What the period calculator does
The period calculator predicts when your next several periods are likely to start, how long each will last, and where your fertile window falls in each cycle. From three simple inputs — the first day of your last period, your average cycle length and your typical period length — it projects six cycles ahead so you can plan around travel, events, or trying to conceive.
How the prediction works
The math is straightforward. Each new period is predicted by adding your cycle length to the previous period's start date: last period + cycle length = next period, and so on. Each predicted period is shown lasting your entered period length. Ovulation for each cycle is estimated about 14 days before that cycle's next period, and the fertile window is the five days before ovulation plus ovulation day itself.
What counts as a normal cycle
Cycle length is measured from the first day of one period to the first day of the next — not from when bleeding stops. A typical cycle runs 21 to 35 days, with 28 as the textbook average, and natural month-to-month variation of a few days is completely normal. Period bleeding itself usually lasts 3 to 7 days, heaviest in the first day or two. If your cycles are consistently shorter than 21 days, longer than 35, or your bleeding regularly lasts more than 8 days, it is worth a conversation with a doctor.
Accuracy and irregular cycles
For regular cycles, these predictions are reliable. For irregular cycles they are rougher, because each forecast is built on the one before it — small month-to-month variation compounds the further out you look. If your cycle length swings widely, treat the later predictions as approximate and consider a tracking app that learns your individual pattern over several months, which adapts better than a fixed-length calendar.
Periods and fertility
The fertile window shown here is useful context, but a word of caution on both sides. If you are trying to conceive, the fertile window is a helpful guide — ideally combined with ovulation predictor kits for precision. If you are avoiding pregnancy, calendar prediction alone is not reliable contraception; typical-use failure rates for calendar methods are high, so use a proven method instead.
When timing changes
Plenty of everyday factors shift period timing: stress, illness, travel across time zones, significant weight change, intense exercise, new medications, hormonal contraception changes and the approach of menopause. An occasional early or late period is normal. A missed period when pregnancy is possible, or a sudden lasting change in your pattern, is worth checking with a provider.
Use with the other tools
To focus on your fertile days, see the ovulation calculator. If you are planning a pregnancy, the conception calculator and pregnancy calculator continue the timeline.