Every other calculator on the internet answers “how much power do these headphones need”. This one runs it the other way: given the amplifier you already own, how loud can it actually make them — and is that a number you should be worried about in either direction.
A note on the NIOSH column: 85 dBA for eight hours is an occupational exposure limit, averaged across a working day, with a 3 dB exchange rate — every 3 dB louder halves the time. It assumes continuous A-weighted exposure, and the levels in this table are computed from an unweighted sensitivity figure at 1 kHz, so treat them as an order of magnitude rather than a dosimeter reading. “Below the criterion” means below the level the limit is written against, not certified safe.
Usually not, and the number above shows why — but it depends on the headphone. Most over-ears are rated to take somewhere between 100 mW and a few watts, and at any volume you could tolerate they are dissipating milliwatts. In-ears are the exception: a balanced-armature driver can be rated in single-digit milliwatts, and a desktop amplifier at full output will destroy one long before it reaches the SPL above. An amplifier capable of 120 dB SPL is not a threat to the driver, because you would have to sit at 120 dB to get near its thermal limit, and 120 dB damages your hearing in well under a minute. The fragile component in this system is you.
Headroom is a good thing. A source that reaches 115 dB when you listen at 80 has 35 dB in hand for the peaks in the music, and spends its life loafing. A source that only just reaches 85 dB is working flat out all the time.
This one is false for headphones, and it is worth knowing where it came from. In loudspeakers there is a real mechanism: a clipped waveform has a higher average power than a clean one, the passive crossover routes the extra high-frequency energy to a tweeter rated for a fraction of the system's power, and the tweeter cooks. Rod Elliott's analysis of exactly this puts the failure down to sustained power, not to distortion itself.
Neither half of that transfers. A headphone is a single full-range driver with no crossover to redistribute anything, and an amplifier clipping at any volume you would tolerate is delivering a small fraction of a watt. The real risks are turn-on thumps and DC faults — a completely different failure — and, once again, your ears, because a clipping amplifier is one you have already turned up too far.
One caveat on the number itself: manufacturers' sensitivity figures vary by around ±3 dB between measurement standards, and quoted power ratings are often taken at a voltage the amplifier cannot sustain. Check the voltage row in the table — plenty of “200 mW into 300Ω” claims quietly need 7.7 volts.
Run the same arithmetic across the whole database and the answer is not “will it be loud enough”. Take the median in-ear here by voltage sensitivity, the Thieaudio Oracle MKII at 15 Ω: the median source in this database can drive it to about 126 dB SPL, 233 of the 280 sources can pass 120 dB on it and 102 can pass 130. The theoretical ceiling in the whole database is the SMSL SH-X on the Simgot EA500 at 164 dB SPL.
Those last numbers are arithmetic, not achievement. They assume the amplifier delivers its rated power and the driver accepts it, and no headphone plays 164 dB — it would be destroyed, and so would you. What the figure actually says is that the amplifier stopped being the limit a long way back. For the great majority of pairings on this site the question is not whether there is enough power but whether anything protects you from it: a volume control with usable resolution at the bottom, and some idea of what level you are listening at. How long that level is safe for is its own calculator, and throwing some level away before the amplifier is the fix when the usable range is a sliver.