Doing a units of measurement program don't know what the formula i need to use to convert the units? Also was wondering how do i get the answer to show at least 3 decimals places im getting to many numbers for my results? Should i declare them as a double or a long?

inch& = 0.0833
rod& = 16.5
yard& = 3
furlong& = 660
meter& = 3.2815
kilometer& = 3281.5
fathom& = 6
miles& = 5280

It is not clear from your question what the "&" is supposed to represent and what units you are converting TO. However, the numbers provide a clue

1 inch = 1/12 = 0.0833 feet
(to three figures)
1 rod = 16.5 feet
1 yard = 3 feet
1 furlong = 660 feet = 1/8 mile
1 meter = 3.2815 feet (to five figures)
1 kilometer = 3281.5 kilofeet
(I've never heard of kilofeet being used)
1 fathom = 6 feet
1 miles = 5280 feet