How long will it take to go from st pancraes to paddington?
Via the tube if there are no delays it’s about a 10 minute ride.
This website is helpful when taking public transport in London. tfl.gov.uk.
Be certain to use proper spelling
( St. Pancras) and be certain you see the London Underground Icon which is a red circle with a thick dark blue going through it. That designates that there is an underground station.
Mind the Gap
Go to the underground station at St Pancras - you want Circle/Hammersmith & City/Metropolitan Lines westbound. Trains from that platform go to several different destinations - only take a train with Hammersmith as final destination, all the others leave the route before reaching Paddington.
The northern part of the Circle can have erratic timekeeping, arising from the junctions at Edgware Road and Baker Street.
The Hammersmith and City platforms @ Paddington are also at the far end of the main line station (indeed it used to have its own name as Bishop's Road).
And if it's the weekend all bets are off. There may be planned engineering works the day you need it.
If you are coming off the Eurostar at St Pancras you need to allow some time more. It will take you time to walk the length of the very long train - unless you are exactly at the right spot - and then there is the long walk of the ramps where you are observed behind one-way glass by Border Control officers all the way down to the frosted glass doors. Then it is a little distance to the tube and down to the platform. And your incoming train may have met with delays.
On the other hand, if it is from Derby the trip may be delayed by the platform reconstruction work there. Most East Midlands trains Derby to London St Pancras are replaced by buses to Nottingham, Beeston or East Midlands Airport where you join a train.
What trip are you trying to plan, and how tight or loose do you like your connections?
It may make sense if it is the Derby trip to consider going via Birmingham New Street depending on where you are going from Paddington.