Period Calculator
This free period calculator predicts your next three periods, your estimated ovulation day, and your fertile window from two numbers you already know: the first day of your last period and your usual cycle length. Everything runs on this page, so nothing you enter is stored or sent anywhere.
These dates are estimates built from averages. Real cycles shift with stress, sleep, travel, illness, and life in general, so treat them as a weather forecast rather than a train timetable. This calculator is not a contraception method and is not medical advice.
How this period calculator works
A period calculator adds your average cycle length to the first day of your last period to predict when the next one starts. Ovulation is estimated by counting back about 14 days from that predicted start, because the second half of the cycle (the luteal phase) is roughly 14 days long for most people. The fertile window covers the several days before ovulation and the day just after it, since that is when pregnancy is most possible.
That is also why the calculator asks for your cycle length instead of assuming 28 days. According to the NHS overview of periods, regular cycles ranging from about 21 to 35 days are typical, and predictions built on your own number beat predictions built on somebody else's average.
What it can and cannot tell you
It can give you a solid planning estimate: when to expect your period, when PMS week is likely to land, and when your body is heading into its higher-energy stretch. If your cycles are fairly regular, the prediction is usually within a day or two.
It cannot confirm ovulation, diagnose anything, or tell you on which exact day you can or cannot get pregnant. Cycle-math alone is not reliable enough to prevent pregnancy: the NHS covers why in its guide to natural family planning, which requires much more than counting days. If your period is late and you are wondering why, our guide to why your period might be late walks through the common causes besides pregnancy.
What the phases in your result mean
Otty describes the cycle in five phases, each paired with its plain name: Storm (your period), Chill (the follicular phase), Glow (the ovulation window), Zen (the early luteal phase), and Pre-Storm (the PMS window). The calculator shows which one today most likely falls in. If you want the full tour of what each phase feels like, start with our guide to the menstrual cycle phases and moods, then go deeper on the luteal phase and what ovulation actually is.
Signs often arrive before dates do: here is how your body signals that your period is coming.
Period calculator FAQ
How accurate is a period calculator?
For people with fairly regular cycles, predictions usually land within a day or two. The further ahead it predicts, the softer the estimate gets, and stress, travel, illness, or sleep changes can move a real period several days. An app that learns your actual cycle history will always beat a one-off calculation.
Can I use this to avoid pregnancy?
No. Counting days is not a reliable contraception method, because ovulation timing shifts and sperm can survive several days. If you want a fertility-awareness approach, the NHS guide to natural family planning explains what it actually involves, and a doctor or nurse can talk through options.
What counts as a normal cycle length?
The NHS describes regular cycles from about 21 to 35 days as typical, with 28 days being an average rather than a rule. What matters more than the number is your own consistency: a steady 24-day cycle is normal, and so is a steady 33-day cycle.
My period did not arrive on the predicted day. Should I worry?
A few days either side of the prediction is completely normal. If you are sexually active and your period is noticeably late, a pregnancy test gives you a real answer. If lateness keeps happening or your cycle changes suddenly, that pattern is worth mentioning to a doctor.
Is my data stored anywhere?
No. The calculation happens entirely in your browser on this page. Nothing you enter is saved, sent, or seen by us.