Distance Unit Conversions Explained
Every distance unit is an exact defined constant, so conversions need no rounding at all. The rounded figures most people carry in memory introduce a small error that compounds.
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Try the Distance CalculatorThe International Mile
Exactly 1,609.344 metres, agreed in 1959 when the UK and US adopted a shared definition. The international yard is exactly 0.9144 metres and the foot exactly 0.3048 metres.
The Nautical Mile Is Different
Exactly 1,852 metres, defined as one minute of arc of latitude rather than by any land survey. It is about 1.15 percent longer than a statute mile, and mixing the two is a common error in navigation contexts.
Imperial Within Itself
A foot is 12 inches, a yard is 3 feet and a mile is 5,280 feet. These internal relationships are exact and need no conversion factor at all.
Why Rounding Drifts
The familiar one mile as 1.6 kilometres is a 0.58 percent underestimate. Converting a mile to kilometres and back returns 0.9958 miles, so the error accumulates across every round trip.
Metric Within Itself
A kilometre is 1,000 metres, a metre is 100 centimetres and a centimetre is 10 millimetres. These are decimal throughout, which is why the metric system was designed this way.
Knots
One knot is one nautical mile per hour, so a vessel at 20 knots covers exactly 20 nautical miles each hour. Since a nautical mile tracks the Earth's curvature, that figure works directly against a chart.
Worked Example
One mile converts to exactly 1.609344 kilometres. Using the rounded figure of 1.6 loses 0.009344 kilometres, which is 9.3 metres per mile.
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