How the distance is calculated

"How far is it?" has more than one answer. This page explains the three figures you see on this site and why they differ.

1. Straight-line distance

Also called "as the crow flies" or great-circle distance. It is the shortest path over the surface of the Earth between two points. We calculate it from the latitude and longitude of the two city centres with the haversine formula, taking the Earth as a ball of radius 6,371 km. The result is right to within about half a percent.

Use it to compare how far apart places really are, and for flights, which follow nearly straight paths.

2. Road distance

The length of the fastest route for a car, worked out from the roads in OpenStreetMap by a route planner. We give it between the cities listed on the Cities page. For any other pair of places, the calculator shows a likely range instead: 1.15 to 1.4 times the straight line, which covers most roads in the plains.

• In the plains the road is usually 15 to 30 percent longer than the straight line.
• In the hills it can be much more: Chandigarh to Manali is about 171 km in a straight line and 271 km by road.
• Routes to the north-east are measured through the Siliguri corridor inside India, not across Bangladesh.
• Islands such as the Andamans have no road figure.

3. Driving time

The route planner gives a time with no traffic, tolls, fuel stops or meals. Real trips in India take longer, so we show a range from that figure up to about one third more. For a long trip, also add your breaks: a common rule is 15 minutes every two hours.

4. Flying time

An estimate of the time in the air on a direct flight: half an hour for take-off and landing, plus the straight-line distance at 800 km an hour. It does not include time at the airport, and many pairs of cities have no direct flight.

Why other sites show a different number

• They may measure from a different point in the city (railway station, airport, city centre).
• They may pick the shortest road instead of the fastest.
• New highways and bypasses change routes; our road figures show the date they were worked out on the About page.

Kilometres and miles

1 mile is 1.609 km, and 1 km is 0.621 miles. To turn kilometres into miles quickly, multiply by 5 and divide by 8.