Miles & Kilometres
Convert between miles and kilometres using the approximate relationship (5 miles ≈ 8 km)
What a learner can do afterwards
- Convert 40 miles to approximately 64 km
- Explain why the conversion factor between miles and km is approximately 1.6
- Use the miles↔km relationship to compare a 10 km race with a 6-mile run
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Distance can be measured in two different units: miles and kilometres. Road signs in the UK and the US use miles. Almost everywhere else, and most science, uses kilometres. Since both units measure the same kind of thing, you can convert between them.
Where does the number 1.6 come from? Divide the two sides of the rule: 8 km ÷ 5 miles = 1.6. That is why multiplying a number of miles by 1.6 gives a good estimate in kilometres. The 5-to-8 rule and the times-1.6 rule are really the same idea.
How far is 40 miles in kilometres? Split 40 miles into groups of 5: that makes 8 groups. Each group is about 8 km, so 8 groups make 8 × 8 = 64 km. Forty miles is about 64 kilometres.
When you compare distances given in different units, convert them to the same unit first. A 6-mile run is about 6 × 1.6 = 9.6 km, so it is a little shorter than a 10 km race.
5 miles is about 8 km, so multiply miles by 1.6, or scale in groups of 5, to estimate kilometres.
More to read
9.1 The Metric System
Britain mixes miles and kilometres every day. Road signs show miles, while races are often measured in kilometres. The bridge between them is 1 mile = 1.609 km, rounded to 1.6 for mental maths. Multiplying miles by 1.6 gives kilometres. A 6 mile run is about 9.6 km, just short of a 10 km race.
3.2: Metric Conversion Factors
Conversion factors work beyond the metric system. The factor 1.6 km per mile turns 40 miles into 64 km. Going back divides instead: 64 km split by 1.6 returns 40 miles. A handy shortcut is 8 km = 5 miles, since 5 times 1.6 = 8 exactly. Factors in both directions keep answers consistent.
3 Ways to Convert Knots to Miles Per Hour
Speed units convert like distance units. Knots, miles per hour and kilometres per hour all measure the same thing with different sizes. The habit is always the same: multiply by the factor when moving to the smaller unit. Checking with a rough factor like 1.6 first catches wild answers. Exact factors can follow for the final value.
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