The APR (Annual Percentage Rate) was primarily used to calculate over loans that are given out for a long period, or for credit cards or even for mortgages. Because payday loan is strictly used for such a short amount of time (generally 30 days), it's an awkward measurement to use. It makes it seem a lot more expensive than it actually is, since the APR shows what the percentage would be if you paid at the same rates for a year. It is a legal requirement for this rate to be shown on all credit and loan products.
Most lenders provide detailed information about their loan interests (APR) on their websites, so read carefully before you sign anything especially the small letters in the Terms and Conditions of the offered loan deal. Another useful option to calculate how much you have to pay as interest over your loan is the online slider and online calculator that loan providers use widely.