While conversion calculators do exist, they're really not necessary- especially for 1/6, which is a relatively easy conversion to do manually.

For ballpark numbers, just remember this: 1 foot in the real world = 2 inches in scale. (So basically, take your number, double it, and replace "feet" with "inches"- a 200 foot hockey rink would be 400 inches long)

For more precise conversions, figure out what unit you want your scaled measurement to be in. In this case, we'll say inches (but pick whatever is best for that project- inches, feet, millimeters, centimeters, etc).

Then, get your source dimensions and convert to that same unit. So 200 x 85 foot real world hockey rink would be 2400 x 1020 in inches.

Then just divide by your scale. 2400/6 = 400 & 1020/6 = 170. So your 1:6 rink would be 400 x 170 inches.