The United States has yet to change to the metric system due, in part, to the challenge in converting to another system because the grids of materials will not simply and rationally convert.  There are two methods for converting: soft conversions and hard conversions.

Soft Conversions

Soft conversions from imperial to metric represent an exact conversion where the material dimension has not changed. For instance, 12 inches would have a soft conversion of 304.8 millimeters.  Occasionally, the soft conversion will be rounded so that 12 inches has a soft conversion of 305 millimeters.

Hard Conversions

Hard conversions from imperial to metric are used in the building industry to make the metric units more rational.  For instance, 12 inches would have a hard conversion of 300 millimeters.  In this case, the actual dimension of the material changes so that building materials can easily conform to metric grids.

Imperial to Metric Soft Conversions

LengthAreaVolume
1 inch = 25.4 mm1 sq. in. = 645.16 mm21 cu inch = 16.387 cm3
1 foot = 304.8 mm1 sq ft = 0.093 m21 cu foot = 28316.847 cm3
1 yard = 914.4 mm1 sq yard = 0.836 m21 cu yard = .7646 m2
1 mile = 1.609 km 1 acre = 4046.86 m21 gallon = 3.785 L
1 sq mile = 2.59 km2

Metric to Imperial Soft Conversions

LengthAreaVolume
1 mm = 0.0394 in1 mm2 = 0.00155 sq in1 m3 = 35.3147 cu ft
1 m = 3.281 ft1 m2 = 10.7639 sq ft1 L = 0.264172 gallons
1 km = 0.621 mi.1 hectare = 2.47105 acres
1 km2 = 0.368 sq miles
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