Period Calculator
Enter the first day of your last period and your average cycle length to predict your next 6 period dates, estimated ovulation days, and fertile windows.
28d
5d
How Does a Period Calculator Work?
A period calculator estimates your future period dates by adding your average cycle length to the start date of your last period. The menstrual cycle is measured from the first day of one period to the first day of the next. While the average cycle is 28 days, anything between 21 and 35 days is considered normal according to the American College of Obstetricians and Gynecologists (ACOG).
This calculator also estimates your ovulation date — typically about 14 days before your next expected period — and your fertile window, which spans roughly 5 days before ovulation through the day after. These estimates are based on the luteal phase method used in clinical practice.
What Affects Your Menstrual Cycle Length?
Several factors can influence the length and regularity of your menstrual cycle:
- Stress — Physical or emotional stress can delay ovulation and shift your period.
- Weight changes — Significant weight gain or loss affects hormone production.
- Exercise — Intense physical activity can disrupt your cycle.
- Hormonal contraception — Starting or stopping birth control can take several cycles to stabilize.
- Medical conditions — PCOS, thyroid disorders, and endometriosis can cause irregular cycles.
When Should You See a Doctor?
Consider consulting a healthcare provider if your cycle is consistently shorter than 21 days or longer than 35 days, you experience very heavy bleeding, your periods suddenly become irregular, or you have severe pain that interferes with daily activities.
Frequently Asked Questions
How accurate are period calculators?
Period calculators provide estimates based on average cycle length and work best for people with regular cycles. Actual dates can vary due to stress, illness, or hormonal changes. For more accurate predictions, track your cycle over several months or use biometric data from a wearable like Apple Watch with the Ovuly app.
What is a normal menstrual cycle length?
A normal cycle ranges from 21 to 35 days, with 28 days being the average. Cycles can vary by a few days each month and still be regular. If yours is consistently outside this range, speak with your healthcare provider.
Can stress affect when my period comes?
Yes. Stress can delay ovulation, which delays your period. The hypothalamus — the brain region that regulates your cycle — is sensitive to cortisol. Significant stress from work, travel, or life changes can cause a late period.
How do I calculate my next period date?
Add your average cycle length to the first day of your last period. For example, if your last period started January 1st and your cycle is 28 days, expect your next period around January 29th.
Medical References
The information on this page is based on peer-reviewed medical sources. This tool is not a substitute for professional medical advice.
Want smarter predictions powered by your body?
Ovuly uses your Apple Watch data — HRV, wrist temperature, sleep — to make cycle predictions that go beyond calendar math.