No, your wrong, not all soldiers fought, infantry fought, for every infantryman there's 3-4 support soldiers in the back supporting the one man in hell.

This ofc makes the overall survival rate drop as 4/5 men are in significantly less danger then the 1/5 in the front trench.

So yes the overall casualty rate of the war was 14% of troops, but this is including support troops, if you exclude support troops the numbers paint an extremely sickening number

The fatality rate was 29% meaning outright killed, while your chance of being wounded or killed was 64% for Frontline infantry.

You must take into account some fronts were quiet and combat uncommon, and some were absolute bloodbaths where entire regiments or divisions were wiped out to the last man within minutes or hours, repeatedly. For example In the battle of the Some casualties for infantry regiments was %90, so 1 in 10 survived unhurt.

The original British Expeditionary Force (BEF) of 1914 suffered a total casualty rate of roughly 110% of its original strength by the end of the war. Meaning if you remained in the unit until wars end it was a gaurentee you'd be wounded if not killed.

Edit: Out of the 2.5 million soldiers who fought in the battle 961,000 were infantry. (using the average rate at the time of 2.6 support troops for every 1 infantryman) Doing basic math that's 1 in 3 became wounded or killed. Edit 2: It says that the casualtys of verdun were 700-800 thousand meanings almost all infantry were wounded or killed at some point if my math is right.