Conversion factors, Metric to Imperial and vice-versa.

Imperial to Metric.

1 inch = 25.4mm
1 foot = 304.8mm
1 yard = 914.4mm

To convert from inches to millimetres, multiply the number of inches by 25.4.
To convert from feet to millimetres, multiply the number of feet by 304.8.
These conversions are exact.

Metric to Imperial.

1mm = 0.03937 inch approximately (about 40 'thou')
10mm = 1 cm = 0.3937 inch (just under four-tenths of an inch)
100mm = 10cm = 0.1m = 3.937 inches (just under four inches)
1000mm = 1m = 39.37 inches (three feet, three and four tenths inches approximately)

To convert from millimetres to inches, multiply the number of millimetres by 0.03937.
To convert from centimetres to inches, multiply the number of centimetres by 0.3937.
To convert from metres to inches, multiply the number of metres by 39.37.
Although approximate, these conversions are within three parts per million so you can treat them as exact for most practical purposes.

For exact conversions, instead of multiplying the number of inches by 0.03937, divide the number of inches by 25.4 or the number of feet by 304.8. If you have feet and inches, you can convert the feet and inches parts separately, then add the two numbers together.

Always try to estimate the answer you expect before you do the arithmetic, so that if you press a wrong button on the calculator you will know if the answer you get is a lemon.

The correct abbreviations for millimetres, centimetres, metres and kilometres are mm, cm, m and km (all are small letters, no capitals).

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