How the period calculator works
This tool predicts when your future periods are likely to arrive using three simple inputs: the first day of your last period, your average cycle length, and how many days your period usually lasts. From those it projects the next six cycles, showing each period’s start and end dates plus an estimated fertile window and ovulation day.
The core idea is repetition. A menstrual cycle repeats on a roughly regular schedule, so once you know one start date and the length of a typical cycle, you can step forward one cycle at a time to estimate the dates ahead.
The menstrual cycle and cycle length
Your cycle length is the number of days from the first day of one period to the first day of the next, not the length of the bleeding itself. Most people have cycles between 21 and 35 days, and 28 days is the common average. The calculator accepts anything from 20 to 45 days so it works for shorter and longer cycles alike. If you have tracked your periods and know your own average, use that number for the best results.
Predicting future start dates
To find the next period, the calculator adds your cycle length to the first day of your last period. To predict the period after that, it adds the cycle length again, and so on for several cycles:
- Next period start = last period start + cycle length
- Second period start = last period start + (2 × cycle length)
- And so on for each future cycle
So a period that began on 1 June 2026 with a 28-day cycle predicts a next start of 29 June 2026, then 27 July 2026, then 24 August 2026. Because each date builds on the last, small differences in your real cycle length add up over time, so dates far in the future are rougher estimates than the next one or two.
Period duration
Duration is how many days your bleeding lasts, commonly 1 to 10 days with a typical value around 5. The calculator uses it to work out each period’s end date:
- Period end = period start + duration − 1 day
The “minus one” is because the first day of bleeding counts as day one. A period that starts on 29 June with a 5-day duration therefore runs 29 June through 3 July.
The fertile window and ovulation
For each predicted cycle the calculator also estimates ovulation and a fertile window. Ovulation is estimated as the cycle length minus 14 days after the period starts, since the phase after ovulation is fairly constant at about 14 days:
- Estimated ovulation = period start + (cycle length − 14 days)
- Fertile window ≈ the 5 days before ovulation through the day after
For a period starting 1 June on a 28-day cycle, estimated ovulation lands around 15 June, with a fertile window of roughly 10 June to 16 June. These are estimates from cycle length alone, so they are a guide rather than a precise measurement.
Why real cycles vary
Cycles are rarely identical month to month. Stress, illness, travel, changes in sleep or exercise, weight changes, and life stages such as adolescence, perimenopause, and breastfeeding can all shift timing. Some medications and health conditions do too. That is why the predictions here should be read as a planning aid: expect your actual period to arrive a few days before or after the estimate, and rely less on dates that are several cycles out.
A note on medical advice
This period calculator is for general information and planning only. It is not a diagnosis and it is not a substitute for professional care. If your cycles are very irregular, if your periods are unusually heavy or painful, or if they stop unexpectedly, please speak with a qualified healthcare provider who can look at your full history.