Ovulation and Period Calculator

How ovulation is estimated

The phase after ovu­la­tion usu­al­ly lasts about 14 days, so ovu­la­tion most often hap­pens about 14 days before your next period. Your fer­tile window is the 5 days before ovu­la­tion plus ovu­la­tion day: sperm can live up to 5 days and the egg about a day (Wilcox et al., NEJM, 1995).

For ir­reg­u­lar cycles, the cal­en­dar method is used: first fer­tile day = short­est cycle − 18, last fer­tile day = long­est cycle − 11. The window is wider — which is more honest than a single date.

Not a method of birth control

Cal­en­dar es­ti­mates don’t pre­vent preg­nan­cy: ovu­la­tion can shift with stress, ill­ness, travel, after child­birth and in per­i­menopause. Talk to your OB-GYN about re­li­able con­tra­cep­tion.

When to talk to a doctor

If your cycle is short­er than 21 or longer than 35 days, your pe­ri­ods are un­pre­dictable, last more than 7 days, are very painful or heavy, if your period is late, or if you’ve been trying to con­ceive for a year (six months if you’re over 35). This cal­cu­la­tor doesn’t di­ag­nose any­thing.

Your privacy

Ev­ery­thing is cal­cu­lat­ed in your brows­er. Your dates are never sent or saved any­where — they’re gone when you close the page.