Impact supports many different types of units:
- Millimetres
- Centimetres
- Feet
- Inches
- Metres
- Points
You can switch between the metric and Imperial Units by going into Options > Environment > Environment > Measurements and Units... In here you can also change the weight format for use with Standards and Case Compression.
After changing the units all existing geometry is converted.
Useful tip: If you use more than one unit type quite often, why not create separate appearance settings.