Question

Units must be consistent. If the unit for p is dollars, the unit for i is dollars and the unit for t is years, what must the units for r be?

Answer 1

Assuming you're concerned with interest rate problems, r is generally involved in something like the formula ...

... i = prt

So, r will have the units of ...

... r = i/(pt) = (dollars)/(dollars·year) = 1/year

That is, r is (some fraction) per year.

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Often, the fraction is expressed as a percentage. The fraction will be unitless (as percentages are), leaving the units of r as year⁻¹.