How dividends are actually taxed
Qualified dividends (most payouts from U.S. companies and funds held more than 60 days) are taxed at 0%, 15%, or 20% depending on where they land in your taxable income. Ordinary (non-qualified) dividends, the kind REITs and bond funds mostly pay, are taxed like wages at your marginal rate. High earners add a 3.8% net investment income surtax on top of either kind.
The subtlety is stacking. Qualified dividends sit on top of your other income, so the same $10,000 of dividends can be taxed at 0% for one household and 18.8% for another. The calculator stacks them the way the IRS worksheet does: ordinary income first, ordinary dividends on top of that, qualified dividends last.