Booking systems will not sell a connection shorter than a published figure for that airport. The number is calculated from the physical steps a transfer requires, and it varies within a single airport.
The figure covers a defined sequence
Minimum connection time runs from scheduled arrival to scheduled departure and must contain taxi time, disembarkation, the walk between gates and boarding.
Each element is measured rather than estimated, using how long these steps actually take at that airport with typical stand allocations.
The result is the shortest transfer the airport is prepared to have sold, not a comfortable one, and it assumes everything runs on time.
It varies by terminal and by journey type
Airports publish separate values for domestic to domestic, domestic to international, international to domestic and international to international transfers.
International arrivals often require immigration and customs clearance before rechecking bags, which adds substantially to the requirement.
Changing terminals adds a transit train or bus and sometimes a second security screening, so two connections at the same airport can differ by an hour.
Baggage sets the floor as often as passengers do
A bag must be unloaded, sorted, transported and loaded onto the departing aircraft, which has its own timeline independent of how fast the passenger walks.
Where two airlines do not have a baggage agreement, the bag must be collected and rechecked, and the connection time reflects that.
This is why a passenger can make a tight connection while the bag does not, and why bags catch up on a later flight rather than being lost.
The published time assumes normal operations
The calculation uses scheduled times, so any arrival delay comes directly out of the transfer, and there is no slack designed into it.
Remote stands, gate changes and long immigration queues at peak hours all consume time the figure does not include.
Weather and air traffic flow restrictions affect the first leg most, which is why early departures generally connect more reliably than late ones.
Choosing a connection deliberately
Adding an hour beyond the minimum removes most of the risk at a large hub, and rather more than that is sensible where a terminal change is involved.
Connecting at an airline's own hub is usually more robust, since the carrier controls the ground operation and has more onward options if the first leg slips.
The last connection of the day is the one to avoid, because a missed one means an overnight rather than a two-hour wait for the next departure.