Sandy Springs Senior Police Officer Tim Sheffield arrests a burglary suspect along Ga. 400. Sheffield was suspicious of the man’s yellow Mustang leaving an office complex late at night.

In 2008, when Billy Grogan started work as Dunwoody’s police chief, he had no senior staff and no officers. He was the new city’s first police hire.

By the time the department opened its doors less than four months later, Grogan said, the department employed 40 officers.

Why 40? Grogan said he and Dunwoody’s city manager worked out the number based on a variety of factors. They looked at how many officers would be needed to police Dunwoody’s streets. They looked at the city budget.

“It came actually from a lot of different things,” Grogan said recently. “We had the best numbers we could get from DeKalb [County police]. We looked at what we thought the population was. … The budget was a big driver. I would have liked to have started with more. I felt we could provide an acceptable level of service with that staff.”

Since the Dunwoody department opened for business in 2009, it has added more officers. It now employs 46 sworn officers and has a total of 53 employees, including civilians. And Grogan has told Dunwoody City Council he believes his department will need another 18 employees over the next five years.

Deciding how many police officers are needed to effectively patrol a community is not a simple calculation, Grogan and other police officials say.

“It’s just not a magic formula,” said Sandy Springs Police Chief Terry Sult. There’re all kinds of magic formulas out there, but it’s more an art than a science.”

The question of how many officers a community needs for adequate patrols has recently arisen in debates over a proposal to create a new city of Brookhaven in north DeKalb County. Brookhaven voters are to decide July 31 whether to start the new city. If approved by the voters, the new city would start its own police department.

The Carl Vinson Institute of the University of Georgia studied the proposed city and reported a city of Brookhaven would be supported by the tax revenues collected in the area that could be diverted to the city. The study, released last November, projected the new city would hire 53 officers. That calculates out to about 1.07 officers per thousand residents in the area.

Critics of the proposed city say that isn’t enough. They argue the city should have more than 80 officers, based on federal Bureau of Justice Statistics averages.

The bureau reported in a recent study that municipal police departments employed an average of 2.3 officers per 1,000 residents. The lowest ratio – 1.8 officers per thousand – was reported among municipalities with 25,000 to 49,999 residents or 50,000 to 99,999 residents, categories that would include Sandy Springs and Dunwoody and, if it comes into existence, Brookhaven.

The number of officers per thousand residents in a community is often used as a way to compare city police departments. But police officials say it doesn’t work to try to determine how many officers a community should have in its department based simply on a calculation of officers per thousand residents. There are many other factors to consider, they say.

“That’s a starting place,” Sandy Springs Police Chief Sult said.

When the Atlanta police reconfigured its police patrol zones in 2011, officials looked at the number of calls for service each officer answered to determine where officers should be assigned.

“We’re dictated by 911 calls,” said APD Capt. Tim Ewing of Zone 2. “If I’m in a rural area, you could have 10 square miles with a population of 100, and you wouldn’t need the same number of officers as Buckhead… . It is workload that drives the number of officers needed.”

Sult said he wants enough officers on patrol so they have time to initiate their own calls. An officer’s day includes answering calls from 911, filling out paperwork and initiating his or her own calls, Sult said. Sult said he wants his officers to initiate their own calls by making traffic stops or looking to prevent crime about 30 percent of the time.

“You’ve got to have time to prevent crime,” Sult said. “Otherwise, you’re just chasing crime.”

Determining staffing levels requires consideration of a combination of factors in addition to the population, including the type of crime that occurs in a community and where it occurs.

And, as in Dunwoody, on what the city can afford.

“It depends on what level of service people really want [and] how much money they want to spend.” Grogan said. “Ultimately, it’ll be up to the city council.”