Period & Ovulation Calculator
Log your periods, see your cycle history, and predict your next periods, fertile window, and ovulation.
Log each period and the tool learns your average cycle for better predictions. Ovulation and the fertile window are estimates — do not use this as birth control. Your log is saved only in this browser (nothing uploaded). General information, not medical advice.
Use on any page (bookmarklet)
Want Period & Ovulation Calculator without leaving the page you're on? Drag the button below to your bookmarks bar, then click it on any website to open Period & Ovulation Calculator right there — it runs entirely in your browser.
Use responsibly: a bookmarklet runs on whatever page you click it on. Avoid sensitive sites such as online banking, payment, or healthcare pages — you run it at your own risk. Everything is processed locally and no data is sent anywhere. See our Terms.
- Show your bookmarks bar if it's hidden — Ctrl+Shift+B (⌘+Shift+B on Mac).
- Drag the button above onto the bookmarks bar.
- Open any website and click the bookmark — the Period & Ovulation Calculator panel appears in the top-right corner. Use ✛ to move it between corners, or ›/‹ to tuck it against the edge and pull it back out.
- Click the bookmark again (or the ✕) to close it.
Note: a few sites with strict security policies may block bookmarklets.
Examples
28-day cycle
Input
Last period Jul 1 · 28 days
Output
Next period ~Jul 29 · ovulation ~Jul 15
Fertile window
Input
Ovulation ~Jul 15
Output
Fertile ~Jul 10–16
Longer cycle
Input
Last period Jul 1 · 32 days
Output
Next period ~Aug 2 · ovulation ~Jul 19
About this tool
This free period and ovulation calculator lets you log each period and then predicts your next periods, estimated ovulation day, and fertile window. It keeps a dated history so you can look back and see when your periods were month to month, and it learns your average cycle length from the log — all saved in your browser for next time.
How to use
- Add the first day of a period to log it — repeat each month to build history.
- Set your period length (cycle length is auto-averaged from your log, or set it manually).
- See your current cycle day, next periods, ovulation, and fertile window.
- Review your dated log below and delete any entry you mistyped.
Important: not birth control
These predictions are estimates that assume a regular cycle. Ovulation timing varies and the fertile window can shift, so this must never be used to prevent pregnancy. It's general information to help you understand your cycle — for contraception, fertility, or any health concern, see a qualified healthcare provider.
Frequently asked questions
How does the period calculator predict my next period?
It takes the first day of your last period and your average cycle length, then adds your cycle length repeatedly to project the next few start dates. So with a 28-day cycle, your next period is estimated 28 days after the last one started.
Can I keep a log of my past periods?
Yes. Add each period's start date and it's kept in a dated log — so when you come back you can see exactly when your periods were in January, February, March, and so on. Each entry also shows how long that cycle was, and the tool averages your logged cycles to make predictions more accurate over time.
Does logging make predictions more accurate?
Yes. With two or more logged periods the tool calculates your real average cycle length from the gaps between them and uses that for predictions, instead of a generic 28 days. You can still type a cycle length manually to override it.
How is ovulation estimated?
Ovulation usually happens about 14 days before your next period, regardless of cycle length. The tool counts back 14 days from the predicted next period to estimate your ovulation day.
What is the fertile window?
It's the days you're most likely to conceive — roughly the 5 days before ovulation plus the day of ovulation, because sperm can survive several days. The calculator highlights this range around the estimated ovulation date.
Can I use this as birth control?
No. Absolutely not. Cycles and ovulation timing vary, and the fertile window can shift — so this must never be relied on to prevent pregnancy. For contraception, use a proven method and talk to a healthcare professional.
What if my cycle is irregular?
Predictions assume a fairly regular cycle, so they're less reliable if yours varies a lot month to month. Using your average cycle length helps, but expect more variation. If your cycle is very irregular, speak to a doctor.
Are my dates saved or private?
Your last period date and cycle settings are saved only in your own browser (localStorage) so the tool remembers them next time. Nothing is uploaded, sent to a server, or shared — clearing your site data removes it.
When should I see a doctor?
See a healthcare provider if your periods are very irregular, unusually heavy or painful, stop unexpectedly, or if you have concerns about fertility or symptoms. This tool is general information only, not medical advice.
Related tools
Sleep Calculator
Find the best bedtime or wake-up time based on 90-minute sleep cycles.
Pet Age Calculator
Save your dogs and cats and watch their age in human years count up live from their birthday.
Water Intake Tracker
Set a daily water goal and tap to track your intake with a progress bar — saved on your device.
Pregnancy Due Date Calculator
Estimate your due date and current week, with saved dates and week-by-week tips and cautions.