Period Calculator
Estimate your next period, ovulation day, and PMS window from your average cycle length, plus a quick check for how many days late your period is running.
Predicted
Next Period
Predicted
Next Ovulation
Instant
Days Away
Your Cycle Details
ⓘ Is My Period Late? (Optional)
If your expected period hasn’t arrived, enter today’s date to check how many days late it is.
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Next Period Starts
Oct 3, 2026
Based on your average 28-day cycle
14
Days Away
Sep 19
Ovulation
📊 Current Cycle Position
Day 15 of your cycle
Oct 3, 2026
Next Period
Sep 19, 2026
Next Ovulation
Sep 26–Sep 30
PMS Window
1–8, 16–28
Safe Days Est.
Next 6 Predicted Periods
Oct 3, 2026
Cycle 1
Oct 31, 2026
Cycle 2
Nov 28, 2026
Cycle 3
Dec 26, 2026
Cycle 4
Jan 23, 2027
Cycle 5
Feb 20, 2027
Cycle 6
How to Calculate Your Next Period
A period calculator projects your next menstrual cycle start date by adding your average cycle length to the first day of your last period. Cycle length is measured from the first day of one period to the first day of the next — it is not the same as your bleeding (period) length, which is usually just 3 to 7 days out of that total.
Understanding Your Cycle Phases
Next Period = Last Period Start + Cycle LengthOvulation Day = Last Period Start + (Cycle Length − Luteal Phase)Fertile Window = Ovulation Day − 5 days through Ovulation Day + 1 dayPMS Window = Next Period − 7 days through Next Period − 3 daysSafe Days (est.) = early cycle days before the fertile window, plus days after ovulation + 2Note: the “safe days” estimate is a rough, lower-fertility approximation based on average timing — it is not a reliable form of contraception, since ovulation timing shifts from cycle to cycle.
Why Periods Can Be Irregular
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Stress
Elevated cortisol can interfere with the hormonal signal that triggers ovulation, pushing it later and stretching out the cycle that month.
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Weight Changes
Rapid weight loss or gain shifts estrogen levels enough to delay or skip ovulation entirely, which can make periods late, light, or absent.
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Intense Exercise
Very high training volume without enough fuel can suppress reproductive hormone signaling, sometimes stopping periods altogether (exercise-induced amenorrhea).
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PCOS & Thyroid
PCOS commonly disrupts ovulation timing, and both an underactive and overactive thyroid can lengthen, shorten, or destabilize cycles — both are treatable with medical care.
— FAQ