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Period Calculator

Predict your next period and upcoming cycle dates for the next 6 months. Enter your last period date and average cycle length for an instant calendar.

Your Cycle Details
First Day of Last Period
Average Cycle Length28 days
days
21 days45 days
Period Duration5 days
days
2 days10 days
Your Result

Fill in the details and
your result appears here.

Next Period
Next Period
Days Until Next Period
Cycle 2
Cycle 3
Cycle 4
Cycle 5 & 6
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Did You Know?
Track 3 cycles for accuracy
Single-cycle predictions can be off. Track 3 months, average the cycle lengths, then use that number. Apps like Flo or Clue do this automatically and improve accuracy with each cycle you log.

How to use this calculator

1

Enter last period start date

The first day of your most recent period — when bleeding began.

2

Set cycle length

Count from day 1 of one period to day 1 of the next. Track 3 cycles and use the average.

3

Set period duration

How many days your period typically lasts. Average is 3–7 days.

The formula explained

Next Period = Last Period Date + Cycle Length

Subsequent cycles are predicted by adding the cycle length repeatedly. The prediction assumes a regular cycle. Actual dates can vary by ±2–5 days depending on stress, illness, travel, and hormonal changes.

Frequently Asked Questions

How accurate is a period calculator?

Accurate within ±2–5 days for regular cycles. Irregular cycles (common with PCOS, thyroid conditions, high stress) make predictions less reliable. Track 3–6 cycles for a better personal average.

What is a normal cycle length?

21–35 days is considered normal. The average is 28 days but this varies significantly between individuals and across life stages. A consistent cycle length, whatever it is, is generally healthy.

Why did my period come early or late?

Stress, illness, travel (especially time zone changes), significant weight changes, intense exercise, and hormonal fluctuations can all shift ovulation timing, which changes the period date.

When should I see a doctor about my cycle?

Cycles consistently shorter than 21 days or longer than 35 days, periods lasting more than 7 days, very heavy bleeding, or sudden significant changes in your pattern are all worth discussing with a gynaecologist.

How accurate are period calculators?

Period calculators are estimates based on average cycle length. Stress, illness, travel, and weight changes can shift periods by several days. Track 3-6 cycles to understand your personal pattern before relying on predictions.

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