To calculate an amount, cost, time, or number as lower than it really is.
We underestimated the cost of the repairs.
I underestimated how long the trip would take.
Use overestimate when your number is too high.
Common phrases & grammar
underestimate the cost · underestimate the time needed · underestimate the amount · underestimate the number of
underestimate the cost of + noun
underestimate how long + noun + will take