About this calculator
Enter the first day of your last period and your usual cycle length to see the next three cycles.
How to use it
- Enter the first day of your last period.
- Enter your usual cycle length.
- Read ovulation, fertile days and next period.
The formula
Ovulation ≈ period start + (cycle length − 14) days
- cycle length
- days from one period start to the next
- fertile window
- 5 days before ovulation to 1 day after
Worked example
Period started 20 September 2026, 28-day cycle
- Ovulation ≈ 4 October 2026.
- Fertile window: 29 September to 5 October.
- Next period expected 18 October 2026.
What the result means
If cycles vary a lot, these dates are less reliable; ovulation test kits help.
Assumptions
- Regular cycles between 21 and 45 days.
Limitations
- Not a method of contraception.
- Irregular cycles may need medical advice.
Frequently asked questions
When am I most fertile?
In the two to three days before and on ovulation day.
Can I use this to avoid pregnancy?
No. Cycles vary and sperm can survive several days, so it is not reliable contraception.
What if my cycles are irregular?
Track several months, use ovulation kits, and see a doctor if cycles are very irregular.
Not medical advice. This calculator is a general screening tool and cannot diagnose any condition. Results are estimates. Please talk to a doctor or a qualified health professional about your health and before making changes to diet, exercise or treatment.
Last reviewed on 1 October 2026. Found a mistake? Tell us.