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Cycle risk

Where pregnancy risk actually sits in your cycle

This is the calendar method drawn across one cycle, with the numbers that usually get left off. Educational only. It is not contraception, and it does not protect against infections.

Your dates stay on this device. Nothing you enter here is saved or sent to Flowy.

Also called safe period calculator, rhythm method calculator, unsafe days calculator. Same calculation, one page.

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Where the risk sits in your cycle

The calendar method needs at least six tracked cycles to mean anything. Enter your shortest and longest from that stretch.

First day of your last period

The calculation

How this estimate works

The calendar method, sometimes called the rhythm method, uses your shortest and longest cycle from at least six months of tracking. Subtracting 18 from the shortest gives the first day to treat as fertile, allowing for sperm surviving up to five days before an early ovulation. Subtracting 11 from the longest gives the last, allowing for a late one.

Everything before that window is the pre-ovulatory lower-risk band, and everything after it is the post-ovulatory one. The post-ovulatory band is the more dependable of the two, because by then ovulation has already happened rather than being predicted.

Good to know

Leave room for real life

This page exists because people search for it, and a page that shows the arithmetic alongside the failure rate is more use than one that shows either alone. It is an educational view of where risk sits in one cycle. It is not birth control and should not be used as any.

The method assumes your next cycle behaves like your last six. Illness, stress, travel, a new medication, recently stopping hormonal contraception, and perimenopause all break that assumption without warning you first.

If pregnancy would be unwelcome, use a method whose failure rate you are happy with. If unprotected sex has already happened, emergency contraception is more effective the sooner it is taken, and a pharmacist can supply it.

Common questions

A little more context

How reliable is the calendar method?

Used perfectly and alone, about 5 in 100 people using calendar-based methods become pregnant within a year. With typical use that rises to roughly 12 to 24 in 100. For comparison, an implant or IUD leaves fewer than 1 in 100 pregnant over the same period.

Are the lower-risk days actually safe?

No. Lower risk is not no risk, which is why this tool does not label any day safe. Sperm survive up to five days, ovulation can arrive earlier or later than usual after stress or illness, and a single unexpected ovulation moves every band on the chart.

How are the bands calculated?

By the calendar method. The fertile window starts at your shortest cycle minus 18 and ends at your longest cycle minus 11. Days before and after that window are the lower-risk bands, and the window itself is the high-risk one.

Can I get pregnant during my period?

Yes, particularly with short cycles. If you ovulate on day 10 and bleed for 6 days, sperm from day 6 can still be present. The pre-ovulatory band narrows as cycles shorten, which is exactly why short cycles make this method less usable.

What works better than the calendar method?

Symptothermal methods that combine temperature and cervical mucus perform considerably better than calendar counting alone, and hormonal methods, implants and IUDs perform better again. A clinician or sexual health service can walk through the options.

Does this protect against sexually transmitted infections?

No. No fertility awareness method does. Condoms are the only method on this list that reduces STI transmission.

Keep exploring

Ovulation calculator

Estimate a six-day fertile window and likely ovulation date.

Fertility calculator

Rank every fertile day by its measured chance of conception.

BBT chart

Log your morning temperature and watch a thermal shift appear.