Straight-Line Distance vs the Route You Actually Drive
The map line and the road you actually take are two different numbers, and only one of them decides your arrival time.
Quick answer: A distance calculator does two jobs: it finds the straight-line gap between two points, and it works out distance from speed and time. Roads do not run straight, so a real drive is normally longer than the map's line. Travel at 55 mph for 2 hours 45 minutes and you have covered 151 miles.
Two numbers describe the same journey and they rarely agree. One is the gap between two points on a map. The other is how far your wheels actually turn. Knowing which one you have been handed saves an argument about arrival times.
Straight line distance
On a flat grid this is Pythagoras and nothing more. From point (2, 3) to point (10, 9) the horizontal gap is 8 and the vertical gap is 6, so the distance is the square root of 64 + 36, which is 10. Surveying, warehouse layouts and school geometry problems all live here.
Over long distances the curve of the earth matters and the calculator switches to a great-circle formula using latitude and longitude. That is why the flight path drawn on a seat-back screen looks like an arc rather than a line.
The same arithmetic measures a diagonal
A room 12 feet by 16 feet has a diagonal of the square root of 144 + 256, which is 20 feet exactly. A plot 30 metres by 40 metres comes out at 50 metres. It is the calculation behind whether a sofa turns a corner and how much fencing a diagonal path swallows, and it is the same one the point-to-point mode runs on coordinates.
Run your own numbers
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Open the Distance CalculatorDistance from speed and time
Distance equals speed multiplied by time, which is easy right up until the units fight each other. 55 mph for 2 hours 45 minutes is 55 × 2.75 = 151.25 miles. The trap is entering 2.45 instead of 2.75. Forty-five minutes is three quarters of an hour, not 0.45 of one.
Run it the other way and 151 miles at 65 mph takes 2.32 hours, which is 2 hours 19 minutes.
Your average speed is always lower than your cruising speed
Say you drive 2 hours 40 minutes at 65 mph and stop for 20 minutes. You covered 173 miles in three hours door to door, an average of 58 mph. Plan a long trip on cruising speed and you will arrive late by roughly the length of your coffee breaks, every single time.
Try it with your figures
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Use the Distance CalculatorUsing the distance calculator
For point-to-point mode you need two coordinate pairs. Any map app will give them: press and hold on a location and it returns decimal degrees, something like 51.5074 and -0.1278 for central London. Paste both pairs in, choose miles or kilometres, and read the result.
For the speed and time mode, enter any two of the three values and the calculator returns the third. Check the unit selector before you trust the answer. Metres per second and miles per hour differ by more than a factor of two, and the calculator will happily compute nonsense if you tell it to.
Common questions
Why is my sat nav's mileage higher than the calculator's? Because roads bend around rivers, fields, towns and one-way systems. Road distance is normally longer than the straight line, sometimes by a little and sometimes by a lot when a mountain range or an estuary sits in the way. Use the straight line for a sense of scale and a routing app for anything you are actually driving.
How do I turn miles into a fuel cost? Divide the distance by your miles per gallon, then multiply by the price. In the US, 151 miles at 32 mpg is 4.7 gallons, and at $3.40 a gallon that is about $16. In the UK the maths runs through imperial gallons: 240 miles at 45 mpg is 5.33 gallons, which is 24.2 litres, and at £1.42 a litre that is roughly £34. The fuel cost calculator does the unit juggling for you.
What is 151 miles in kilometres? One mile is 1.609 kilometres, so 151 miles is 243 km. Worth working out before you set off in a hire car with a speedometer in the other unit.
Does a hill add to the distance? Barely. Climb 1,000 feet over five miles and the slope length is the square root of 52 + 0.1892, because 1,000 feet is 0.189 of a mile. That works out at 5.004 miles, roughly 19 extra feet across the whole climb. Hills cost time and fuel, not mileage, which is why the road sign still quotes the flat figure.
Can I use this for running or cycling? For the distance itself, yes. For anything involving a target time, a pace calculator is the better tool, because it works in minutes per mile rather than miles per hour and handles splits.