Where a marriage bonus comes from
Marrying changes your tax because a progressive system taxes a household differently from two individuals. Filing jointly pools the two incomes and applies one set of brackets to the total, with thresholds that are usually, but not always, twice the single ones.
When the two incomes are very different, that pooling helps. The higher earner's income is effectively taxed partly in the lower earner's brackets, which are lower. One person on a large salary and one person on a small one usually pay less filing jointly than they would apart, and that difference is the marriage bonus.
When the two incomes are similar, the pooling does nothing useful, and any threshold that is not exactly doubled starts to bite. Two people earning the same amount can find the joint calculation lands them in a bracket neither of them reached alone. That is the marriage penalty.
The single most useful thing to do with this page is to move the two income fields while keeping the total constant. The total tax changes, and watching it change is the clearest picture of how the system treats couples.
What is being compared
Three calculations, from the same rules, differing only in filing status. The first person's income filed alone. The second person's income filed alone. Both incomes filed together. Everything else, allowances, dependants, locality, is held identical across the three, because a comparison in which two things move at once answers nothing.
The figure in the headline is the difference in income tax between the third calculation and the sum of the first two. A positive difference is a penalty; a negative one is a bonus.
Why only income tax moves
A payslip has two kinds of deduction on it, and only one of them can respond to how a couple files.
Income tax is worked out on a return. File jointly and there is one return covering both incomes, with one set of brackets applied to the total. That is the whole mechanism behind a marriage bonus or penalty, and it is what the table's tax row shows.
Payroll contributions are different. Social Security, Medicare, national insurance and their equivalents are assessed on each person's own wages, usually up to a ceiling that applies per person. Two people have two of those ceilings whether or not they are married, and nothing about filing jointly changes either one. That is why the contributions row on this page shows the same figure in all three columns: it is two people's wages, either way.
Adding the two incomes together and running a single calculation over the total would get the first part right and the second badly wrong. It would apply each per-person ceiling once instead of twice, quietly delete thousands of real contributions, and report the missing money as a saving from marrying. On two large salaries that is enough to turn a small penalty into a large bonus, which is to say enough to get the answer backwards.
One contribution that does depend on filing status
There is a single exception to the paragraph above, and it is worth naming because this page does not model it. In the United States the Additional Medicare Tax starts at $200,000 of wages for a single filer and at $250,000 for a couple filing jointly. An employer withholds it above $200,000 whatever you filed, because your employer does not know what your spouse earns; the difference is reconciled on the return.
So the contributions row here follows the employer's rule rather than the return's. For two people earning well over $200,000 each, the return will charge a little more than this page shows. Everything else in that row is genuinely unaffected by how you file.
What this is not
These are payroll withholding calculations for a full year, not filed tax returns. That distinction matters more here than on most pages, because the decisions couples actually face are filing decisions.
So the comparison leaves out everything that only appears at filing: itemised deductions, investment and rental income, credits that phase out on joint income, retirement account limits that change with filing status, and the separate question of filing as married but separately, which in most systems is a worse outcome than either option shown here and exists for reasons other than tax.
It also assumes both people are employees with no other income. Self-employment changes the arithmetic substantially in every system this site covers.
Not every country has this choice
Joint filing is a feature of some tax systems and not others, and the place list on this page only offers the ones where the choice exists in the withholding rules.
The United Kingdom taxes each person separately; there is no joint return, and the marriage allowance is a small transfer of unused personal allowance rather than a joint calculation. Germany does have joint assessment, and the splitting method behind it produces some of the largest marriage bonuses in Europe, but it is reached through tax classes rather than through a filing status on a payroll form, so it needs its own tool rather than a column in this one.