Estimate Your Next Period From Your Cycle Length
The calculation runs in your browser. Your dates are not sent to Loopa.
Three steps, and where each one is uncertain
Calendar arithmetic, done in the open. It cannot see ovulation, hormones or anything about your body, only the three numbers you give it.
1. Project the start dates
Each estimated start is your last start date plus your average cycle length, once per cycle.
start(n) = last start + n × cycle length
Cycle length is clamped to 15 to 90 days and period length to 1 to 14.
2. Widen the range
Your regularity choice sets how many days a cycle can move: 0, 2, 5 or 9. That margin is added once per cycle, so the sixth estimate is six times less certain than the first.
range(n) = start(n) ± n × variation
3. Estimate a fertile window, carefully
It counts back 14 days from each estimated start, the usual calendar convention for ovulation, then takes the five days before to the day after, widened by your variation. The six-day span follows Wilcox, Weinberg and Baird (New England Journal of Medicine, 1995).
When cycles vary by more than a week it shows no window at all.
Not contraception. Calendar estimates cannot pinpoint ovulation, because the day it happens varies for any given cycle length. Johnson, Marriott and Zinaman compared cycle apps and calendar methods against hormone-confirmed ovulation in 949 women and reported in Current Medical Research and Opinion (2018) that accuracy was “no better than 21%”. No day this calculator shows is a safe day for unprotected sex.
Methodology and sources last reviewed: September 16, 2026. This is the date the method and its sources were checked, not a medical approval.
The ones worth asking
Is anything I type here sent to Loopa?
No. The calculation runs in your browser, and the page does not send your dates, cycle length or results anywhere. We test for it: an automated check fails if any of them appears in a network request or an analytics event. You can check it yourself by turning your connection off once the page has loaded.
How is a cycle counted?
From the first day of one period to the first day of the next, not from the day your period ends. The first day of bleeding is day 1.
What cycle length is typical?
A wide range is. The FIGO definitions of normal menstrual bleeding put a normal cycle at every 24 to 38 days, lasting 2 to 8 days; other clinical sources use 21 to 35. What matters more is what is typical for you.
How accurate is the fertile window?
Less accurate than most apps suggest. Johnson, Marriott and Zinaman found in Current Medical Research and Opinion (2018) that accuracy was “no better than 21%”, because the day of ovulation varies considerably for any given cycle length. That is why this calculator widens the window when your cycles vary, and hides it when they vary a lot.
Can I use this as birth control?
No. This is a calendar estimate, not a contraceptive method, and it should never be used to prevent pregnancy. If you want contraception, talk to a healthcare professional.
When should I talk to a doctor?
Speak to your healthcare team if your periods stop for more than 90 days and you are not pregnant, become irregular after being regular, last more than eight days, fall outside the 24-to-38-day range the FIGO definitions of normal menstrual bleeding describe as normal, are unusually heavy, or come with severe pain or bleeding between periods.