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.
Worked example: a 28-day cycle starting 1 June 2026
Suppose your last period started on Monday 1 June 2026, your average cycle is 28 days and your period lasts 5 days. The calculator predicts the upcoming cycle as follows:
| Prediction | Date |
|---|---|
| Next period starts (last period + 28 days) | Mon 29 June 2026 |
| Next period ends (5-day period) | Fri 3 July 2026 |
| Ovulation (next period − 14 days) | Mon 15 June 2026 |
| Fertile window (5 days before ovulation + ovulation day) | 10 June – 15 June 2026 |
Enter your own dates above to see your next six periods, each with its ovulation day and 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)
Signs Your Period Is Coming (PMS)
Premenstrual symptoms usually appear in the week before your period and ease once bleeding starts. Common signs include:
- Abdominal cramps and lower-back ache
- Bloating and water retention
- Tender or swollen breasts
- Mood swings, irritability or low mood
- Tiredness and trouble sleeping
- Food cravings, headaches or breakouts
Most people have some PMS, but symptoms severe enough to disrupt daily life can point to PMDD or another condition and are worth raising with a doctor.
How to Track an Irregular Cycle
If your cycle length changes from month to month, log the first day of every period for three to six months and use the average as your cycle length here. Pairing the calendar method with ovulation signs — clear stretchy cervical mucus, a small rise in basal body temperature, or ovulation predictor kits — gives a more reliable picture than dates alone.
Frequently Asked Questions
How is my next period predicted? Your next period start date is your last period start date plus your average cycle length. If your last period started on the 1st and your cycle is 28 days, the next starts 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 normal, and 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. The fertile window is the five days leading up to ovulation plus ovulation day itself — six days in 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 and 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 see a GP if your cycle becomes very irregular.
Can I use this calculator if my cycle is irregular? You can, but treat the predictions as a rough guide. Enter the average of your last three to six cycles. For very irregular cycles the calendar method is less reliable, so combine it with logging your actual start dates and tracking ovulation signs.
What are common signs that a period is coming? PMS usually appears in the week before bleeding and may include cramps, bloating, tender breasts, mood changes, tiredness, food cravings, headaches and acne. They ease once the period starts. Severe symptoms that disrupt daily life are worth discussing with a doctor.
Last reviewed: June 2026. Estimates based on the calendar method only — not medical advice and not suitable for contraception. Consult a qualified healthcare provider for personalised cycle, fertility or contraception guidance.