This post is for educational purposes and is meant to be used as a template or free-to-use ranks for any American WW2 group.
The ranks are only accurate for 1941 and going forward
United States Marine Corps 1941+ Line Enlisted Ranking
Grade 1 (E7 in present day terms) Sergeant Major
Grade 2 (E6) First Sergeant
Grade 3 (E5) Platoon Sergeant
Grade 4 (E4) Sergeant
Grade 5 (E3) Corporal
Grade 6 (E2) Private First Class
Grade 7 (E1) Private
No insignia
United States Army 1920-1942 Line Enlisted Ranking
Grade 1 (E7) Master Sergeant
Grade 2 (E6A) First Sergeant
Grade 2 (E6B) Technical Sergeant
Grade 3 (E5) Staff Sergeant
Grade 4 (E4) Sergeant
Grade 5 (E3) Corporal
Grade 6 (E2) Private First Class
Grade 7 (E1) Private
No insignia
United States Navy 1941+ Seaman Enlisted Ranking
Grade 1 (E7A) Chief Petty Officer
Grade 1 (E7B) Acting Chief Petty Officer
Grade 2 (E6) Petty Officer 1st Class
Grade 3 (E5) Petty Officer 2nd Class
Grade 4 (E4) Petty Officer 3rd Class
Grade 5 (E3) Seaman 1st Class
Grade 6 (E2) Seaman 2nd Class
Grade 7 (E1) Apprentice Seaman
Army & Marine Corps Officer Ranks
O10 General
O9 Lieutenant General
O8 Major General
O7 Brigadier General
O6 Colonel
O5 Lieutenant Colonel
O4 Major
O3 Captain
O2 First Lieutenant
O1 Second Lieutenant
Navy Officer Ranks
O10 Admiral
O9 Vice Admiral
O8 Rear Admiral
O7 Commodore
O6 Captain
O5 Commander
O4 Lieutenant Commander
O3 Lieutenant
O2 Lieutenant Junior Grade
O1 Ensign
Message me on the devforum if you want more ranks from different eras or different countries