In luminescence and ESR dating methods, total count rate from thick source alpha counting is commonly used for estimating annual dose with assumption of equal activities for both uranium and thorium decay chains. This is equal to a Th/U weight ratio of 3.2. The systematic error in total dose rate due to uncertainty of the ratio is calculated. It is found that the error is insignificant for uniformly distributed samples such as sediment, but can be significant for some extreme circumstances.