Crime victimization risk is usually calculated by dividing the number of crimes by the size of the local population. This procedure does not account for the mobility of the residents. We develop and apply an alternative procedure that divides the number of crimes by the number of people actually present: the ambient population. The ambient population includes commuters, tourists and other visitors. The regular and the alternative crime rate measures of the four largest cities in the Netherlands are compared using data of the ‘AD Misdaadmeter’. We find that ambient populations are larger than residential populations, in particular in Amsterdam. The differences are small however, and the increased effort of calculating ambient populations does not compensate the increased precision in crime measures. For comparisons between annual crimes rates of cities or municipalities, it is sufficient to divide numbers of crimes by populations.