Right-click the map where you want to start measuring, select "Measure distance" and click the map to add new places to your path.
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Google Maps shows distances on the map in kilometres or miles, but you can't switch between metric and imperial units.
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You can adjust the path using drag and drop: drag a point to move it, click a point to remove it.
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There's a "measure distance" card that shows the exact distance. Right-click the map and select "clear measurement" to remove your path or click "x" in the sidebar card.
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The new distance measurement tool is better than the old feature:
* it's a standard feature, it's not hidden in Google Maps Labs
* it's easier to find: use the contextual menu instead of clicking the small ruler icon
* it's more flexible: you can drag any point to move it, click it to remove it
* it shows more information on the map
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{ via +Google Maps }