How the menstrual cycle works
A cycle is counted from the first day of one period to the first day of the next. The average is around 28 days, but anywhere from about 21 to 35 days is common and normal. This calculator takes your last start date and your usual cycle length and projects the dates forward.
What the fertile window means
Ovulation — when an egg is released — typically happens about 14 days before your next period starts. The worked example puts it concretely: a period starting 29 October 2026 gives ovulation on 15 October, and the fertile window runs 10–16 October, five days before ovulation through the day after. The window is that wide because sperm can survive for several days, so conception is possible from intercourse days before the egg is released.
Why these are estimates
The table below projects 29 October, 26 November and 24 December from one addition — a 28-day cycle, repeated. A month that runs longer pushes ovulation later, and the next period moves with it, because ovulation sits 14 days before the next period starts. Stress, travel, illness and sleep all do exactly that kind of pushing. Read each projected date as the calendar's best guess for a regular month, not a promise for the month you are actually having.
An important note
This tool is for general information and planning only. It is not a reliable form of contraception and is not medical advice. If your cycles are very irregular, unusually painful, or you have concerns about your health or fertility, please speak with a doctor or gynaecologist.
Worked example: three periods ahead
Take a last period starting on 1 October 2026, a 28-day cycle, and a 5-day period length. The next period is 1 October + 28 days = 29 October 2026. Ovulation is 29 October − 14 days = 15 October 2026, and the fertile window runs from 15 October − 5 days = 10 October through 15 October + 1 day = 16 October. Projecting forward by the same 28-day step gives the next few starts: 29 October, then 29 October + 28 days = 26 November, then 26 November + 28 days = 24 December 2026. The calculator shows each start with its end from the period length you entered, so the table below is the skeleton it fills in.
| Period starts | Est. ovulation | Fertile window |
|---|---|---|
| 29 October 2026 | 15 October 2026 | 10–16 October |
| 26 November 2026 | 12 November 2026 | 7–13 November |
| 24 December 2026 | 10 December 2026 | 5–11 December |
The ovulation and window columns for November and December follow the same rule applied forward: ovulation 14 days before each projected start (26 November − 14 = 12 November; 24 December − 14 = 10 December), with the window five days before ovulation through the day after (7–13 November; 5–11 December). The pattern is deliberately mechanical — add the cycle length, count back 14 days, widen by five and one — because the calculator's job is projection, not prediction. Real months will drift around these dates, which is the subject of the next section.
When the cycle is not 28 days
A cycle is counted from the first day of one period to the first day of the next, and while the average is around 28 days, anywhere from about 21 to 35 days is common and normal. The length changes every date downstream. From the same 1 October start, a 24-day cycle gives a next period of 1 October + 24 days = 25 October and ovulation of 25 October − 14 days = 11 October. A 35-day cycle gives a next period of 1 October + 35 days = 5 November and ovulation of 5 November − 14 days = 22 October. Eleven days of cycle difference move ovulation by eleven days. That is why entering your usual length matters more than any other input, and why someone with a 35-day cycle who assumes a 28-day pattern will look a full week too early. It is also why the Ovulation Calculator exists as a separate tool: it focuses on the fertile days themselves, while this page projects the periods and derives the windows from them.
Why these are estimates, and what moves them
Real cycles vary from month to month, affected by stress, travel, illness, sleep and many other things, and even people with regular cycles see some drift. A stressful month can push ovulation — and therefore the next period — later; travel and illness can do the same. The calculator cannot see any of that: it adds your cycle length to your last start date and counts back, which is the best a calendar can do. Treat every date here as a best guess, not a certainty, and treat a single late period as information, not an alarm — unless cycles are very irregular, unusually painful, or you have health or fertility concerns, in which case the page advises speaking with a doctor or gynaecologist. The same caution applies in the other direction: this tool is for general information and planning only, is not a reliable form of contraception, and must never be relied on as birth control. Fertility-awareness methods need proper training and daily tracking to be used safely, which is a different undertaking from reading a projected calendar.
Frequently asked questions
Can I use this to avoid pregnancy?
No. Fertility-awareness methods need proper training and daily tracking to be used safely. This estimate should never be relied on as birth control.
My cycle is irregular — is this accurate?
Less so. The more your cycle varies, the rougher the estimate. A period-tracking app that learns your pattern, or a chat with your doctor, will serve you better.
Why did my period come a week late when my cycle is usually regular?
Because real cycles vary even when they are usually regular. Stress, travel, illness and sleep all move ovulation, and the next period moves with it, since ovulation sits about 14 days before the next period starts. A single late month is usually just that variation, not a sign the calculator is broken — the calculator projects from your usual length and cannot see the month you are having. If lateness becomes a pattern, or cycles become very irregular or unusually painful, the page advises speaking with a doctor or gynaecologist.
How is this different from the Ovulation Calculator?
This page projects your next few periods and derives the fertile window from each projected start, while the Ovulation Calculator focuses on the fertile days themselves from your last period and cycle length. The underlying rule is the same — ovulation about 14 days before the next period, window five days before through the day after — but the two tools answer different questions: when will it start, versus when am I most fertile. Use this page for planning ahead and the Ovulation Calculator for the fertile window in detail.