How is my next period predicted?

Your next period start date is calculated by adding your average cycle length to the start date of your last period. If your last period started on the 1st and your cycle is 28 days, your next period will start on the 29th. The calculator projects six future cycles.

What is a normal cycle length?

A typical menstrual cycle is 21–35 days, measured from the first day of one period to the first day of the next. 28 days is the population average but anything in this range is considered normal. Cycle length can vary by a few days month to month.

How is the fertile window calculated?

Ovulation typically occurs 14 days before the next period starts. The fertile window is the five days leading up to ovulation plus ovulation day itself — six days total — because sperm can survive in the reproductive tract for up to five days.

Is this calculator a reliable form of contraception?

No. Cycle-based predictions are estimates, not contraception. Cycle length varies and stress, illness, travel, hormonal changes and breastfeeding can shift ovulation. Use a recognised contraceptive method if you want to avoid pregnancy.

Why does my cycle vary each month?

Stress, sleep changes, weight changes, intense exercise, illness, travel across time zones, hormonal birth control, perimenopause and certain medications can all shift cycle length by several days. Variations of ±5 days are usually nothing to worry about — but speak to a GP if your cycle becomes very irregular.

🌺 Period Calculator

Predict your next six periods, ovulation day and fertile window from the start date of your last period and your average cycle length.

First day of last period

Average cycle length (days)

Average period length (days)

⚠️ For information only. Predictions are estimates based on cycle averages and are not a contraceptive method. Speak to a GP about irregular cycles or fertility planning.

Understanding Your Menstrual Cycle

The menstrual cycle is the monthly hormonal cycle that prepares the body for pregnancy. It is measured from day 1 — the first day of bleeding — to day 1 of the next period. The average cycle is 28 days but anything from 21–35 days is medically normal. This calculator uses your last period date and average cycle length to predict the next six cycles, including ovulation and the fertile window.

The Four Phases of the Cycle

Menstrual phase (days 1–5): the uterine lining is shed if no pregnancy occurred.
Follicular phase (days 1–13): the body prepares an egg for release. Oestrogen rises.
Ovulation (around day 14 in a 28-day cycle): a mature egg is released and is viable for ~24 hours.
Luteal phase (days 15–28): if the egg is not fertilised, hormone levels drop and the cycle starts again.

Fertile Window Explained

The fertile window covers the five days before ovulation plus ovulation day itself, because sperm can survive up to five days inside the reproductive tract. Highest chance of conception is the two to three days immediately before ovulation.

When to Speak to a GP

Cycles consistently shorter than 21 or longer than 35 days
Periods that suddenly become much heavier or longer
Bleeding between periods or after sex
Severe period pain that interferes with daily life
Trying to conceive for over 12 months without success (or 6 months if over 35)

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Important Note: This tool provides cycle estimates only and is not a substitute for medical advice or contraception. For fertility planning, contraception or irregular cycles, please consult a qualified healthcare professional.

Related Calculators

Ovulation Calculator — Predict your fertile window, ovulation day and next period from your last period and average cycle length.
Pregnancy Due Date Calculator — Estimated due date, current week and trimester from last period.
Sleep Calculator — Best bedtime and wake-up times based on sleep cycles.