Overview
What this calculator does
Estimate your fertile window, ovulation day, and next period from the first day of your last period and your average cycle length. It shows the most fertile days across three upcoming cycles so you can plan ahead. Everything is calculated in your browser.
The cycle
How ovulation timing works
A menstrual cycle has two phases around ovulation. The luteal phase — from ovulation to your next period — is fairly constant at about 14 days. So ovulation happens roughly cycle length − 14 days after your period starts:
- 28-day cycle: ovulation around day 14
- 30-day cycle: ovulation around day 16
- 26-day cycle: ovulation around day 12
That's why the day you ovulate shifts with your cycle length, even though the luteal phase stays about the same.
The 6 key days
The fertile window
Five days before, plus ovulation day
Beyond the estimate
Tracking for accuracy
This calculator uses average timing, which is a good starting point but not exact — ovulation varies between cycles and people. For more precision:
- Ovulation predictor kits (OPKs) detect the LH surge 24-36 hours before ovulation
- Basal body temperature (BBT) rises slightly after ovulation, confirming it happened
- Cervical mucus becomes clear and stretchy near ovulation
- Tracking several cycles gives a better average cycle length to use here
Not contraception
Important note
Not a birth control method
Behind the scenes
Privacy and how it runs
Your data stays private
Common questions
When am I most likely to get pregnant?
During the fertile window — the five days before ovulation and ovulation day. The highest chance is in the two to three days right before and including ovulation.
How do I know my cycle length?
Count from the first day of one period to the first day of the next. Do this over a few cycles and average them. Most people fall between 21 and 35 days.
What if my periods are irregular?
Calendar-based estimates are less reliable with irregular cycles because ovulation timing varies more. Ovulation predictor kits or tracking basal body temperature give a clearer picture in that case.
How long should I try before seeing a doctor?
General guidance: see a healthcare provider after about 12 months of trying if you're under 35, or after 6 months if you're 35 or older. Talk to a provider sooner if you have known fertility concerns.
Can this predict the sex of the baby?
No. Methods claiming to time intercourse for a specific sex are not supported by strong evidence. This tool only estimates fertile days.
Related calculators
Pregnancy and health tools that pair with the ovulation calculator.
Last reviewed: · Methodology based on US building code standards, contractor pricing surveys, and manufacturer specifications.