How is ovulation calculated?
After ovulation there is a fairly fixed phase of about 14 days until the next period. So we first estimate the next period (last period + cycle length), then count back 14 days to find the likely ovulation day.
Example: with a 28-day cycle that started on the 1st, the next period is expected on the 29th and ovulation on the 15th. With a 32-day cycle, ovulation moves to the 19th, not the 14th as many people assume.
What is the fertile window?
Sperm can survive in the body for up to 5 days, while the egg can be fertilized for about 24 hours after release. So the fertile window is about 6 days: the 5 days before ovulation and ovulation day itself. The chances of pregnancy are highest in the 2 days before ovulation and on the day itself.
Signs that help confirm ovulation
- Ovulation test kits: detect the LH surge that comes 1–2 days before ovulation.
- Cervical mucus: becomes clear and stretchy, like raw egg white, near ovulation.
- Basal body temperature: rises slightly after ovulation, confirming it has happened.
Frequently Asked Questions
Does this work if my cycle is irregular?
It’s less accurate, because ovulation shifts when cycle length changes. You can enter the average of your last few cycles, but ovulation tests or advice from your doctor are more reliable in this case.
Can I use this calculator for birth control?
No. Ovulation can come days earlier or later in any cycle, even in usually regular ones, so the calendar method alone isn’t a reliable form of contraception.
When should I see a doctor?
If your cycles are consistently shorter than 21 days or longer than 35 days, clearly irregular, or if you haven’t conceived after a year of trying (or 6 months if you are over 35).
I’m pregnant. How do I find my due date?
Use our due date calculator to estimate your due date and how far along you are.