How many killometres would make one billion millimetres?

One billion millimetres is equal to one million kilometres. This is because there are 1,000 millimetres in a metre and 1,000 metres in a kilometre, so you can convert by dividing one billion by 1,000,000 (the number of millimetres in a kilometre). Therefore, 1,000,000,000 mm ÷ 1,000,000 mm/km = 1,000 km.