Period Calculator
Estimate your next period date, current cycle day, and estimated fertile window (NHS guidance: ovulation ~14 days before your next period) from the first day of your last period and your average cycle length. Dates only — not a contraceptive method.
Jul 29, 2026
Next period expected (in 11 days) · cycle day 18 of 28 today
Estimated fertile window: Jul 10, 2026 – Jul 17, 2026 · estimated ovulation around Jul 15, 2026
Next 6 cycles
| Cycle | Period starts | Estimated fertile window |
|---|---|---|
| #1 | Jul 29, 2026 | Aug 7, 2026 – Aug 14, 2026 |
| #2 | Aug 26, 2026 | Sep 4, 2026 – Sep 11, 2026 |
| #3 | Sep 23, 2026 | Oct 2, 2026 – Oct 9, 2026 |
| #4 | Oct 21, 2026 | Oct 30, 2026 – Nov 6, 2026 |
| #5 | Nov 18, 2026 | Nov 27, 2026 – Dec 4, 2026 |
| #6 | Dec 16, 2026 | Dec 25, 2026 – Jan 1, 2027 |
Dates only — this is not a contraceptive method and should not be used to prevent pregnancy. Fertile-window estimate follows NHS guidance (ovulation ~14 days before the next period). Educational estimate only, not medical advice. How we calculate →
How your next period is predicted
The calculator takes the first day of your last period and adds your average cycle length (the number of days from the start of one period to the start of the next) to estimate when your next period will start. It defaults to a 28-day cycle and a 5-day period, but you can adjust both to match your own pattern.
The estimated fertile window
According to NHS guidance, ovulation typically happens around 10 to 16 days before your next period — commonly cited as around day 14 for a standard 28-day cycle. The 'fertile window' is the five days before ovulation plus the day of ovulation and the day after, roughly seven days in total, since sperm can survive in the fallopian tubes for up to a week.
The calculator marks this as days 10-17 of your cycle (counting the first day of your period as day 1), which covers that typical window for a regular cycle. If your cycles are irregular, this estimate becomes less reliable — the further from a textbook 28-day, 14-day-ovulation pattern your body runs, the wider the real uncertainty.
Why this is not a contraceptive method
This calculator estimates dates from averages — it does not track your actual hormone levels, basal body temperature, or cervical mucus, all of which are needed for a clinically validated fertility-awareness method. Ovulation timing can shift cycle to cycle even in people with generally regular periods, due to stress, illness, travel and many other factors. Do not use this tool, or any date-based estimate, to avoid or achieve pregnancy without proper guidance from a healthcare provider.
Planning ahead with the cycle calendar
The calculator projects your next six cycles based on your average cycle length, so you can see upcoming period and estimated-fertile-window dates at a glance — useful for planning, packing, or simply knowing roughly what to expect. Because it's a forward projection, accuracy naturally decreases the further out you look, especially if your cycle length varies.
What this calculator does not do
It doesn't diagnose irregular cycles, PCOS, or any other condition, and it isn't a pregnancy test or a form of contraception. If your cycles are consistently irregular, if you miss periods, or if you're trying to conceive or avoid pregnancy, speak to a doctor or nurse — they can offer clinically validated tracking methods and personalised guidance.
Frequently asked questions
How is my next period date calculated?
The calculator adds your average cycle length to the first day of your last period. For a standard 28-day cycle, that means your next period is estimated 28 days after the last one started.
When is the fertile window in my cycle?
NHS guidance places ovulation around 10-16 days before your next period, commonly cited as day 14 of a 28-day cycle. The fertile window - the days pregnancy is most likely - spans roughly the 5 days before ovulation plus ovulation day and the day after.
Can I use this calculator as birth control?
No. This is a dates-only estimate based on averages, not a clinically validated fertility-awareness method. It should never be used to prevent or achieve pregnancy - speak to a doctor or nurse about reliable contraception or fertility guidance.
Why did my period not arrive on the predicted date?
Ovulation timing varies naturally cycle to cycle due to stress, illness, travel and many other factors, so a date-based prediction is an estimate, not a guarantee. If your period is significantly late and you might be pregnant, take a pregnancy test or speak to a healthcare provider.
What if my cycles are irregular?
The wider your cycle length varies month to month, the less reliable a fixed-cycle-length prediction becomes. If your cycles are consistently irregular, tracking your actual cycle over several months, or speaking to a doctor, will give more reliable insight than a single averaged estimate.
How many days is a normal menstrual cycle?
A typical cycle ranges from about 21 to 35 days, counted from the first day of one period to the first day of the next, with periods themselves usually lasting 2-7 days. The calculator defaults to 28 and 5 days but both can be adjusted.
Researched & verified by the Calcuris Data & Research Team. How we build and check our tools →