Period Calculator
Predict your next few periods, ovulation and fertile window from your last period.
Results update as you type.
About this calculator
A period calculator predicts when your next several periods are due from the first day of your last period and your average cycle length. Counting forward one cycle length gives each period start; ovulation falls about 14 days before the next period, and the fertile window is the five days before ovulation plus the day itself. It also lays out a schedule of upcoming cycles with their period, ovulation and fertile dates.
On a 28-day cycle, your next period is due 28 days after the last one began, the one after that at 56 days, and so on for as many cycles as you ask it to show. Ovulation for each cycle is placed 14 days before that period start, with the fertile window running the five days before it. Enter a longer or shorter average and every projected date shifts by the same amount.
Use it to know when to expect your period over the coming months, to plan around it, or to see your fertile days alongside each cycle. Set your usual period length so the schedule shows start and end dates. Real cycles vary by a few days month to month with stress, illness and hormones, so the predictions are estimates; a consistently missed period is worth a pregnancy test or a clinician’s advice.
Frequently asked questions
How do you predict when my period will come?
Each period is one cycle length after the previous one, so the tool adds your cycle length (28 days by default) to the first day of your last period, and repeats that to project the next several months.
How accurate is a period prediction?
It assumes regular cycles of a fixed length. Real cycles vary by a few days month to month — and more with stress, illness or hormonal changes — so treat the dates as an estimate rather than a guarantee.
What if my cycles are irregular?
Enter your average cycle length for a best guess, but the prediction will be less reliable. Tracking several cycles or using an ovulation kit gives a clearer picture than a fixed-length calendar estimate.
When will my next period start?
It is one cycle length after your last period began, so on a 28-day cycle your next period is due 28 days after the previous one started. The tool projects several months ahead using the average cycle length you enter.
How do I work out my average cycle length?
Log the first day of each period for a few months, then average the gaps between them. Entering that average gives more accurate predictions than assuming a fixed 28-day cycle.
Can this tool also show my fertile days?
Yes. Alongside each predicted period it lists the estimated ovulation day, about 14 days before the next period, and the six-day fertile window ending on it.
API — use this calculator from code
Call this calculator as a free JSON endpoint — no key required. Send the field values below as query parameters or JSON. Read the full API docs →
Endpoint
GET https://calculator.free/api/v1/period/curl
curl "https://calculator.free/api/v1/period/?cycle=28"JavaScript fetch()
const r = await fetch(
"https://calculator.free/api/v1/period/?" + new URLSearchParams({
"cycle": "28"
}));
const data = await r.json();
console.log(data.results);Results are estimates for general guidance only, not financial, medical or tax advice.