The question most checkers actually answer
Type a passport and a destination into visa check software and you get an entry answer: visa-free, visa on arrival, eVisa, or apply in advance. That is one question, and it is the one this site's requirement directory answers across its passport matrix. It is a genuinely useful answer, and it is not the whole trip, because the flight you booked to reach that destination may connect somewhere your passport is not welcome, even for three hours airside.
The second question: the layover
Transit rules are a separate rulebook, hub by hub. Our Transit Visa Checker covers 199 passports across 16 major hubs, built from official-source rules only. Where a hub's rule is conditional, the answer is Depends with the conditions printed, not a confident green light; where a passport and hub pair is not covered, it says so instead of guessing. That honesty is the feature: a checker that guesses wrong at a connection does not cost you a refund, it costs you the trip.
What no checker can promise you
Three limits apply to every tool in this category, ours included. Rules change, sometimes with weeks of notice, so a check run at booking time should be run again before departure. The airline's check-in system is the final referee, and it reads its own database, not ours. And a checker reads rules, not your documents: an expiring passport, a name mismatch or a missing onward ticket fails trips that the rulebook would have allowed. Good software narrows the risk; it does not abolish it.
Run both checks before you pay
Entry first in the directory, transit second in the Transit Visa Checker if your route connects. Two minutes at booking time is the cheapest insurance in travel, and it is free.