Rhythm Method Calculator (Calendar Method, Ogino-Knaus)
Use your shortest and longest cycle to find your fertile days with the calendar rhythm method (shortest − 18, longest − 11), the dates for this cycle and whether the Standard Days Method fits you.
How it works
First fertile day = shortest cycle − 18 · Last fertile day = longest cycle − 11 · Standard Days Method (cycles 26–32 days only): days 8–19
The calendar rhythm method (Ogino-Knaus) assumes ovulation happens 12–16 days before the next period and adds the 2–3 days sperm can survive, so it uses your shortest recorded cycle to find the first possibly fertile day and your longest cycle to find the last. Record at least 6 (ideally 12) cycles first. The simpler Standard Days Method marks days 8–19 as fertile but only works if every cycle lasts 26–32 days. With typical use, calendar-based methods fail in roughly 12–24 of every 100 women a year, so combine them with temperature and cervical-mucus signs or another method if avoiding pregnancy matters.
Worked example
Your shortest cycle was 26 days and your longest 31, and your last period started on October 1, 2026: first fertile day = 26 − 18 = 8, last = 31 − 11 = 20, so your fertile days are Day 8 to day 20 — October 8, 2026 to October 20, 2026, 13 days in total. Because all cycles fall within 26–32 days, the Standard Days Method also fits.
Sources: WHO & Johns Hopkins CCP (2018) Family Planning: A Global Handbook for Providers — Fertility awareness methods · NHS — Natural family planning (fertility awareness)
Frequently asked questions
How does the rhythm method work?
Track the length of at least 6 cycles. Subtract 18 from the shortest to get the first fertile day and 11 from the longest to get the last fertile day. Day 1 is the first day of full bleeding. Avoid unprotected sex — or use a barrier method — on all days in between.
How effective is the rhythm method?
Used perfectly, calendar methods are about 91–95% effective, but with typical use around 12–24 of every 100 users become pregnant in a year. Combining the calendar with basal body temperature and mucus observations (symptothermal method) is more reliable.
Does it work with irregular periods?
Poorly. The more your cycle lengths vary, the longer the fertile window becomes, and an unusually long or short cycle can fall outside the calculation. It is not recommended after childbirth, while breastfeeding, near menopause or after stopping hormonal contraception until cycles are regular.
What is the Standard Days Method?
A simplified calendar method developed by Georgetown University for women whose cycles are always 26–32 days long: days 8 to 19 are treated as fertile. A string of colored CycleBeads helps keep track.
Related calculators
This calculator is for education only and is not a diagnosis or a method of contraception you can rely on by itself. Talk to your doctor, nurse or midwife about your cycle, bleeding or family planning.