Lee was filling a new sandbox and estimated how many bags of sand he would need. He used 8 bags of sand to fill the sandbox, but his estimate was 25% more than that. How many bags of sand did Lee estimate he would need?

Let \( x \) be the number of bags of sand that Lee estimated he would need.

According to the problem, Lee's estimate was 25% more than the actual amount he used, which was 8 bags. This means we can express the relationship as:

\[ x = 8 + 0.25 \cdot 8 \]

Calculating \( 0.25 \cdot 8 \):

\[ 0.25 \cdot 8 = 2 \]

Now, substituting back into the equation for \( x \):

\[ x = 8 + 2 = 10 \]

Therefore, Lee estimated that he would need

\[ \boxed{10} \]

bags of sand.