An understanding of the measurement units used in Islamic societies in the medieval era would provide an insight to the distances travelled in the middle ages and the volume of trade in their economies. Measurements have always been important tools used in daily life: in trade, in travel, in the use of water, and in construction. It was therefore essential for scientists to devise units of measurement, such as large and small "spans", a "palm length", and the finger, and to use formulas which relate them to each other accurately. This paper attempts to explain how such ancient units of measurement can be converted to metric units, demonstrating many ways in which this knowledge can be used to inform extensive research into medieval Islamic societies. Interestingly, research shows that the large span, which was the base for commercial and social transactions in Islamic countries and was roughly equivalent to 23.1 centimeters, was possibly used in ages after the middle ages.