Conversions will vary according to the country in which you did your undergradaute work. Your first step should be to contact your undergraduate institution and see whether it has a standard translation. A strong international school will have had many graduates go on to US graduate schools.

You can also try asking the school to which you are applying how they usually go about translating grades. Most large US schools will have reviewed applicants from graduates from all over the world and may have their own resources or recommendations for you.

Finally, I would check the internet. There are a number of sources that claim to translate grades from different academic systems to the US scale. Unfortunately more than a few of these contradict each other, but you may find enough guidance to help you with an approximation.