Period Calculator
Period start dates are predicted by adding your average cycle length to the first day of your last period, repeatedly fo...
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How It Works
Period start dates are predicted by adding your average cycle length to the first day of your last period, repeatedly for each future cycle. The fertile window for each cycle is also shown, based on ovulation occurring 14 days before the next period.
Quick Reference
Normal cycle
21–35 days
Within normal range
Average cycle
28 days
Most common assumption
Fertile window
6 days per cycle
5 before + ovulation day
Period duration
2–7 days
Normal range
Tips & Shortcuts
Track 3–6 actual period dates to establish your true average cycle length — do not rely on a fixed 28-day assumption.
Cycles naturally vary by 1–3 days month to month. If variation exceeds 8 days regularly, consult a doctor.
Travel across time zones, illness, and intense exercise can temporarily shift your cycle.
Common Mistakes
Assuming periods always arrive on the predicted date
Period predictions have a ±2–4 day margin for regular cycles. Track actual dates and update your average cycle length regularly.
Using a single past period to calculate future dates
Average your last 3–6 cycles for a reliable estimate. A single unusual cycle will skew all future predictions.
Frequently Asked Questions
Predictions are most accurate for regular cycles. Stress, illness, and significant weight changes can shift your period by several days.
A normal cycle ranges from 21 to 35 days. The 28-day average applies to only about 13% of women. Both shorter and longer regular cycles are completely normal.
Consult a doctor if your cycle is consistently shorter than 21 or longer than 35 days, if you miss 3+ periods without pregnancy, or if bleeding is unusually heavy.
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