What percentage of US Civil War soldiers were in combat?

There are lots of sites about Civil War casualty rates, but I can’t find the answer to the related question of how many (what percentage) actually saw action.

What was the chance of a being in a battle, in combat, for Civil War soldiers? I assume it was higher for the Confederates. I assume also that for both sides it was higher than in any other conflict involving the US military.

Any estimates?

Thanks,