How accurate is the prediction?

It is only accurate if your cycles are regular. With a 28-day cycle the estimate is usually close, but a single early or late ovulation moves the fertile window by several days. Track several cycles to see your own pattern.

Can I get pregnant during my period?

It is unlikely but possible. Sperm survive up to five days, so in a short cycle with early ovulation, intercourse near the end of bleeding can overlap with the fertile window.

What is a normal cycle length?

For adults, 21–35 days is considered normal, with bleeding lasting 2–7 days. Cycles that are consistently shorter, longer or very irregular are worth discussing with a clinician.

📅 Menstrual Cycle Calculator

Calculate next period date and ovulation date from your menstrual cycle.

Last Period Start Date Cycle Length (days) Period Length (days)

How the Cycle Is Calculated

The menstrual cycle runs from the first day of one period to the day before the next. Ovulation usually happens about 14 days before the next period starts — not 14 days after the last one — so the fertile window is estimated backwards from the predicted next period. Cycle length varies between people and between cycles; 21–35 days is considered normal for adults.

What is Menstrual Cycle?

The menstrual cycle is the period from the start of one period to the day before the next. Average is 28 days; 21-35 days is normal.

Ovulation Calculation

Ovulation Date = Next Period Date - 14 days

The fertile window is 5 days before and 1 day after ovulation.

Safe Days / Fertile Window

Fertile Window Start: Fertile window: 5 days before to 1 day after ovulation

Fertile Window End: Safe days: Periods outside the fertile window (not 100% safe)

Next period = last period start + cycle length. Ovulation ≈ next period − 14 days. The fertile window is the five days before ovulation plus the day after, because sperm survive up to five days while the egg lives about 24 hours.

Worked Examples

Last period 1 May with a 28-day cycle → next period 29 May, ovulation around 15 May.

Fertile window: 10–16 May.

An Estimate, Not Contraception

This is a calendar estimate, not a contraceptive method or a fertility diagnosis. Ovulation shifts with stress, illness, travel and sleep changes, and cycles shorter than 21 or longer than 35 days deserve medical advice. If you are trying to conceive or avoid pregnancy, combine the estimate with ovulation tests, basal body temperature or medical guidance.

Frequently Asked Questions

How accurate is the prediction?: It is only accurate if your cycles are regular. With a 28-day cycle the estimate is usually close, but a single early or late ovulation moves the fertile window by several days. Track several cycles to see your own pattern.
Can I get pregnant during my period?: It is unlikely but possible. Sperm survive up to five days, so in a short cycle with early ovulation, intercourse near the end of bleeding can overlap with the fertile window.
What is a normal cycle length?: For adults, 21–35 days is considered normal, with bleeding lasting 2–7 days. Cycles that are consistently shorter, longer or very irregular are worth discussing with a clinician.