Professional estimates of straw usage are hard, but not impossible, to find. Yet, the number this fourth grader came up with in 2011, as part of a personal environmental conservation campaign, has proved surprisingly durable, working its way to the heart of the debate over plastic straws.

“It is honestly a little surprising,” said Mr. Cress, who turned 17 on Thursday. “I came up with this statistic because I couldn’t find anything else about it. If there are other statistics on how many straws we use that are based on more rigorous research than the research that I did, I’m happy to embrace those.”

Fact check: The claim that 500 million straws are used by Americans is an estimate above the ranges of more rigorous studies. Market research firms put the figure between 170 million and 390 million per day, or 63 billion to 142 billion straws per year.

One market research firm, Freedonia Group, determined that the nation used about 390 million straws a day last year. Another, Technomic, puts the number closer to 170 million, though that count excludes some types of straws.

But Mr. Cress, who then lived in Burlington, Vt., had access to neither of those figures when he developed his estimate in 2011, when he started “Be Straw Free,” a campaign to persuade restaurants to offer straws optionally rather than automatically.

After failing to find reliable statistics online about straw usage, he decided to call a handful of manufacturers himself.

“The average of those was 500 million,” he said, adding that, being 9, he had not thought to document the process closely. “It’s likely that the number has changed since then, and I would hope that the number has gone down.”