How does a period calculator work?
Your menstrual cycle is counted from the first day of one period to the first day of the next. If your cycles are fairly regular, adding your usual cycle length to the start of your last period predicts the next one. This free period calculator does that for the next six cycles, and also estimates your ovulation day and fertile window — useful whether you are planning a pregnancy, a holiday, Umrah or Ramadan fasting days.
- Next period start = last period start + cycle length
- Period end = start + period length − 1 day
- Ovulation ≈ next period − 14 days; fertile window = ovulation − 5 days to + 1 day
Example: last period started 1 January 2026, 28-day cycle, 5-day period → next periods start around 29 January, 26 February and 26 March 2026.
What is a normal menstrual cycle?
| Measure | Short / light | Normal | Long / heavy |
|---|---|---|---|
| Cycle length | Under 21 days | 21 – 35 days (adults); 21 – 45 days in the first years after periods start | Over 35 days |
| Period length | Under 2 days | 2 – 7 days | Over 7 days |
| Blood loss | Spotting only | About 30 – 40 ml (up to 80 ml) | Soaking a pad or tampon every 1–2 hours, or clots larger than a coin |
| Variation | — | Up to 7–9 days between cycles | Very unpredictable |
Benefits of tracking your period
- Be prepared for your next period and plan travel, sport and events.
- Spot changes early — missed, heavy or very painful periods can point to PCOS, thyroid problems, anaemia or other conditions.
- Know your fertile days when trying to conceive (see the ovulation calculator).
- Support your energy: heavy periods drain iron; tracking helps you and your doctor see the pattern.
Health warnings
- Predictions are estimates and not a method of contraception.
- See a doctor for periods that stop for 3 months or more (and you are not pregnant), bleeding between periods or after sex, very heavy bleeding, severe pain, or periods more often than every 21 days.
- Heavy periods often cause iron deficiency anaemia — symptoms include tiredness, breathlessness, hair loss and pale skin.
- Sudden weight loss, very low body fat or over-exercising can stop periods.
Nutrition and your cycle
Iron-rich foods (red meat, lentils, chickpeas, spinach with vitamin C), steady blood sugar and enough magnesium and omega-3s can help energy and cramps. For irregular cycles linked to PCOS, see our PCOS meal plan; for heavy periods, read the iron deficiency anaemia diet.
Frequently asked questions
How do I calculate my next period?
Add your usual cycle length to the first day of your last period. For example, if your last period started on 1 March and your cycle is 28 days, your next period should start around 29 March.
What is a normal cycle length?
For adults, anything from 21 to 35 days is normal. Teenagers often have longer, less regular cycles in the first few years.
Why is my period late?
Common reasons include pregnancy, stress, travel, illness, weight change, intense exercise, breastfeeding, PCOS and thyroid problems. If there is a chance you are pregnant, take a test.
Can I use a period calculator as birth control?
No. Ovulation can shift from cycle to cycle and sperm can survive for several days, so calendar predictions are not reliable contraception.
How long should a period last?
Most periods last 2 to 7 days. Periods longer than 7 days or very heavy bleeding should be checked by a doctor.
Sources
- American College of Obstetricians and Gynecologists. Menstruation in girls and adolescents: using the menstrual cycle as a vital sign. Committee Opinion 651.
- NHS. Periods – heavy periods and irregular periods.
- NICE. Heavy menstrual bleeding: assessment and management (NG88).