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Use standard measurements except in a quote or in natural history and science articles where metric is appropriate. If metric measurements are used, provide enough conversions to help the reader.

40-mile journey
11-minute walk
measured six feet two inches tall
measured six feet two, measured six two
six-foot-two-inch logger
a 2-by-4 eight feet long
a 15-to-20-foot killer whale
a 15-to-20-foot-long whale 
15-, 20-, and 25-ton whales

six-gram lump of gold
15-foot-6-inch ladder
five hours ten minutes
a three-year-old child
a three-year-old
16-mm film, 16 mph
0.5 cubic meter
three-by-five card or a three-by-five
4 x 6 inch print or 4 x 6 print
Unless the text specifies otherwise, miles are statute not nautical and tons are short, or net, tons (2,000 pounds) not long tons (2,240 pounds) or metric tons (2,204.6 pounds).

Nautical miles are often used in marine or aeronautical context, but specify when using nautical miles. One nautical mile equals 1.15 statute miles or 1.852 kilometers.

A knot is a measurement of speed equivalent to one nautical mile per hour:
            The boat reached a top speed of 20 knots.

See also ABBREVIATIONS, METRIC MEASUREMENTS, mph, nautical mile, knot, NUMBERS, per, ton