About this tool
The Period Calculator predicts your upcoming cycle from the first day of your last period, your average cycle length and how long your period lasts. Every date is worked out live in your browser and nothing is saved or sent anywhere.
Each future period start is your last start plus a whole number of cycles (last + cycle, + 2× cycle, + 3× cycle). Estimated ovulation is taken as 14 days before your next period, and the fertile window spans from 5 days before ovulation to 1 day after it. All arithmetic is done with UTC day counts, so a daylight-saving change can never shift a predicted date by a day.
This is a general estimate for information only — it is not a contraception method and not medical advice. Real cycles vary from month to month, and factors like stress or health can shift ovulation, so do not rely on it to avoid or achieve pregnancy. Speak with a healthcare professional for anything important.