Map Units to Scale Ratio
When adding scale to a map, round numbers are important. It's good style, but more importantly, it makes it easier for the user to measure out units on the map.

 1 in = 200 feet good 1 in = 206.9813 feet bad For the paper distance between 0 and 2,000 on this scale bar to equal an inch... ...the map ratio needs to be 1:24,000
It is equally important to make sure your scale bar equals (or is divided into) inches (or whatever reference unit you are using). People will often assume that the first division in a scale bar equals and inch. Having an uneven number on the scale can decrease the accuracy of the information others derive from the map, as well as making the map frustrating to work with. Different GIS programs have different settings, but in ArcMap the best way to ensure a single unit measement is to choose "adjust width" in the properties and make sure your division value is divisible by your ground unit. In the case of "1 in= # feet", this number should be divisible by 12. The calculators below make it easy to adjust map scales on the fly.

1 inch equals...

Feet

1 inch = feet.    1:

Miles

1 inch = miles.    1:

Meters

1 inch = meters.    1:

Kilometers

1 inch = kilometers.    1:

1 centimeter equals...

Feet

1 cm = feet.    1:

Miles

1 cm = miles.    1:

Meters

1 cm = meters.    1:

Kilometers

1 cm = kilometers.    1:

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