   The absolute minimum would be 20 calories per jumbo prawn of this size, and probably the maximum 40 or more calories if it is very fatty with very little water weight.    30 calories per jumbo prawn for a 1 ounce prawn is a good rule of thumb, so 6 jumbo prawns will have about 180 calories total.   Of course, if you jumbo prawns weigh 2 ounces each, you need to double the calorie count.We are using a 1 ounce per prawn base since that is a common size for so called 'jumbo prawns' or 'jumbo shrimp'.  Of course, if you jumbo prawns weigh 2 ounces each, you need to double the calorie count. We are using a 1 ounce per prawn base since that is a common size for so called 'jumbo prawns' or 'jumbo shrimp'.