Ovulation Calculator
Estimate ovulation, the fertile window and the next periods from your cycle.
Your Likely Fertile Days, Worked Out From Your Cycle
Enter the first day of your last period and your usual cycle length, and see your estimated ovulation day, your fertile window and your next three periods, laid out on a calendar strip so the most fertile days stand out.
Ovulation usually happens about 14 days before the next period. The fertile window is the five days before ovulation and the day itself, because sperm can survive up to five days. Everything stays in your browser — nothing about your cycle is sent or stored anywhere.
Key features
- Ovulation day estimated from your own cycle length.
- Six-day fertile window, with the two most fertile days marked.
- Next three cycles — periods, windows and ovulation dates.
- Adjustable luteal phase if you know yours.
- Completely private — no account, no storage, no upload.
This tool runs entirely inside your browser using native Web APIs. Your files and text are never uploaded to a server, never logged and never shared with third parties.
How to use: Ovulation Calculator
- Enter your last period
The first day of bleeding, not the last. - Set your cycle length
Count from one period’s first day to the next; 28 is only an average. - Read the calendar
The fertile window and likely ovulation day are highlighted. - Look ahead
The next three cycles are listed for planning.
Technical specifications
| Processing location | Entirely in your browser — no server round trip |
|---|---|
| Data uploaded | None. Files and text never leave your device |
| Price | Free — no account, no trial, no usage cap |
| Category | Calculators |
| Works offline | Yes, once the page has loaded |
| Browser support | Chrome 90+, Edge 90+, Firefox 90+, Safari 15+ |
| Interface languages | English, 中文, हिन्दी, Español, العربية |
Frequently asked questions
How accurate is an ovulation calculator?
It is an estimate from dates alone. Cycles vary month to month, and stress, illness and travel shift ovulation. Ovulation predictor kits, basal body temperature and cervical mucus tracking are much more precise, and a calculator is best used alongside them.
When is the fertile window?
The five days before ovulation and the day of ovulation. The two days before ovulation and the day itself are the most fertile. An egg survives only about 24 hours, but sperm can survive up to five days.
What if my cycles are irregular?
Use your shortest and longest recent cycles as two separate estimates; your fertile days are likely somewhere across both windows. Very irregular cycles are worth discussing with a doctor.
Can I use this as contraception?
No. Calendar-based estimates are not reliable enough to prevent pregnancy. Please talk to a healthcare professional about contraception.
What is the luteal phase?
The time from ovulation to the next period — usually 12 to 16 days and fairly constant for each person. If you know yours from tracking, entering it makes the estimate better.