Period Calculator — Predict Your Next Period & Ovulation
Enter your last period date and average cycle length to get instant predictions for your next periods, ovulation day, and 6-day fertile window.
Track your menstrual cycle, fertile days, and ovulation. Plan with confidence.
When did your last period start?
Most cycles are 21-35 days (average 28)
Typical period lasts 3-7 days
Quick Facts About Your Menstrual Cycle
The average menstrual cycle is 28 days, but cycles between 21-35 days are normal.
Ovulation typically occurs 14 days before your next period, regardless of cycle length.
Your fertile window is the 5 days before ovulation plus the day of ovulation (6 days total).
Irregular cycles (varying by 8+ days) may indicate hormonal imbalances — consult your doctor if concerned.
Understanding Your Menstrual Cycle
Your menstrual cycle is counted from the first day of one period to the first day of the next. A typical cycle lasts 28 days but can range from 21 to 35 days in adults and 21 to 45 days in teens. Understanding your cycle helps you predict periods, identify your fertile window for conception or contraception, and recognize irregularities that may need medical attention.
Tracking your menstrual cycle is essential for understanding your body's natural rhythms, reproductive health, and overall well-being. Whether you're planning to conceive, avoid pregnancy, or simply want to know when your next period will arrive, this calculator provides accurate predictions based on your personal cycle data.
The 4 Phases of Your Menstrual Cycle
Menstrual Phase (Days 1-5)
Period bleeding occurs as the uterine lining sheds. Hormone levels (estrogen and progesterone) are at their lowest. You may experience cramping, fatigue, and lower energy levels during this phase.
Follicular Phase (Days 1-13)
Estrogen rises as follicles develop in the ovaries. This phase overlaps with the menstrual phase and lasts until ovulation. Energy increases, and you may feel more motivated, social, and creative.
Ovulation (Day 14)
A mature egg releases from the ovary. This is your most fertile day. You might notice increased energy, libido, and a slight rise in basal body temperature. Cervical mucus becomes clear and stretchy.
Luteal Phase (Days 15-28)
Progesterone rises to prepare the uterus for pregnancy. If no conception occurs, hormone levels drop and your period begins. You may experience PMS symptoms like mood changes, bloating, breast tenderness, or food cravings.
Why Track Your Period?
1. Predict Your Period with Confidence
Know when to expect your next period so you can plan vacations, events, and daily activities without surprises.
2. Understand Your Fertile Window
Whether trying to conceive or avoid pregnancy, knowing your ovulation date helps you make informed decisions about your reproductive health.
3. Identify Irregular Patterns
Tracking your cycle helps you notice irregularities like missed periods, heavy bleeding, or unusual spotting that may require medical attention.
4. Monitor Hormonal Health
Regular cycles often indicate balanced hormones, while irregular cycles may signal conditions like PCOS, thyroid issues, or chronic stress.
5. Plan Medical Appointments
Many gynecological procedures are best scheduled at specific cycle phases. Tracking helps you book appointments optimally.
Period Calculator FAQs
How do I calculate when my next period will start?
To calculate your next period, add your cycle length (typically 28 days) to the first day of your last period. For example, if your last period started on January 1 and you have a 28-day cycle, your next period should start around January 29. Our calculator does this automatically and shows your next 3 cycles.
What if my cycle is irregular?
For irregular cycles (varying by 8+ days month to month), use your shortest cycle length for the most conservative estimate. Track 3-6 cycles to find your average. If cycles vary widely (21 days one month, 40 the next), consult a healthcare provider to rule out conditions like PCOS or thyroid disorders.
When am I most fertile during my cycle?
Your fertile window is the 6 days leading up to and including ovulation. Sperm can survive 5 days in the reproductive tract, while the egg lives 12-24 hours after release. For a 28-day cycle, your fertile window is typically days 9-14, with ovulation on day 14.
Can I use this calculator if I'm on birth control?
Hormonal birth control (pill, patch, IUD, implant) typically stops ovulation and creates withdrawal bleeding, not true periods. This calculator is designed for natural cycles. If you've recently stopped birth control, wait 2-3 cycles for your natural pattern to resume before using this calculator.
How accurate is a period calculator?
Period calculators are 70-85% accurate for women with regular cycles (varying by ±2 days). Accuracy decreases with irregular cycles, stress, illness, or hormonal changes. Use this as a guide, not a guarantee. Track 3+ cycles for better predictions.
What's a normal cycle length?
A normal cycle for adults is 21-35 days (average 28 days), counted from the first day of one period to the first day of the next. Teen cycles can be 21-45 days. Cycles shorter than 21 days or longer than 35 days may warrant a doctor visit.
What causes my period to be late?
Common causes include pregnancy, stress, significant weight changes, excessive exercise, hormonal imbalances (thyroid, PCOS), illness, travel, or perimenopause. A late period is typically defined as 5+ days past your expected date. Take a pregnancy test if sexually active, or consult your doctor if periods are frequently irregular.
Can stress delay my period?
Yes. Stress triggers cortisol release, which can interfere with reproductive hormones and delay or skip ovulation, pushing back your period. Chronic stress may cause irregular cycles. Managing stress through exercise, sleep, and relaxation techniques can help regulate your cycle.
Should I see a doctor about irregular periods?
Consult a healthcare provider if you experience: cycles shorter than 21 days or longer than 35 days, missed periods (not pregnant), bleeding for more than 7 days, severe cramps affecting daily life, bleeding between periods, or sudden changes in cycle pattern. These may indicate treatable conditions.
Medical Review
This period calculator uses evidence-based methods recommended by the American College of Obstetricians and Gynecologists (ACOG) to estimate cycle phases and fertile windows. While calculators are helpful tools, individual variations exist. Consult your healthcare provider for personalized advice.
Based on guidelines from ACOG, National Institute of Child Health and Human Development (NICHD), World Health Organization (WHO), NHS UK, and CDC Reproductive Health Guidelines.
Last updated: January 11, 2025 | Next review: July 11, 2025