Need a formula

If quantity is 1000 and rate is 260 then amount is 260 because the rate is per 1000.
Please note the rate is per 1000, if i put 2000 in quantity and 260 in rate it should automatically double in amount coloumn.
If i put 3000 in quantity and 260 in rate it should automatically come 3 times of 260 in amount coloumn.