A Roth is taxed at the other end
Money goes into a Roth after tax and, if the rules are met, comes out with no tax on it at all — not on the contributions and not on the growth. A traditional account does the reverse: a deduction now, income tax on everything later. Every honest comparison of the two comes down to one question, which is whether your tax rate is higher today or on the day you take the money out.
That question has no general answer, and anybody who gives you one is guessing about tax law decades out. What this page can do is show the size of the difference at whatever pair of rates you name, so the guess you are making is explicit rather than buried.
Income can stop you contributing at all
Unlike the traditional account, where income limits only the deduction, a Roth has an income limit on the contribution itself. Above the top of the range you may not pay in directly at all. Inside it, the allowance tapers in proportion to how far through the range you are, with the same two rules the statute attaches everywhere in this area: the reduction is rounded down to the nearest ten dollars, and the result never falls below two hundred dollars until it disappears altogether.
The page tells you which of the three bands you are in and what is left of the allowance. If it is nothing, that is worth knowing before you make a contribution you would then have to withdraw.
The projection then holds that allowance at the same share of the ceiling for every year it runs, which is the same as assuming your income stays where it is relative to the range. It is an assumption rather than a forecast, and it is the only one available: nobody can tell a calculator what they will earn in twenty years. If your income is about to change materially, run the page at both figures.
Why the comparison here is on the same amount paid in
The two accounts are compared on the same contribution, because the allowance is written on the amount that goes in rather than on what it cost you to earn it. On that footing the Roth always comes out ahead, and it is important to see why rather than to treat it as a verdict: a dollar in a Roth is a dollar you keep, while a dollar in a traditional account is a dollar minus whatever tax applies when you withdraw it.
What the Roth costs is the tax the traditional contribution would have deferred today, and the page shows that figure beside the comparison. If you would invest that deferred tax, the gap narrows; if you would spend it, it does not. Working out which is true of you is the actual decision, and it is not one a calculator can make.
The things a straight comparison leaves out
Three of them can matter more than the rate arithmetic. A Roth has no required minimum distributions during the original owner's lifetime, so the money can stay invested rather than being forced out on a schedule. Contributions — though not growth — can generally be withdrawn without tax or penalty, which makes a Roth a far more flexible emergency reserve than a traditional account. And an inherited Roth arrives without the income tax bill attached to an inherited traditional account.
Against that, a deduction now is certain and a tax rule decades out is not. Anybody who tells you the direction of tax rates in thirty years is telling you about their opinions.
What this does not do
It uses one flat rate for today and one for retirement. It does not use a bracket table, does not model state tax, does not know about the five-year rules that govern when a Roth withdrawal counts as qualified, does not model conversions from a traditional account, and does not touch the treatment of earnings withdrawn early. It assumes one steady rate of return throughout.