overestimate
🇺🇸
/ˌəʊvərˈestɪmeɪt/
🇬🇧
/ˌəʊvərˈestɪmeɪt/
to think something is bigger, better, or more important than it really is
Don't overestimate how much time you need for the project.
a guess that is too high about size, cost, or value
My guess of 100 people was an overestimate; only 50 came.