Enter your headphone's impedance and sensitivity. The calculator tells you how much power your amp needs and whether a typical portable or desktop amp will drive them properly.
The power figure is what your amplifier must deliver at the jack to hit your target peak level. Judge it together with the voltage figure, because most headphones that "need an amp" are voltage-limited, not power-limited: a decent dongle manages roughly 100 mW and 2 Vrms, a real desktop amp several times that. The difficulty rating on the right compares your headphones against those two limits at once.
This is why a 600Ω Beyerdynamic can run happily from a dongle at sane listening levels and still be called "hard to drive" on forums — the forum is quoting numbers for ear-splitting peak levels almost nobody actually uses.
Every figure in the headphone list is stored as dB per milliwatt, because that is what this calculation needs. That sounds like a detail. It is the single most common reason online headphone calculators give wrong answers.
Manufacturers split roughly into two camps. Beyerdynamic, Grado and most Japanese brands quote dB/mW. Sennheiser, AKG, Focal and nearly every Chinese IEM maker quote dB per volt — SPL measured at 1 Vrms instead. The two are not interchangeable, and the gap between them grows as impedance drops. Convert with:
dB/mW = dB/V − 10 × log10(1000 / Z)
Worked example: AKG quotes the K702 at 105 dB and 62Ω. Read as dB/mW, that makes it look easy to drive. It is actually 105 dB per volt, which works out to about 93 dB/mW — roughly 12 dB less sensitive than the spec sheet appears to say, and exactly why the K701/K702 family has a reputation for needing a real amp despite a modest 62Ω rating.
The list here was rebuilt with that conversion applied throughout. Where a figure could not be pinned to a manufacturer sheet or a trustworthy measurement, an approximation from the closest documented variant was used. Anything published as a nominal spec — by anyone, including the manufacturer — is a starting point, not a measurement of the pair on your head: production tolerance alone runs a few dB. Treat the output as a sizing guide, not a verdict.
Voltage swing matters as much as power. High-impedance headphones are current-efficient but voltage-hungry — they need an amplifier with a high voltage rail, which is why many dedicated headphone amps run from ±15V or higher power supplies.