download this app called : Poke Genie
just upload ur pokemon screenshot and it will auto calculate for u

or

follow this in-game Appraisal info

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Once you get yourself familiar with the appraisal, you will be quickly analyze your pokemon IV . Usually the first statement prompted already know whether your pokemon is above 80% or not then only you filter it out and check more in detail. So get familiar with those appraisal term will save you a lot of time

first statement - you will know above 80% or not
second - your stat for each att/def/stamina
third - analyze your stat

if 1st = best , 2nd = all stat mentioned, 3rd = best = 100%
if 1st = best , 2nd = 2 stat metioned, 3rd = best = 90-100%
and so on....

the time when you need to measure it in detail when you have to decide whether to power up or not then only you use tools. if during farm, just see appraisal is enough