Period Calculator
Forecast your next periods, ovulation and fertile window from your cycle length — track your menstrual cycle in India.
Enter your last period date to forecast your next periods and ovulation.
Last updated: February 2026
How this calculator is verified
Checked by True Calculator automated test suite on
- Formula verified against a published worked example in the automated test suite
- Edge cases (zero, negative, boundary and unit-mismatch inputs) covered by unit tests
The full verification method is on our how we verify page. Found an error? Tell us and we will re-check it.
When to Use This Calculator
Use the period calculator to forecast upcoming periods so you can plan holidays, travel, exams and busy work weeks around your cycle with less guesswork. It also shows the ovulation day and fertile window between periods, which is valuable both for couples planning a pregnancy and for women who want to understand their monthly rhythm. Many women in India juggle irregular schedules, shift work and travel, where an unexpected period is inconvenient — a forward-looking calendar eases that. Over several months, comparing predictions with reality reveals whether your cycle is regular enough to rely on, and flags when it drifts into the 21-day or 35-day extremes that warrant a gynaecologist's opinion. It is a simple planning tool, not a medical assessment.
How to Use This Calculator
- Step 1: Enter the first day of your most recent period to start the cycle forecast.
- Step 2: Enter your average cycle length (the days between period starts).
- Step 3: Enter how many days your period usually lasts.
- Step 4: Read the next period date, its expected end, and the ovulation window in between.
Worked Example
With a period starting 1 August 2026, a 28-day cycle and a 5-day period, the calculator predicts the next period on 29 August, ending around 2 September. Ovulation falls on 15 August and the fertile window spans 10–16 August. The tool also lists the following two periods: 26 September and 24 October, assuming the cycle stays regular. These are simple calendar additions from the LMP, so a cycle that drifts even a few days will shift every later date accordingly — which is normal biology rather than a flaw in the calculation.
Tips and Common Mistakes
- •Tip 1: Log the start of each period for at least three months and use the average of those gaps as your cycle length.
- •Tip 2: Watch for patterns — stress, travel and exams commonly delay periods by a few days.
- •Tip 3: Use the predicted dates to plan trips, swims and heavy event days around your usual cycle.
- ✗Mistake 1: Confusing the cycle length with the number of days you bleed — a 5-day period on a 28-day cycle is still a 28-day cycle.
- ✗Mistake 2: Assuming one missed or early period is a problem; an occasional off-cycle is common, while persistent irregularity deserves a doctor's check.
Frequently Asked Questions
How does the period calculator predict my next period?
It adds your average cycle length to the first day of your last period. For example, a 28-day cycle starting on 1 August predicts the next period on 29 August.
Why is my predicted date sometimes wrong?
Hormones shift with stress, sleep, travel, illness and exercise, so cycles naturally vary by a few days. The estimate is most reliable when your cycle has been regular over several months.
What does the fertile window shown mean?
It is the six days around the estimated ovulation date (14 days before the next period) when pregnancy is most likely. It helps with both planning and understanding your cycle.
When should I see a doctor about irregular periods?
If your cycles are regularly shorter than 21 days or longer than 35 days, if they stop for three months or more, or if bleeding is unusually heavy or painful, check with a gynaecologist.
Is 'period length' the days of bleeding?
Yes — the number of days bleeding usually lasts, typically 3–7 days. The calculator uses it to show when your next period is expected to end.
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