What This Calendar Estimates
The tool projects one future cycle from the last period start date and an average cycle length. It estimates the next period, an approximate ovulation date, and a six-day fertile window.
These are calendar estimates. Ovulation can shift even when previous cycles appeared regular, and bleeding is not always a menstrual period.
Formula And Variables
Estimated ovulation = next period - 14 days
Fertile window = ovulation - 5 days through ovulation + 1 day
The 14-day luteal-phase assumption is a planning shortcut, not a measurement of ovulation. The entered bleeding duration is shown as cycle context but does not change the predicted ovulation date.
Worked Example
With an LMP start on June 1 and a 28-day average cycle, the next period is projected for June 29. Ovulation is estimated around June 15, with a fertile window of roughly June 10 through June 16.
Why Predictions Move
Stress, illness, travel, sleep changes, breastfeeding, perimenopause, medications, weight change, and hormonal conditions can alter cycle length or ovulation timing. Averaging several recent cycles is more informative than relying on one month.
When To Get Medical Advice
Seek qualified care for possible pregnancy, very heavy bleeding, severe pain, bleeding between periods, cycles that change persistently, or periods that stop unexpectedly. Adolescents, postpartum users, and people approaching menopause may need different context.
Frequently Asked Questions
When does ovulation usually happen?
Ovulation often happens about 14 days before the next period starts, but this varies by person and cycle consistency.
What is a normal menstrual cycle length?
Many adult cycles fall between 21 and 35 days, with bleeding often lasting about 2 to 7 days. Persistent changes deserve clinical attention.
Can this be used as contraception?
No. Calendar estimates are not reliable birth control. Cycle timing can shift because of stress, illness, travel, medication, and hormonal conditions.