Solution 34592: Calculating Logs of Bases Other Than 10 on the TI-84 Plus Family of Graphing Calculators.
How do I calculate logs of bases other than 10 on the TI-84 Plus family of graphing calculators?
The logbase( function can be used to perform log calculations with bases other than 10 on the TI-84 Plus family of graphing calculators.
Please Note: The TI-84 Plus and TI-84 Plus Silver Edition require Operating System (OS) v 2.55MP to access the logbase function below. To verify what OS is installed press [2nd] followed by [+] then in the menu that appears select [1:About]. If you do not have this version of OS please upgrade to this version.
Example: Calculate log4160
• Press [ALPHA] [F2]  to access the FUNC menu and insert the logbase( function.
• Input 4 for the base.
• Scroll to the right and input 160 for the log value.
• Press [ENTER] to complete the calculation.
Please see the TI-84 Plus family guidebooks for additional information.