To use the headphone calculators, you’ll need to know the sensitivity of your headphones, either in dBSPL @ 1 milliWatt (more common) or dBSPL @ 1 Volt. Different manufacturers use different numbers. Sometimes they specify a number without stating which measurement they are using.

You’ll also need the headphone impedance in Ohms. Many people think that Ohms measure how hard headphones are to drive, but this is incorrect. Higher impedance headphones require a higher voltage, but low impedance headphones require more current – actual power. Driving a headphone well requires ample amounts of both.

The desired loudness is not the maximum volume you intend to listen at, but the peak volume that the device you plug your headphones into needs to output. Depending on the music you listen to, even if the average volume you listen at is, say, 80dB, it is possible peaks in the music may go up to 100dB, or louder (if the music has a very large dynamic range, such as some orchestral music).