One date — the first day of your last period — gives you your ovulation day, fertile window, next periods and due date

How to use it

1. Pick a tab: Ovulation & fertile window, Next period or Due date & weeks. All three share the same inputs, so you only fill them in once.
2. Enter the first day of your last period (not the last day). Cycle length defaults to 28 days and period length to 5 — change them to match your body.
3. If your cycle varies, tick irregular and type your last three cycle lengths. The calculator uses their average.
4. Press Calculate. You get a table of the next three cycles with predicted period, ovulation day and fertile window, a two-month mini calendar, six upcoming period dates with countdowns, and on the due-date tab your estimated due date, current week and a checkpoint schedule.
5. Copy result puts a one-line summary on your clipboard. Inputs are kept only in this browser and can be wiped with Clear saved data.

How ovulation is estimated

The most stable part of a menstrual cycle is the luteal phase — the stretch between ovulation and the next period, which is usually about 14 days (12 to 16). So the calculator predicts ovulation as the next period date minus 14 days. On a 28-day cycle that is day 14; on a 32-day cycle it is day 18. Differences in cycle length mostly come from the phase before ovulation, which is why an accurate cycle length matters more than anything else here. This is the classic calendar (rhythm) method, useful for timing when you are trying to conceive.

Why the fertile window is 5 days before and 1 day after

Sperm can survive inside the body for up to 5 days, while an egg can only be fertilised for about 12 to 24 hours after release. Together that makes a window of roughly seven days ending the day after ovulation. The highest chance of conception is generally in the one or two days just before ovulation. The calendar shades those days and highlights the predicted ovulation day itself.

Irregular cycles

If your cycle length moves around by several days each month, a calendar estimate alone will miss. Use the average of your recent cycles, and if you want a safer range, calculate with your shortest and longest cycle and treat everything in between as potentially fertile. Ovulation test strips (LH surge), basal body temperature charting and cervical mucus observation give a much more reliable picture. Cycles shorter than 21 days, longer than 45, or skipped for three months or more are worth discussing with a doctor.

Due date: Naegele's rule

The estimated due date is the first day of your last period plus 280 days (40 weeks). The shortcut is add 7 days, subtract 3 months, add 1 year. This assumes a 28-day cycle with ovulation on day 14, so a longer cycle can push the real date later. That is why clinics confirm the dating with an early ultrasound (about 8–13 weeks) that measures the embryo's crown-rump length; if it differs from the calendar by more than about a week, the ultrasound date wins. If you were told "8 weeks 2 days" at a scan, switch to From ultrasound dating and enter that date and week count.

How pregnancy weeks are counted

Weeks are counted from the first day of your last period, which is week 0, day 0 — not from conception. Ovulation and conception happen around week 2, and a missed period with a positive test usually lands at week 4 or 5. "12 weeks 3 days" means 87 days have passed since that first day. Trimesters are commonly split as weeks 0–13 (first), 14–27 (second) and 28 onward (third). Only around 4–5% of babies arrive exactly on the due date; most are born between 37 and 41 weeks, so treat the date as the centre of a two-week range.

FAQ

I don't remember the exact start date

Use your best guess and start noting the first day of each period from now on. Two or three months of notes are enough to see your real cycle length. If you are already pregnant, use the ultrasound dating mode instead.

My period is late — am I pregnant?

A few days late is common. If it is more than a week late, take a pregnancy test; testing before the missed period can give a false negative. Persistent lateness without pregnancy can come from stress, weight change, thyroid issues or PCOS and is worth a check-up.

Do most babies come on the due date?

No — only about one in twenty. Full term begins at 37 weeks, so have your hospital bag ready from then.

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