External condoms are 98% effective at preventing pregnancy when used correctly, every single time you have sex. But in real life, people aren’t perfect, so condoms are about 85% effective. That means that for every 100 people who are using condoms as their only form of birth control, 15 of them will get pregnant every year.
This breakdown is a little different for internal condoms. They’re 95% effective when used correctly, but in the real world, they work about 79 out of 100 times.