become less reliable. Using the average of several recent cycles helps, and a clinician can look for causes if the pattern is very variable.
Can this predict ovulation?
It estimates it as 14 days before each predicted period, which is only an approximation.
When should I take a pregnancy test?
A test is most reliable from the day your period is expected, or later. Testing too early can give a false negative.