With that premise, we have two census figures. In 1910, Silver Bow County counted 56,848 people, and in 1920, the number was 60,313. It’s really challenging to get to 100,000 from either of those numbers, but let’s look at some other sources.

The 1917 city directory estimates 91,000 people in Butte, and for 1918 they say 93,000. The city directories make their total population estimates based on a factor of, for Butte in those days, 2.25. The actual count of names in the directories was multiplied by that factor to get the 91,000. Wives and children were not listed separately, so that’s the basis for the factor, and the Polk Directories have a pretty good rationale and justification for the value. Through the 1910s, estimates in the directories climb from 74,000 in 1910 to 86,000 in 1914, 82,000 in 1916, and 91,000 in 1917, and down to 60,000 in 1923, in line with the 1920 census.

There also exists an official US Census estimate for 1917, which lists Butte (city) at 93,981, and Silver Bow County at 36,212. Are we to presume that the county number is everyone outside the city, so the total county population would be 130,193? Based on the statewide total, that’s exactly what it appears we are to believe; the two numbers are separately part of the state total, and one does not include the other.

It’s common to find references in the press of the day to Butte as a city of 90,000. It’s also pretty well established that censuses typically under-counted some populations, such as the Chinese, prostitutes, and transients, which made up a large part of the population in 1917. The census shows no more than 400 or so Chinese in Butte, but Rose Hum Lee estimates as many as 2,500 at the peak. So it is probably fair to see the 1910 and 1920 census counts as noticeably low.

There’s also no doubt that 1917-1918 was the peak of everything in Butte. Copper production records are clear that the metal output was greatest then. Company figures give a total of 14,500 underground mine workers in 1917, on the order of 40% more than in 1910.

Could the population have fallen from 94,000 in 1917 to 60,000 in the 1920 census? It is true that all of Butte’s mines were closed over many months in the late 1910s and early 1920s. The end of World War I meant the demand for copper plummeted along with its price, and many people must have left Butte in the 1918-1922 time frame. But a third of the population, 34,000 people? It’s not impossible, but that many in that short a time seems a stretch. What if the 1920 census is low by as many as 8,000 or so, and maybe the 94,000 census estimate for 1917 is high by that much. Then the exodus would have amounted to 18,000 people — a lot, but not really too unreasonable in the boom-and-bust economy Butte has always suffered from.

The bottom line is, we cannot be sure. There is no proper count of Butte’s people at the peak in 1917-1918. I am comfortable saying Butte reached “close to 100,000” in those years and was the largest city in the region from Minneapolis to Denver to Salt Lake City to Spokane, since Spokane — at 104,000 in 1910 — was almost certainly always larger than Butte. But I could be wrong!