Period calculator
Enter the first day of your last period, your usual cycle length and how long your period lasts. You get the next six start dates, the estimated ovulation day in each cycle, and how many days late you are today, if you are.
Updated October 4, 2026. Sources listed below.
Nothing you enter leaves this page. The result is an estimate for information only, not medical advice.
How the dates are worked out
A period calculator is calendar arithmetic. It takes the first day of your last period and adds your usual cycle length to find the next expected start date, then does the same again for the following cycles. The estimated ovulation day in each cycle is counted back 14 days from the next period, the standard assumption for the luteal phase. If the predicted date has already passed, the calculator shows how many days late the period is by that arithmetic.
The numbers it rests on are averages. In the largest published analysis of app-logged cycles, more than 600,000 of them, the mean cycle length was 29.3 days rather than the textbook 28, and length varied with age and between cycles for the same person. The NHS describes regular cycles anywhere from 21 to 40 days. So a prediction is a centre point, not a deadline.
What it cannot tell you
It cannot tell you why a period is late or early, and it cannot confirm ovulation. Those come from tracking your own cycles over time, and from your provider when something worries you. If you are trying to conceive, the ovulation calculator and the when-to-test calculator continue from here.
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Frequently asked questions
How does the period calculator work?
It adds your usual cycle length to the first day of your last period to predict the next start date, then repeats that for six cycles. Each cycle also shows an estimated ovulation day, counted back 14 days from the following period. It is arithmetic on the dates you enter, not a prediction from your body.
How late is "late"?
Cycle length varies between people and from cycle to cycle. In a study of more than 600,000 cycles the mean length was 29.3 days, and the NHS describes regular cycles anywhere from 21 to 40 days as common. A period a few days past the predicted date is within ordinary variation; the calculator shows the count, and your provider is the person to ask about anything that concerns you.
Can I use this with irregular cycles?
You can, but the dates will be rough. Calendar maths assumes every cycle is the same length. If yours vary a lot, logging each period and tracking ovulation signs gives a better picture than any calculator.
Does the calculator store my dates?
No. Everything is calculated in your browser and nothing is sent anywhere or saved.
Get Ovulation Tracker
Ovulation Tracker logs each period as it happens and adjusts its predictions to your real cycles, with a PCOS mode for irregular ones.
