Ovulation Calculator – Predict Your Fertile Window Online (Free & Private)
Use this free ovulation calculator as an online ovulation predictor: enter your last period and cycle length to see your estimated fertile window. All calculations run in your browser—no data is sent or stored.
For educational and planning purposes only. Not a substitute for medical advice.
Why you can trust this tool
- Your data never leaves your device.
- 100% browser-based
- No tracking
- No signup
- Free forever
Educational note
Ovulation typically occurs 12–16 days before your next period; this calculator uses a 14-day average for estimation. The fertile window includes the 5 days before ovulation and the day of ovulation (and sometimes the day after), because sperm can live several days and the egg about 24 hours. Your cycle length can vary—using your average over the last few cycles can help improve the estimate.
This tool is for planning and education only. It is not medical advice. For fertility concerns or irregular cycles, we recommend speaking with a healthcare provider.
This calculator is built for educational planning using standard menstrual cycle research referenced by organizations like ACOG, Mayo Clinic, and Planned Parenthood.
Educational basis and accuracy
Ovulation timing in this calculator is based on widely used cycle biology: ovulation typically occurs 12–16 days before the next period. We use a 14-day average for estimation because it fits most regular cycles. Actual timing can vary due to stress, illness, travel, or hormonal factors. This tool is for educational and planning purposes only and is not a substitute for medical advice or diagnosis.
Sources referenced for educational accuracy:
- American College of Obstetricians and Gynecologists (ACOG)
- Mayo Clinic
- Planned Parenthood
Why use this calculator?
- 100% browser-based — no data stored or sent
- Ovulation day and fertile window in one view
- Instant results
- Free, no signup, no ads
What is a free ovulation calculator?
A free ovulation calculator (or online ovulation predictor) estimates your ovulation day and fertile window based on the first day of your last period and your average cycle length. Ovulation typically occurs 12–16 days before the next period; we use a 14-day average for estimation, so in a 28-day cycle that is around day 14. The fertile window predictor shows the 5 days before ovulation and the day of ovulation (and sometimes the day after), when conception is most likely.
This tool is for education and planning only. Actual ovulation can vary. For more precise tracking, consider ovulation predictor kits (OPKs) or basal body temperature. Always consult a healthcare provider for medical advice.
A private ovulation calculator that stays on your device
Many people look for a private ovulation calculator that doesn’t require an account or send their dates to a server. This tool is built for that: it’s a no-login fertility calculator that runs entirely in your browser. Your last period date and cycle length never leave your device, and we don’t use cookies or tracking. You get a simple ovulation predictor and fertile window estimate without signing up or sharing data.
Whether you call it a browser ovulation tool or a simple ovulation predictor, the idea is the same: enter your dates, see your estimated ovulation day and fertile window, and use the visual calendar to plan. No apps to install, no accounts, and no data stored—just a lightweight, private estimate you can use anytime.
Ovulation calculator for irregular periods and simple tracking
If you have an ovulation calculator irregular periods need, use the “My cycle length varies” option and enter your last three cycle lengths. We use the average and show an estimate range so you get a private fertility calculator experience without apps or accounts. This is an ovulation calculator without app installs: a browser fertility calculator that works on any device, leaves your data on your device, and stays free.
Frequently asked questions
How is ovulation day calculated?
Ovulation typically occurs 12–16 days before the next period. We use a 14-day average: ovulation date = last period date + (cycle length − 14). So for a 28-day cycle, ovulation is around day 14 from the first day of your last period.
What is the fertile window?
The fertile window is the 6 days when conception is most likely: the 5 days before ovulation and the day of ovulation (we include 1 day after as well). Sperm can live several days; the egg lives about 24 hours.
Is this calculator accurate?
It gives an estimate based on your average cycle using a 14-day ovulation rule (ovulation typically occurs 12–16 days before the next period). Real ovulation can vary. For precise tracking, use OPKs or BBT. This tool is for education and planning only—not medical advice.
Is this medical advice?
No. For fertility concerns or irregular cycles, see a healthcare provider. Related tools: Pregnancy Due Date Calculator, Age Calculator, Date Calculator.
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