Health & Fitness Calculators
Period Calculator
Project one estimated period start and inclusive end date from cycle and period lengths.
Period projection will appear here
Enter one period start and the two average lengths. Only the next cycle is projected.
What period projection represents
The calculator adds an entered average cycle length to one entered period start. It then uses the entered period length to show one inclusive estimated start-to-end range.
Worked example
A January 1, 2025 start with a 28-day cycle projects January 29. With a five-day period length, January 29 is day 1 and February 2 is the inclusive estimated end.
Normal variation and common mistakes
An average does not guarantee the next interval. Use the first day of bleeding as the start, do not add period length to the cycle length, and remember that projections become less useful when cycles vary.
Safety and trust
Period dates are calendar estimates. This tool does not determine pregnancy status, diagnose reproductive health, or replace professional assessment.
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.
Reference check
Sources and references
These references provide background context for the topic. They do not replace official advice or documents for personal decisions.
- Your menstrual cycle
U.S. Office on Women's Health
- Period problems
U.S. Office on Women's Health
How to Use This Tool
Use these steps to enter the right inputs and interpret the result correctly.
Enter the first day of a period, an average cycle length from 24 to 38 days, and a period length from 1 to 8 days.
Confirm that the entered start is not after the displayed validation date.
Select Calculate period projection to project one following cycle.
Read the projected start and inclusive end as estimates based on past averages.
Related Tools
Other helpful tools in the Health & Fitness Calculators category.
Frequently Asked Questions
Common questions about Period Calculator and how to read the result.
What does the period length change?
It sets the inclusive projected end date. A five-day projection beginning January 29 includes January 29 through February 2.
Why does this tool show only one future cycle?
Uncertainty compounds across repeated cycles. One projection keeps the arithmetic clear without presenting a long schedule as medically certain.
Why are ovulation dates not shown here?
This route is limited to period timing. The separate Ovulation Calculator explains its own assumptions and fertile-window method.
Will the next period occur on the projected date?
Not necessarily. Cycle timing can vary because of normal variation, stress, illness, medication, pregnancy, and health conditions.