Kilometres to miles in your head: three quick methods

To turn kilometres into miles in your head, multiply by 5 and divide by 8; it is within 1% of the exact factor, 0.621. Taking 60% is easier but about 3% low. For miles to kilometres, multiply by 8 and divide by 5, or × 1.6. So 100 km is about 62 miles, and 50 miles about 80 km.

Method 1: × 5 ÷ 8

0.625 is very close to the true 0.6214, so this overstates by about 0.6%. It is easiest with numbers that divide by 8:

• 40 km → 40 ÷ 8 × 5 = 25 miles (exact: 24.85)
• 120 km → 15 × 5 = 75 miles (exact: 74.56)
• 16 km → 2 × 5 = 10 miles (exact: 9.94)

Method 2: take 60%, then add a little

Halve the kilometres and add a tenth: 100 km → 50 + 10 = 60 miles. The exact answer is 62.1, so add about 3% if you need more accuracy. This method is quick for round numbers and speeds.

Method 3: the Fibonacci trick

Consecutive Fibonacci numbers — 2, 3, 5, 8, 13, 21, 34, 55, 89 — have a ratio close to 1.618, which is near the 1.609 kilometres in a mile. So each number in miles is roughly the next one in kilometres:

• 3 miles ≈ 5 km (exact 4.83)
• 5 miles ≈ 8 km (exact 8.05)
• 8 miles ≈ 13 km (exact 12.87)
• 13 miles ≈ 21 km (exact 20.92)
• 55 miles ≈ 89 km (exact 88.51)

Other numbers can be split into Fibonacci numbers and added: 20 miles = 13 + 5 + 2 → 21 + 8 + 3 = 32 km.

Which method to use when

• Road signs and route planning: × 5 ÷ 8 or 60% plus a bit — both are close enough to plan a journey.
• Running and cycling: the Fibonacci pairs cover common distances, and race distances are worth memorising (below).
• Anything written down: the exact factor.

Speeds

The same factors work for km/h and mph:

• 30 mph = 48.3 km/h
• 50 km/h = 31.1 mph
• 70 mph = 112.7 km/h
• 100 km/h = 62.1 mph
• 130 km/h = 80.8 mph

Driving abroad, round the limit down rather than up; speed limits are legal maxima, and a few percent of conversion error is not a defence. The Speed Converter gives exact figures in km/h, mph, m/s and knots, and turns running pace into speed.

Race distances

• 5K = 3.11 miles
• 10K = 6.21 miles
• Half marathon = 21.0975 km = 13.1 miles
• Marathon = 42.195 km = 26.2 miles

For pace per kilometre and per mile, see running pace and speed.

Why miles and kilometres are related this way

The kilometre was defined from the size of the Earth — originally, the metre was one ten-millionth of the distance from the equator to the North Pole. The mile comes from Roman paces and later English furlongs. The two were never meant to fit together, which is why the factor is an awkward 1.609344. Since 1959 the inch has been defined as exactly 2.54 cm, which fixes the mile at exactly 1,609.344 metres; the mental methods above are just ways of approximating that number.

When to use the exact figure

Mental methods are for estimates. For anything recorded — mileage claims, distances in a report, training logs — use the exact factors: 1 mile = 1.609344 km and 1 km = 0.621371 miles. The Length Converter uses the exact definitions and also covers nautical miles, which are a different unit (1.852 km) used at sea and in the air.

Mistakes to avoid

• Multiplying by 1.6 when going from km to miles, which makes the answer bigger, not smaller.
• Confusing statute miles with nautical miles in sailing or aviation figures.
• Rounding a speed limit up after converting.

Checklist

• Kilometres → miles: the number gets smaller (× 5 ÷ 8).
• Miles → kilometres: the number gets bigger (× 8 ÷ 5).
• Need precision? Use 1.609344.