Ovulation Calculator
Find your fertile window and most fertile days each cycle.
A free ovulation calculator — estimate the day you're most likely to ovulate and your fertile window (the ~6 days each cycle when conception is possible) from the first day of your last period, your cycle length and your luteal-phase length. It also predicts your next several periods and your most fertile days, and estimates a due date if you conceive. 100% in-browser — educational estimate only, not medical advice or contraception.
Ovulation Calculator features
- Estimate your ovulation day each cycle
- Full fertile window and most-fertile days
- Predicts upcoming periods and cycles
- Adjustable cycle and luteal-phase lengths
- Private — runs entirely in your browser
How to use Ovulation Calculator
Enter your last period
Select the first day of your most recent period on the calendar — this is the anchor the calculator uses to work out the rest of your cycle.
Set your cycle length
Adjust the average cycle length to match your body. If you're unsure, leave it at 28 days; tracking a few cycles gives the most accurate figure.
Fine-tune the luteal phase
Optionally set your luteal-phase length (the gap between ovulation and your period, usually 12–14 days) for a more personalised ovulation estimate.
See your fertile window
The calculator shows your estimated ovulation day, the six-day fertile window and the two most fertile days when conception is most likely.
Plan upcoming cycles
Review the table of upcoming cycles to see future fertile windows and period dates, making it easy to plan around them.
Frequently asked questions
How is ovulation calculated?
Ovulation is estimated as your next period date minus your luteal-phase length (about 14 days). Your next period is your last-period date plus your cycle length, so a longer cycle shifts ovulation later.
What is the fertile window?
It's the roughly six days each cycle when pregnancy is possible — the five days before ovulation plus ovulation day itself — because sperm can survive several days while the egg lives about 24 hours.
How accurate is an ovulation calculator?
It's a helpful estimate based on averages. Real ovulation can vary from cycle to cycle, especially with irregular periods. Ovulation predictor kits or basal body temperature tracking give more precise, real-time signals.
Can I use this for birth control?
No. The calendar method is not a reliable form of contraception on its own. If you're avoiding pregnancy, use a proven contraceptive method and speak to a healthcare professional.
What if my cycles are irregular?
The calculator assumes a fairly regular cycle. With irregular cycles the estimate is less reliable — tracking several cycles and using ovulation tests alongside it will improve your picture.
When am I most likely to get pregnant?
The highest chance is on the day of ovulation and the two days before it, within the roughly six-day fertile window this calculator highlights. Because sperm survive several days, timing around the days leading up to ovulation matters most.
How do I calculate ovulation with an irregular cycle?
Calendar estimates are less reliable with irregular cycles because ovulation shifts month to month. Enter your average cycle length here for a best guess, but combine it with ovulation predictor kits or basal body temperature tracking for a clearer signal.
Is my data private?
Yes. Everything is calculated in your browser. Your dates and cycle details are never uploaded, logged or stored anywhere.
Related tools — Health · Pregnancy & cycle
- Pregnancy & Due Date Calculator — Estimate your due date and how far along you are, week by week.