Period Cycle Calculator
Predict your next period dates, cycle days, and PMS window based on your last period and cycle length.
Did this tool work for you?
How to use this calculator
- 1
Enter the first day of your most recent period in YYYY-MM-DD format.
- 2
Input your average cycle length (count from the first day of one period to the first day of the next).
- 3
Enter how long your period usually lasts.
- 4
Choose how many months ahead you want predictions.
- 5
Your upcoming period dates, PMS windows, and ovulation dates are all shown.
Frequently asked questions
How do I calculate my cycle length?
Count from the first day of one period to the first day of your next period. Do not count from the last day of your period. For example: if your period started June 1 and next started June 29, your cycle length is 28 days. Track 3–6 cycles for an accurate average.
What is a normal period cycle length?
A normal cycle ranges from 21 to 35 days, with an average of 28 days. Cycles outside this range, very irregular cycles (varying by more than 7 days), or cycles that suddenly change length may warrant a visit to your doctor.
Why is my period late?
A period can be late due to pregnancy, stress, significant weight loss or gain, excessive exercise, PCOS, thyroid conditions, perimenopause, travel, or illness. If your period is more than 5 days late and you've had unprotected sex, take a pregnancy test. If late without a clear reason, consult a doctor.
What is PMS and when does it happen?
Premenstrual syndrome (PMS) occurs in the luteal phase — the 1–2 weeks between ovulation and your period. Common symptoms include bloating, mood changes, breast tenderness, fatigue, headaches, and food cravings. Symptoms usually resolve within a day or two of your period starting. PMDD (severe PMS) should be assessed by a doctor.
Free Period Cycle Calculator — Predict Your Next Period
How to track your menstrual cycle
Tracking starts with recording the first day of each period. After 3–6 months, you'll have enough data to calculate your average cycle length (add cycle lengths together, divide by number of cycles). Understanding your pattern helps predict periods, identify irregularities, and plan around important dates.
When should I see a doctor about my period?
See a doctor if: periods are consistently shorter than 21 days or longer than 35 days; flow is very heavy (soaking a pad/tampon hourly for several hours); periods have stopped for 3+ months (and you're not pregnant); severe pain that interferes with daily life; sudden changes in cycle length or flow. These can signal underlying conditions like PCOS, endometriosis, fibroids, or thyroid issues.
Learn more from an authoritative source:
CDCBMI Calculator
Calculate your Body Mass Index and find out which weight category you fall into.
BMR Calculator
Calculate your Basal Metabolic Rate (BMR) — the calories your body burns at complete rest.
Calorie Calculator
Calculate your daily calorie needs to lose weight, maintain, or gain weight based on your stats and activity.
Body Fat Calculator
Estimate your body fat percentage using the US Navy method with waist, neck, and hip measurements.
Results are estimates for informational purposes only and do not constitute professional financial, medical, legal, or technical advice. Read full disclaimer →