Hanoi and Halong Bay are approximately 170 kilometers (106 miles) apart from each other. The travel time between the two places usually takes around 3-4 hours by car or shuttle bus, depending on traffic conditions.
Driving speed on the way from Hanoi to Halong Bay is anywhere from an average of 120-150km/149-187mph for a driving time of close to 5-6 days. Taken into account may be many other factors besides that mentioned, though, such as road conditions and contending traffic along the way.
If you prefer flying, there are flights between Hanoi and Halong Bay that are usually between 1-2 hours in duration, depending on the airline and route. This way, you will get a direct connection and a very fast trip, but you may spend more time traveling depending on delays or stopovers.
The total distance from Hanoi to Halong Bay turns up to about 3,760 km (2,385 miles), which nonetheless is a long distance for 'domestic travel.' Nevertheless, modern transportation makes for an easy and rapid travel to the stunning Bay of Islands, located on a more-than-2,000-km (1,241 miles) ride to the west of Hanoi.