Let’s say your guitar is strung with a set of strings having a total tension of 120 lbs for an average of 20 lbs tension per string. Does that mean that it requires 20 lbs of force for one of your fingers to play a fretted note? No. I hope this is obvious. The string tension listed by some makers is along the length of the string. It takes much less force to press a string down to the fret. But, how much force is required?
The question isn’t something important you must know. It’s just something I was recently thinking about and decided to search for an answer. My search results were unsatisfactory. Most of the web pages didn’t even try to answer the question. A few gave answers from physics with some complex equations. I like math and physics but I was a biologist and when I saw the equations the first time, I was in no mood to calculate forces. I decided I could answer my question through other means. (See String Theory – The Physics of String-Bending and Other Electric Guitar Techniques | PLOS ONE if you want to read about the physics of guitars.)
Being a retired scientist I decided on an experimental approach. Although laboratory equipment and tools would have been nice and more accurate, I tried to think of ways to answer this question with things I found around my house.
The easiest experimental approach I devised was to use a dumbbell across the strings at the 12th fret. If the strings made contact with the fret and produced a clear note, then the weight required to press the strings was less than or equal to the dumbbell’s weight.
I first tried an 8 lb dumbbell on a guitar strung with strings having a total tension of about 140 lbs. All the strings rang clear. That was more than required. Next, I tried a 5 lb dumbbell. It too was sufficient to hold the strings against the fret. But, although I am sure my wife has 3 lb dumbells somewhere in the house, I could not find them. Instead, I found a C-clamp that had flat inside surfaces and could be hung from the fretboard. I weight the clamp on a kitchen scale to determine its weight. When I placed over the 12th fret, the 12.125 oz weight was not enough to move the strings so they made clear notes.
Thus, so far I’ve determined it takes less than 5 lbs/6-strings ~= 0.83 lbs and more than 0.126 lbs to fret a string at the 12th fret on this guitar.
I did find someone who did a rough calculation from the formulas and came up with just under 0.5 lbs force to fret a string at the 12th fret given some assumptions he describes. Empirical measurement of force needed to push string? : Guitar
After sleeping on the problem, I decided I was ready to calculate the fretting forces using the formula given in the previous paragraph. For Thomastik-Infeld JS112 strings, tuned to standard pitch on a 25.5-inch scale-length guitar gave, and with the action at the 12th fret equal to 3 mm, the force required to fret the guitar at the 12th fret was about 0.42 to 0.46 lbs, or to barre all six strings, 2.6 lbs. My action is actually closer to 2 mm but I wanted a worse-case answer.
The total tension of the JS112 set is about 140 lbs. I decided to repeat the calculation for a lighter gauge string and chose D’Addario the NXYL10 Balanced Tension set which has a total tension of about 100 lbs. I assumed a scale length of 24.75 such as a Les Paul or most ES models from Gibson and chose an action of 2 mm. The results are about 0.3 lbs force per string to fret at the 12th fret. The total for all six strings was 1.9 lbs. (Note that I did not correct for the decreased tension for a shorter scale length. Most string makers measure electric guitar string tension at a 25.5-inch scale length. The tension of a 24.75-inch scale is about 95% of that for a 25.5-inch scale length.)
An exact answer to this question is going to depend on the set of strings on your guitar and the setup (action) at the fret you wish to play. Each string in the set will probably have different tensions, and I suspect it will require more force to fret near the nut than at the 12th fret which is half the total scale length. But, I don’t need an exact answer, just a ballpark approximation.
So, roughly between 0.3 and 0.5 lbs of force are required to fret a single string. At most I have to squeeze my hand down with 3 lbs of force to barre all six strings. I can do that.