ASPEN — The hotels are full, private jets crowd the tarmac and a prime-time dinner reservation is harder to come by than a downtown parking space, but the sure sign Aspen is packed for the holidays isn’t measured by who’s coming in, but what’s going out.

At the city’s wastewater treatment plant, operators know when the town is hopping. They just check the flow meter. As Aspen’s population swells, so does the sewage it produces.

On Christmas Day, the plant measured 1.78 million gallons of sewage coming into its system. Dividing that total by 98 — the standard assumed production of wastewater per person, per day, in gallons — produces a rough estimate of the number of people in town: 18,163. That’s close to triple Aspen’s full-time, year-round population of about 6,300 people, according to the latest census data.

By Wednesday, the town’s population had swelled to 19,795, judging from its sewage output. If recent history is any indication, there will be more than 20,000 people in town for the coming weekend.

Last year, the holiday population peaked on Dec. 29, with 22,142 people, according to the treatment plant’s daily tally of incoming flows. Two years ago, Aspen’s peak holiday population hit 26,222 on Dec. 31.

Granted, calculating the Christmas rush based on sewage isn’t an exact indicator — not everyone who flushes in Aspen spends the night in Aspen — but it’s one way to gauge the number of individuals who have been in town over the course of 24 hours.

Flows into the treatment plant include what is produced by locals and visitors, as well as workers who come the resort each day and leave at night. And, the plant’s service area extends beyond the core of town. It stretches up to Aspen Highlands, and takes in the Aspen Business Center and Red Mountain, for example.

The 98 gallons per day standard reflects an individual’s use of water, including daily showers and toilet flushes, which then becomes wastewater.

janet@aspentimes.com