Change of rate<span>. If the unit or quantity in respect of which something is changing is not specified, usually the rate is </span>per unit of measurement<span>. However, a rate of change can be specified per unit of </span>time<span>, or per unit of </span>length<span> or </span>mass<span> or another quantity. The most common type of rate is "per unit of time", such as </span>speed<span>, </span>heart rate<span> and </span>flux<span>.</span>