How effective is spermicide at preventing pregnancy?

If spermicide is used alone, it is only somewhat effective—around 82% when used perfectly and 72% with typical use.

In other words:

Of those women who use spermicide exactly as directed, about 18 in 100 will experience an accidental pregnancy during the first year of using this method.

Of those women who do not use spermicide exactly as directed, about 28 in 100 will experience an accidental pregnancy during the first year of using this method.