[KFHph]

I've done alot of Googling and looking at threads but every time I try to do the calculation the answer doesn't make sense.
 
My headphones have 30 Ohms resistance and have a sensitivity of 100 dB @ 1mW. Source is an iPhone 6 at 63%. Don't really know the maximum output because it is different depending on different loads which I don't really understand. Any help would be appreciated.

 

[MindsMirror]

The power output of your phone will be different with different loads because the impedance/resistance is part of the equation for power.
P = V^2/R
 
1mW of power deliverd to 30Ohms equals 0.173VRMS for your headphones to reach 100dB. You just need to figure out what the level of your phone's output is.
 
It would be easiest if you had a multimeter to measure a sine wave that is at the same average loudness as your music. If you can't do that, you need to find out what the max level is, and what gain value 63% corresponds to (it's not a linear volume control so it does not mean 63% of the maximum). Here it looks like it is 3Vpp max, which is 1.060VRMS. Then you also need to know the RMS level of your music relative to full scale.
 
Again, it would be much easier to measure it directly with a multimeter.
 
If the output impedance is substantial you will need to consider that as well, though from the page I linked before it looks like it's around 5Ohms which is not significant enough to impact this calculation.

 

[billybob_jcv]

The only thing I need to know is how to turn the volume knob...