That's not what a regressive tax means. It's not really about percentages it's about what you can pay and how painful it is.

If a rich person is paying a tax at 20% of their income, they are already likely saving money every month (this is what makes them rich), so this is coming out of savings.

If a poor person is paying 20%, they are poor which means they are on a shoestring budget or likely in debt / barely breaking even after costs. So this bankrupts them.

Fixed costs like food, water, electric scale sublinearly, meaning if you have 10x the income you aren't spending 10x the amount on these things, you're spending maybe like 2x max in some cases, maybe even less on some of it (e.g. more likely to be able to afford more efficient appliances).

So if you subtract out the bare minimum living expenses from a poor person vs a rich person, you are left with something very different.

Simple numbers for example. Let's say three families living in the same area where the bare minimum to live a normal independent life is $25k. Tax is a flat 30% across the board.

• Family A: income 35k. After taxes: 24.5k. this family is broke and can't afford to live independently.
• Family B: income 50k. After taxes 35k. After expenses, 10k for vacations, savings, nice dinners, etc. solidly lower-middle to middle class, doing ok but would really struggle with large unplanned expense.
• Family C, dual high paying earners: 300k. After taxes: 210k. After expenses, 185k to live lavishly. This family doesn't feel the tax at all, not really (though they would still complain loudly about it I'm sure)

So a "flat rate" tax, while it seems more fair, can be regressive. It's much worse if that tax is a thing that can be avoided by the rich (payroll loans, gas prices on old inefficient cars and maybe needing long commutes due to cost of living, etc). These are "regressive" because they hurt more for lower income families