How is ovulation estimated?

Ovulation is estimated as roughly 14 days before your next predicted period, because the luteal phase after ovulation is fairly consistent at about 14 days. The fertile window covers the 5 days before ovulation through the day itself.

Can I rely on this for contraception?

No. Cycle-based predictions are estimates only and cycles vary month to month. Do not use this tool as a contraceptive method or a replacement for medical advice.

How is my next period date predicted?

It adds your average cycle length to the first day of your last period. With a 28-day cycle, the next period is predicted 28 days after the start of the last one.

What cycle lengths can I enter?

The tracker accepts cycle lengths from 20 to 45 days and period lengths from 1 to 10 days, covering the normal range. Values outside those bounds are clamped to keep predictions sensible.

What is the fertile window?

It is the span when conception is most likely — the five days before ovulation plus ovulation day itself. Sperm can survive several days, so intercourse in this window has the highest chance of meeting the egg.

Why might my real cycle differ from the prediction?

Stress, illness, travel, hormones and many other factors shift cycle length and ovulation timing from month to month. A calendar estimate cannot capture these, so treat it as a guide, not a certainty.

Is my data saved anywhere?

No. Every calculation runs in your browser and nothing is uploaded, stored or shared.

Period & Cycle Tracker

Period & cycle tracker

Enter the first day of your last period and your usual cycle length to predict your next period date, estimated ovulation day and fertile window. It helps with planning, understanding your cycle, and timing for or against conception — as an estimate, not a medical or contraceptive tool.

How it works

The tracker works forward from your last period using simple date arithmetic and the standard luteal-phase assumption:

next period = last period + cycle length
ovulation   = next period − 14 days        (luteal phase ≈ 14 days)
fertile     = ovulation − 5 days  …  ovulation day

Cycle length is clamped to 20–45 days and period length to 1–10 days. Because ovulation is anchored 14 days before the next period, a longer cycle pushes ovulation later.

Example

Last period started on a given date, 28-day cycle, 5-day period:

Next period: last period + 28 days
Ovulation: next period − 14 days (so day 14 of the cycle)
Fertile window: days 9 to 14 of the cycle

Cycle length	Ovulation (cycle day)	Fertile window
28 days	day 14	days 9–14
30 days	day 16	days 11–16
35 days	day 21	days 16–21

Everything is calculated privately in your browser — nothing is stored or uploaded.