This is false, and is an (unfortunately) common misunderstanding about tax brackets generally.

You pay 0% up to $40K, and the 15% starts applying only to income greater than $40K. You absolutely do not pay 15% on the whole thing.

You can verify on the IRS worksheet itself line 11 "This amount is taxed at 0%":

https://apps.irs.gov/app/vita/content/globalmedia/capital_ga...

Here's a blog entry describing:

"Notably, because the 0% long-term capital gains rate only applies until crossing the threshold of... taxable income..., the reality is that the opportunity for 0% capital gains is inherently limited – as with other low tax brackets, it only applies until there’s enough income to cross out of that bracket, and any additional income falls in the next higher bracket."

https://www.kitces.com/blog/understanding-the-mechanics-of-t...

This isn’t correct. The calculation can be found as the worksheet on page 33 of https://www.irs.gov/pub/irs-pdf/i1040gi.pdf — regular income gets taxed first, and then long-term gains after. The 15% bracket only applies to the portion in excessof $40k, not the whole amount (lines 11-20 on the worksheet).

Be careful giving tax advice here, because you're partly wrong and missing key details.

For instance, I'm married and I can make $120k (with no other income) of long term capital gains and pay $2.5k in federal taxes on it.

The cap gains bracket is progressive.

Source:https://turbotax.intuit.com/tax-tools/calculators/taxcaster/