To calculate rate per 1,000, place the ratio you know on one side of an equation, and place x/1,000 on the other side of the equation. Then, use algebra to solve for "x."
For example, if you know there are 27 murders for every 100,000 people who live in a certain city, start with the equation 27/100,000=x/1,000. Solve the equation by multiplying both sides by 1,000. At that point, you have 27/100=x. That fraction becomes the decimal 0.27, and the ratio is 0.27 murders per 1,000 people.
If you do not have a ratio to start with, you need to create a ratio. For instance, if you are trying to figure out how many property crimes per 1,000 residents occur in your town, find the total number of property crimes and your town's population. If there are 182 property crimes and 142,000 citizens, create the ratio 182/142,000. Then, use the steps explained above to determine the rate per 1,000. In this case, the rate is 128 property crimes per 1,000 residents.
Another way to look at this problem is to divide the bottom number of the original ratio by 1,000. Then, divide that number into the top number of the ratio. The quotient is the rate per 1,000.