In relation to the US:

The Effective Annual Rate (EAR) is the interest rate that is annualized using compound interest, as opposed to using simple interest in the case of the Annual percentage rate (APR). The EAR is the annualized equivalent of interest with shorter compounding periods. It can be calculated from the APR as follows:

$EAR\; =\; (1\; +\; (APR\; /\; m))\; ^\; m\; -\; 1$

or

$EAR\; =\; ((1\; +\; r)^m)\; -\; 1$

where m is the number of times (or periods) interest is compounded during the year and r is the interest rate per period. For example, if interest is compounded monthly, m = 12.

In Canada, interest on mortgages is compounded semi-annually instead of monthly. For Canadian EAR calculations, use m=2.

In relation to the UK:

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Interest rates