Your period dates
I start with the date you entered and move forward by your average cycle length. Then I repeat it for six cycles so you have something useful to plan around.
Plan the month.
Keep it yours.
Estimate your next period and see the next few cycles on a clear calendar. Three simple details go in. Nothing gets uploaded, saved, or turned into an awkward ad later.
Your dates stay in this tab. This calculator does not upload them, save them to an account, or store them for your next visit.
Your calendar will show up here.
Start with the first day of your last period. I’ll handle the date math from there.
Next estimated period
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Estimate
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Expected period end—
Next estimated ovulation—
Estimated fertile window—
Cycle used—
The safety note is included on the image.
03
Three months at a glance
Pink marks your estimated period. Gold is the estimated fertile window. The teal outline marks estimated ovulation.
Estimated periodEstimated fertile windowEstimated ovulationToday
04
A little farther ahead
Bodies change, so the dates naturally get less certain farther out. Think of these as helpful planning windows.
| Cycle | Expected start | Likely window | Estimated ovulation |
|---|
Here’s what I do with your dates01 02 03
Simple math, with a little humility.
Your ovulation estimate
I place this estimate 14 days before the next expected period. It’s a practical midpoint, but your body gets the final say—and it may choose another day.
Your fertile window
I mark the five days before estimated ovulation and the estimated ovulation day. That’s a planning window, never a guarantee or a “safe day.”
A GENTLE BUT IMPORTANT NOTE
If something feels off, it’s worth asking.
You know your usual pattern better than a calendar does. Consider talking with a health care professional if your periods suddenly become irregular, come more often than every 21 days or less often than every 35 days, last longer than 7 days, or get in the way of daily life. Very heavy bleeding with chest pain, shortness of breath, dizziness, or lightheadedness deserves urgent medical care.
Good questions. Straight answers.
Can I use this calculator to avoid pregnancy?
Please don’t. A calendar can’t promise a “safe day,” because ovulation can move around even when your periods seem regular. Use a reliable birth-control method, and talk with a health care professional if you’d like help choosing one.
What if my cycles are irregular?
Choose the widest variation option and take the result lightly. Pregnancy, postpartum recovery, breastfeeding, perimenopause, hormonal birth control, stress, illness, and plenty of ordinary life changes can all move a cycle around.
How is the fertile window estimated?
The tool estimates ovulation 14 days before the next period, then marks the five preceding days plus that day. The Office on Women’s Health notes that the fertile window is about six days, but the actual timing varies.
Does HandyPebble keep my period dates?
No. The math happens in your browser, and the date is not sent to HandyPebble or tucked away for later. I only make a PNG if you press the button asking for one.
Is this medical advice?
No. Think of it as a tidy planning calendar, not a clinician. It can’t diagnose anything, confirm ovulation or pregnancy, or replace care from a qualified health professional.
Health information sources
I checked the health guidance, too.
I used public information from the American College of Obstetricians and Gynecologists, the U.S. Office on Women’s Health, and CDC contraception guidance to shape the explanations and safety notes. These organizations are sources, not sponsors of this calculator.
Health information reviewed August 22, 2026.