Weight and balance
Enter each station's weight and arm. This runs the moment arithmetic and checks the result against the envelope you type in from your own aircraft's flight manual.
This is arithmetic, not authority. Every number that defines your envelope comes from your aircraft's own approved flight manual and its current weighing record, and both are specific to that airframe and its installed equipment. A tool on a website is not an approved source and computing a legal loading here does not make a flight legal. Fly the numbers in the book.
The empty weight in the book is usually wrong. Avionics upgrades, added soundproofing, a different interior and thirty years of paint all add weight nobody re-weighed for. If the last weighing record is decades old, the aircraft is almost certainly heavier and its empty CG has moved.
Fuel burn moves the CG. A loading legal at takeoff can leave the envelope as tanks empty, depending on where the tanks sit relative to the CG. Check the landing condition too, not just the takeoff one.
The pre-purchase inspection kit for your aircraft
A pre-buy is signed by an A&P, and what you bring decides which questions get answered. The kit is the logbook checklist, the AD sweep, and the walk-away triggers for the type.
See the inspection kits