Measure Distance on Map

How the Map Distance Measurer works

Why measure distance on map routes yourself

Routing apps tell you how far it is by the route they chose. To measure distance on a map along a path you choose, you need to trace it yourself. Common cases:

• a running loop from the front door before you commit to it
• a canal towpath or bridleway that no car router knows
• a fence line, a hedge or a property boundary, where contractors quote per metre
• a sailing passage between headlands, in nautical miles
• a claim on an expenses form, where most employers pay mileage on the shortest practical route, not the one you happened to drive

Click the points and the page adds up the legs. Nothing leaves your browser except requests for map tiles.

How the distance is calculated

Each leg is a great-circle distance: the shortest path between two points on the surface of a sphere. It is found with the haversine formula, using the Earth's mean radius of 6,371,008.8 metres. The total is the sum of the legs.

The Earth is slightly flattened, its polar radius about 21 km shorter than the equatorial, so a spherical model is off by up to about 0.5% on long lines and usually far less. Survey-grade tools use an ellipsoid such as WGS84 with Vincenty's or Karney's method. For anything clicked by hand, the click is the bigger error. A point placed at city zoom can be 20 m from where you meant, and missing one bend in a lane costs more than the formula does.

Reading the result

• Total distance is the sum of every leg, in the unit chosen above the map. Kilometres show three decimals above 1,000 m and whole metres below. Feet are rounded to the whole foot.
• Kilometres and Miles always appear to three decimals, whatever the unit, so a run can be logged in one and a race entry filled in the other.
• Straight line is the crow-flies distance from the first point to the last. Divide the total by it for a detour ratio: 1.0 is perfectly direct, and 1.2–1.4 is typical for roads. After Close loop it drops to zero, because the route ends where it started.
• Points counts your clicks. Close loop adds one more, a copy of the first.
• The leg list under the map gives each leg's length and the coordinates of its end, to five decimals, about a metre.

Conversions used

A statute mile is exactly 1,609.344 m, set by the 1959 international yard agreement. A nautical mile is exactly 1,852 m, adopted internationally in 1929 and close to one minute of latitude. A foot is exactly 0.3048 m. Common race distances look like this:

Tips for an accurate route

• Click every bend. Legs are straight lines. A winding lane measured end to end can read 30% short. Zoom until individual buildings show, then click each corner. A dozen clicks on a 5 km run usually lands within a few per cent of a GPS watch.
• Use Undo point freely. It removes the last click only, so a misplaced point costs one click, not the whole route.
• Close the loop for laps. Close loop needs at least three points and joins the end back to the start, so a circuit is measured without clicking the start twice.
• Mind the unit. Nautical miles suit sea and air passages, and feet suit short boundary runs. The leg list and headline follow the selector.
• Check elevation separately. This is horizontal distance. A steep hill adds very little length. A 10% gradient adds 0.5%, but it adds a lot of effort.

Taking the route with you

Download GPX saves the points as a GPX 1.1 route, which Garmin Connect, Strava, Komoot, OsmAnd and most sports watches import. It holds only the clicked coordinates, in order, to six decimal places. To open or tidy a file you already have, use the GPX viewer. To plan a finish time from the distance, try the GPX pace calculator. To cost a drive, use the fuel cost calculator. To switch units for a whole list, use km to miles. For a circle of fixed distance around one point, use the radius map.

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