Menstrual Cycle Calculator
Knowing your cycle length allows you to anticipate your next period and estimate your ovulation day well in advance. This calculator uses your cycle length and days since your last period to provide both predictions.
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Formula
Next period = Cycle length − Days since last period; Ovulation = Cycle length − 14 − Days since last period
The next period arrives when the full cycle length has elapsed from the first day of the last period, so the remaining days equal cycle length minus days already elapsed. Ovulation typically occurs 14 days before the next period — a reflection of the consistent luteal phase length — making the ovulation estimate cycle length minus 14 minus days elapsed.
How to use the Menstrual Cycle Calculator
- 1
Enter your cycle length
Value should be in days.
- 2
Enter your days since last period
Value should be in days.
- 3
Read your results instantly
Results update in real time as you type.
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Normal cycle variation
A textbook menstrual cycle is 28 days, but healthy cycles range from 21 to 35 days. Cycle length can also vary month to month by up to 7–9 days in otherwise healthy individuals. Variation is largest in the follicular phase (from period to ovulation), while the luteal phase (ovulation to period) is more consistent at 10–16 days. Tracking your cycle over 3–6 months gives you a reliable average length for more accurate predictions.
Factors that disrupt cycle regularity
Stress is the most common disruptor of cycle regularity, acting through the hypothalamic-pituitary-adrenal axis to delay or suppress ovulation. Significant weight changes, intense exercise, illness, travel across time zones, and hormonal contraceptive use or discontinuation can all alter cycle timing. Thyroid dysfunction and polycystic ovary syndrome (PCOS) cause chronic cycle irregularity and warrant medical evaluation if cycles consistently fall outside the 21–35 day range.
Tips & Insights
Use a period tracking app for historical data
Apps like Clue or Flo analyze your logged cycle history to improve predictions. Even entering just start dates gives them enough data to identify your average cycle length and variability.
Identify your personal luteal phase length
Track your BBT (basal body temperature) for a few cycles to identify your specific ovulation day. Subtracting that day from your cycle length gives your personal luteal phase length, improving ovulation predictions.
A negative days-until-ovulation result is normal
If you are past your expected ovulation date in the current cycle, the result will be negative — this simply means ovulation has already occurred this cycle.
Worked Examples
28-day cycle, 10 days since period
Next period in 18 days; ovulation in 4 days
32-day cycle, 5 days since period
Next period in 27 days; ovulation in 13 days
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Frequently Asked Questions
What is a normal menstrual cycle length?
Healthy cycles range from 21 to 35 days. Cycles consistently shorter than 21 or longer than 35 days warrant a conversation with a healthcare provider.
Is it normal for cycle length to vary month to month?
Yes. Variation of up to 7–9 days is normal. Larger variation or sudden changes may indicate hormonal imbalance and should be discussed with a doctor.
How do I know if I have PCOS?
PCOS is characterized by irregular or absent periods, signs of elevated androgens (acne, excess hair), and polycystic ovaries on ultrasound. Diagnosis requires clinical evaluation — a calculator cannot diagnose it.
Can I use this to prevent pregnancy?
Calendar-based methods have high failure rates because ovulation timing varies. This calculator should not be used as contraception. Consult a healthcare provider about reliable contraceptive options.
Does menopause affect cycle prediction?
Perimenopause causes progressively irregular cycles in the years before the final period. Standard cycle-length predictions become unreliable during this transition.
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