Health & Fitness Calculators
Period Calculator
Estimate your next period, ovulation day, and fertile window.
Enter your values and press Calculate.
Results and breakdowns will appear here after a valid calculation.
Calculation assumptions
- *Ovulation is estimated at about 14 days before your next predicted period.
- *Fertile window estimates are planning guides and can vary from cycle to cycle.
- *Predicted dates are most useful for regular cycles and do not replace medical advice.
- *This result should not be used to diagnose, treat, prevent, or manage a medical condition.
How Period Dates are Estimated
This calculator adds your average cycle length to the first day of your last period to estimate the next period start date. It also estimates ovulation at about 14 days before the next period.
Example: if your last period started January 1 and your average cycle length is 28 days, the next period is estimated around January 29. Ovulation is estimated around January 15, with a fertile window around January 10 to January 16.
Safety notes and common mistakes
Cycle estimates can shift. Stress, illness, medication, travel, pregnancy, postpartum changes, and health conditions can affect timing.
- Do not assume every cycle will be exactly the same length.
- Do not use the estimate to confirm or rule out pregnancy.
- Do not ignore missed periods, unusual bleeding, pain, or major cycle changes.
- Use professional guidance for pregnancy concerns, irregular cycles, medication questions, or symptoms.
Transparency note
Accuracy and limitations
Calzivo tools are built for practical estimates, conversions, and checks. Some tools use standard formulas or simplified assumptions, and results can be affected by input accuracy, rounding, units, local rules, or changing official requirements.
Health and fitness results are estimates, not medical advice. Formulas simplify real bodies and cycles, so important health decisions should be checked with a qualified clinician.
How to Use This Tool
Use these steps to enter the right inputs and interpret the result correctly.
Select the start date of your last period.
Enter your average cycle and period lengths.
The calculator will estimate your next period dates, ovulation day, and fertile window.
Related Tools
Other helpful tools in the Health & Fitness Calculators category.
Related Guides
Background reading and explanations related to Period Calculator.
Ovulation and Fertile Window Explained
Learn what ovulation and the fertile window usually mean and why both are cycle-based estimates.
Menstrual Cycle Length Explained
Understand cycle length, day 1, and why regular and irregular cycles affect planning estimates.
Due Date Calculation Methods
See how LMP-based due date estimates work and why cycle length can shift the timeline.
Frequently Asked Questions
Common questions about Period Calculator and how to read the result.
What is a normal cycle length?
A typical cycle is 28 days, but anything from 21 to 35 days is considered normal for adults.
How does this calculator estimate my next period?
It adds your average cycle length to the first day of your last period, then estimates future cycles from that pattern.
Is this accurate for irregular cycles?
It is less reliable for irregular cycles because cycle length and ovulation timing can change from month to month.
Can this calculator tell if I am pregnant?
No. It cannot confirm pregnancy or rule it out. Use appropriate testing and professional guidance for pregnancy concerns.
Why is my period later than the estimate?
Stress, illness, travel, medication, pregnancy, postpartum changes, and health conditions can all shift period timing.
When should I get professional guidance?
Consider professional guidance for missed periods, unusual bleeding, severe pain, pregnancy concerns, irregular cycles, or medication-related questions.