<prompt> Human: I have recently taken a greater interest in my finances. Im am examining how one of my credit cards calculates my interest rate. I have read my statement and found my previous balance, my last payment amount and my APR, though cannot understand how they used this information to determine the interest charge that is listed. For the 30 day billing period, if my previous balance was 1000, and on the 15th day I paid 300, how do I figure out what the interest charge will be if the APR is 22.90?  <response>: Interest is calculated daily on the average daily balance and is compounding. So basically your daily interest rate would be .229365. Calculate that by your average daily balance and thats your daily interest charge.Day Beginning BalanceChargesPaymentsAverage Daily BalanceCalculated InterestEnding Balance-------11000.000.000.001000.000.631000.6321000.630.000.001000.630.631001.2631001.260.000.001001.260.631001.8841001.880.000.001001.880.631002.5151002.510.000.001002.510.631003.1461003.140.000.001003.140.631003.7771003.770.000.001003.770.631004.4081004.400.000.001004.400.631005.0391005.030.000.001005.030.631005.66101005.660.000.001005.660.631006.29111006.290.000.001006.290.631006.92121006.920.000.001006.920.631007.55131007.550.000.001007.550.631008.19141008.190.000.001008.190.631008.82151008.820.00300.00708.820.44709.2616709.260.000.00709.260.44709.7117709.710.000.00709.710.45710.1518710.150.000.00710.150.45710.6019710.600.000.00710.600.45711.0520711.050.000.00711.050.45711.4921711.490.000.00711.490.45711.9422711.940.000.00711.940.45712.3823712.380.000.00712.380.45712.8324712.830.000.00712.830.45713.2825713.280.000.00713.280.45713.7326713.730.000.00713.730.45714.1727714.170.000.00714.170.45714.6228714.620.000.00714.620.45715.0729715.070.000.00715.070.45715.5230715.520.000.00715.520.45715.97Your credit card statement and activity wont show you the daily calculations. Itll just package it up for you and present it to you as a single sum on your statement. 