Period Calculator
Free Period Calculator with medically-sourced formulas. Enter your measurements for personalized, accurate health insights.
Formula
Next Period = Last Period Start + Cycle Length
Your next period is predicted by adding your average cycle length to the first day of your last period. Ovulation is estimated at (Cycle Length - 14) days after period start, based on the relatively consistent 14-day luteal phase. The fertile window spans 5 days before ovulation through 1 day after. These are estimates — actual cycles can vary.
Worked Examples
Example 1: Regular 28-Day Cycle Prediction
Problem: Last period started March 1. Cycle length is 28 days, period duration is 5 days. Predict the next 3 periods and fertile windows.
Solution: Cycle 1: Period March 29 - April 2\nOvulation: April 12 (Day 14)\nFertile window: April 7-13\n\nCycle 2: Period April 26 - April 30\nOvulation: May 10\nFertile window: May 5-11\n\nCycle 3: Period May 24 - May 28\nOvulation: June 7\nFertile window: June 2-8
Result: Next 3 periods: Mar 29, Apr 26, May 24 | Periods per year: ~13 | ~65 bleeding days/year
Example 2: Shorter 25-Day Cycle
Problem: Last period started March 5. Cycle length is 25 days, period duration is 4 days. When are the next periods?
Solution: Ovulation day = 25 - 14 = Day 11\n\nCycle 1: Period March 30 - April 2\nOvulation: April 10\nFertile window: April 5-11\n\nCycle 2: Period April 24 - April 27\nOvulation: May 5\nFertile window: April 30 - May 6\n\nPeriods per year = 365 / 25 ≈ 15
Result: Next periods: Mar 30, Apr 24, May 19 | Ovulation on Day 11 | ~15 periods/year
Frequently Asked Questions
How long does a normal period last?
A normal period (menstruation) lasts between 3 and 7 days, with the average being 5 days. Bleeding is typically heaviest during the first 2-3 days and gradually lightens. Normal blood loss during a period is 30-80 mL (about 2-5 tablespoons) total. If you soak through a pad or tampon every hour for several hours, pass clots larger than a quarter, or your period lasts more than 7 days, you may have menorrhagia (heavy menstrual bleeding), which affects about 1 in 5 women. Light periods (fewer than 2 days or very light flow) can also indicate hormonal issues. Period duration and flow can change with age, after pregnancy, with certain medications (especially hormonal contraceptives), and during perimenopause. Keeping a period diary helps you notice changes and provides valuable information for your healthcare provider.
When should I see a doctor about my period?
Consult a healthcare provider if you experience: Absent periods (amenorrhea) — no period for 3+ months without pregnancy; could indicate hormonal imbalances, PCOS, thyroid issues, or premature ovarian failure. Very heavy bleeding — soaking through products every 1-2 hours, periods lasting more than 7 days, or passing large clots. Severe pain — cramps that don't respond to OTC pain relievers or significantly impact daily activities (could indicate endometriosis or fibroids). Irregular cycles — cycles consistently shorter than 21 days or longer than 35 days, or significant variation between cycles. Bleeding between periods — spotting or bleeding outside your normal period. Post-menopausal bleeding — any bleeding after 12 months without a period. Sudden changes — significant changes in flow, duration, or symptoms from your normal pattern. These symptoms may indicate conditions like PCOS, endometriosis, fibroids, thyroid disorders, or other treatable conditions.
How accurate are period prediction calculators?
Period prediction calculators provide estimates based on your average cycle length and are most accurate for women with regular cycles that vary by only a few days each month. For someone with a consistent 28-day cycle, predictions can be accurate within one to two days. However, many factors can cause deviations including stress, illness, travel, changes in exercise or diet, and hormonal fluctuations. Studies show that only about 13 percent of women have a perfect 28-day cycle. For the most accurate predictions, track your cycle for at least three to six months to establish your personal average cycle length.
What is the difference between spotting and a period?
Spotting refers to light bleeding or brown discharge that occurs outside of your regular menstrual period. It is typically much lighter than a period, often requiring only a panty liner rather than a pad or tampon. Spotting can occur mid-cycle around ovulation due to the drop in estrogen, during early pregnancy as implantation bleeding, as a side effect of hormonal contraceptives, or due to cervical irritation. A period involves heavier bleeding that lasts three to seven days and follows a predictable cyclical pattern. If you experience frequent or unexplained spotting between periods, it is worth discussing with your healthcare provider to rule out underlying conditions.
Is my data stored or sent to a server?
No. All calculations run entirely in your browser using JavaScript. No data you enter is ever transmitted to any server or stored anywhere. Your inputs remain completely private.
Can I share or bookmark my calculation?
You can bookmark the calculator page in your browser. Many calculators also display a shareable result summary you can copy. The page URL stays the same so returning to it will bring you back to the same tool.