We are asked to find the value of \( \log_3 81 \), which means we need to determine the exponent to which the base 3 must be raised to produce 81.
Step 1: Write 81 as a power of 3:
\[
81 = 3^4.
\]
Step 2: Thus,
\[
\log_3 81 = \log_3 3^4.
\]
Step 3: Using the logarithmic property \( \log_b (a^n) = n \log_b a \), we get:
\[
\log_3 3^4 = 4.
\]
Thus, the value of \( \log_3 81 \) is \( 4 \).