Ovulation Calculator
Estimate when you're most likely to ovulate. Enter the first day of your last period and your usual cycle length to see an approximate ovulation window for your next few cycles. This is an estimate, not a method of birth control.
This is NOT a method of birth control
Most likely ovulation day
November 21, 2026
Check the highlighted fields to see your result.
- Estimated ovulation window
- November 19 – 25
- Next period expected
- December 5, 2026
Next cycles (estimated)
| Cycle starting | Ovulation window |
|---|---|
| November 7, 2026 | November 19 – 25 |
| December 5, 2026 | December 17 – 23 |
| January 2, 2027 | January 14 – 20 |
Results are estimates for information only and do not constitute financial, medical, or legal advice.
How it works
Ovulation usually happens about 10 to 16 days before the next period starts. The calculator adds your cycle length to the first day of your last period to estimate the next period, then counts back to find that window. The most likely single day is 14 days before the next period. Real cycles vary, so treat the dates as approximate.
The formulas
Next period = last period + cycle length
Ovulation window = next period − 16 days to next period − 10 days
Worked example
Your last period started on March 1, and your cycles are usually 28 days long.
- Next period: March 1 + 28 days = March 29.
- Ovulation window: March 13 to March 19.
- Most likely day: March 29 − 14 days = March 15.
Sources
- NHS.Periods and fertility in the menstrual cycle(ovulation usually happens around 10 to 16 days before the next period).
Frequently asked questions
Can I use this to avoid getting pregnant?
No. A calendar estimate is not a reliable method of birth control. Ovulation can happen earlier or later than expected, and sperm can survive for several days. Talk to a health professional about contraception that suits you.
When does ovulation usually happen?
In most people it happens around 10 to 16 days before the next period. With a regular 28-day cycle, that is often around day 14, but this won’t apply if your cycles are shorter, longer, or irregular.
My cycles are irregular. Does this still work?
Less well. The calculator assumes each cycle has the same length. If yours vary a lot, the estimate can be far off. Ovulation tests, tracking body temperature, or advice from a health professional can give more information.
When should I see a doctor?
If you've been trying to get pregnant for a while without success, if your periods are very irregular or painful, or if you have any concerns, talk to a health professional. They can tell you when it's a good idea to get checked.