Did you make any attempt to solve it yourself?

Googling: Amazon warehouse worker $14/hour. 2080 hours per year = $29K. US Adult population 252 million. Multiply: 7.4 Trillion. US GDP (2017) 19.4 Trillion. Taking only US citizen adults, the figure is still >90% of the above.

Yeah, I could do it myself, I was being sarcastic but maybe no one could tell. One question is always what is the UBI amount. Say 1k for kids, 2k for adults. Wikipedia says 18% are 14 y.o. or under, so say 25% will get 1k, 75% get 2k each month. Use 325 mil as population. 325m * (1/4 * 12k + 3/4 * 24k) = 6.8 trillion so basically the same as you. I want people to say what their vision of the size of UBI funding is. It's ambiguous, kind of like what is brexit?

The other side of it is we are a consumer spending driven economy. If consumers, especially poor people, had more money they'd spend it (unlike say overpaid software engineers like me). We'd get a big boost in the economy. Just like reducing unpayable college loans will free up much more money for the economy.

Instead of thinking about the % of the gdp, think about tax revenue. It's 3.6 trillion a year. Tripling tax overhead to fund this new thing would actually hurt the economy, even I can see that.