When is my next period?
Answer three questions and you get your next six periods. The calculator runs in your browser. The dates you type never reach us, and you can check that by turning your connection off once the page has loaded.
Why the window moves
A wider fertile window is a more useful one
Most apps draw the same confident six-day band whether your cycles run like clockwork or all over the place. Calendar arithmetic cannot pinpoint ovulation, because the day it happens varies for any given cycle length. Johnson, Marriott and Zinaman tested cycle apps against hormone-confirmed ovulation in 949 women and reported in Current Medical Research and Opinion (2018) that accuracy of ovulation prediction was “no better than 21%”.
So this one gets wider as your cycles vary. Tell it your cycles are very irregular and it shows no fertile window at all, and says why. Drawing a tight ring over two weeks of uncertainty would just be guessing. None of it is contraception, and it should never be used as any.
Questions
The ones worth asking
Is anything I type here sent to Loopa?
No. The calculator runs entirely in your browser. The dates you enter are never sent, never logged and never stored, by us or anyone else. Turn your connection off after the page loads and it still works.
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. There is no single correct number, and 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 compared cycle apps against hormone-confirmed ovulation in 949 women and 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.
Let Loopa keep the calendar
A calculator needs you to remember the date. Loopa remembers it for you, works out your own average, and tells you where you are without being asked. It sits alongside your weight, food and mood, so you can see how they move together. Cycle tracking is opt-in, encrypted to you, and never shared unless you switch it on.
Not medical advice, and not a contraceptive method. See Vitals tracking and progress tracking for what else Loopa records.