Calculate Resistors for an Amplifier-to-Headphone Adapter

If you have an amplifier without a dedicated headphone output, you can build an adapter using resistors that gets connected to the speaker output.
Using this tool, you can calculate the resistor values based on your headphones, your amplifier and the speaker load, the attenuation and the damping factor you want to achieve.

Based on the Headphone Resistor Network Calculator by Rob Robinette.