What this page answers that a salary calculator does not
A yearly figure tells you what a job is worth. It does not tell you what arrives in your account on the fifteenth. Those are different numbers, and the gap between them is the reason this page exists: the tax is worked out for a whole year, then divided by however many times a year you are paid.
So the first thing to check is not the tax at all. It is the pay cycle. The same salary, taxed identically, arrives as twenty-six modest packets or twelve larger ones, and a budget built on one rhythm falls apart on the other.
Weekly, fortnightly, twice a month, monthly
Four cycles cover almost every employer. Weekly is fifty-two pay days a year and is most common where work is paid by the hour. Fortnightly, every second week, is twenty-six pay days and is the most common arrangement for salaried work in the United States. Twice a month is twenty-four pay days, usually the fifteenth and the last working day. Monthly is twelve and is the norm across most of Europe.
Fortnightly and twice a month look like the same thing and are not. Twenty-six packets against twenty-four means each fortnightly packet is smaller, and it means two months a year carry three pay days instead of two. People who switch employers between the two cycles often think their pay changed. It did not; the arithmetic of the calendar did. The comparison table on this page puts all four side by side so the difference is visible rather than surprising.
What comes out of a packet
The deductions shown here are the ones an employer withholds at source. Which ones apply depends entirely on where you are taxed, and the page lists them by name for the place you picked rather than assuming a single country's payslip.
In the United States that usually means federal income tax withholding, Social Security and Medicare, and state income tax where the state has one; a few states add disability or paid family leave contributions, and a few cities add their own income tax. In Germany it means wage tax, the solidarity surcharge where it still applies, church tax if you are a member, and four separate social insurance contributions. In the United Kingdom it means income tax through PAYE, National Insurance, and student loan repayments if you have a plan running.
What none of them include is anything your employer deducts after tax on your instructions: pension contributions beyond the statutory ones, health premiums, union dues, salary sacrifice arrangements. Those come off the figure shown here, which is why a real payslip is usually a little lower than any calculator.
Why the packets do not add up to the year exactly
Divide a yearly tax bill by twenty-six and you will rarely get a whole number of cents. Every pay day rounds, and twenty-six roundings do not cancel out. This page divides the annual figure and rounds to the nearest cent, which leaves the sum of the packets a cent or two from the yearly column.
Real payroll systems have the same problem and solve it the same way: they let the difference accumulate and settle it in the final run of the year. If your December packet is a few cents off what you expected, that is usually what happened.