Disclaimer: I'm not a chartered accountant or a licenced tax advisor.

In the US, personal earnings are subject to two categories of tax: (1) payroll (Social Security and Medicare) tax and (2) income tax. These work very differently.

Payroll tax is assessed on the top line of your income (e.g. the $100k) and is a flat rate (15.3% for self-employed individuals). Income tax is assessed on that $100k after itemised and/or standard deductions. Most states also have income tax, and you can deduct what you paid to the state from your taxable income for federal tax purposes and so forth. You can also deduct half of your self-employment tax. Some top-tier cities also have municipal income tax (e.g. SF, NYC). It gets complicated.

So, in a worst-case scenario (the one I ran with for years out of laziness and ignorance), you make $100k, you pay $15,300 of payroll tax on it, then you pay some additional amount of income tax based on a figure lower than $100k, and subject to graduated tax brackets at the state and federal level. The federal tax will unquestionably be higher. But the amount you pay income tax on will be adjusted downward based on tax-deductible factors like mortgage interest, dependents (children) and so forth.

Wage and salary _employees_ do not pay 15.3% in payroll tax. They pay 7.65% and their employer pays the other half.

As a practical matter, what most small business owners smarter than me do is that they choose an incorporation structure which allows them to be employees of that entity, rather than self-employed. An Subchapter S corporation ("S-Corp") is very popular. This allows them to take some portion of that $100k as salary and some of it as a "distribution" (kind of like a dividend), with the latter not subject to payroll tax at all, but only income tax.

There are no definite rules on how much you can take as salary and how much as distribution. Extremes obviously won't fly with the IRS, such as paying yourself a $10k salary and the other $90k in distributions. However, in general, the salary one can get away with paying onesself can be rather generously low, as it usually takes into account the "just starting out" characteristics of the business and is based on the lower end of the spectrum for market-rate compensation in the given field. So, a good accountant will typically set up a salary of something like $30k (+/-) in this hypothetical scenario. Or less if it's a coffee shop, since baristas aren't highly paid. The general rule is to pay yourself the smallest salary you can get away with, since only salary is subject to payroll tax. You'll pay 7.65% payroll tax on that (with the company kicking in the other 7.65%), and just income tax on $100k, graduated and after deductions.

This leads to considerable savings. A number of people I know, as well as myself, pay(paid) far less in income tax than they do in payroll tax, once deduction-maximising strategies are considered. As a hypothetical example, $2300 in payroll tax on the salary, then something like $15k-$20k in income tax.

Actual income tax varies wildly with available personal deductions, but that would be fairly typical for someone married, with children, a lower-earning or non-working spouse, and a house on whose mortgage they pay interest. Single people who don't have many itemised deductions to exploit on the personal side of their tax return are disadvantaged and may pay more (e.g. $25k-$30k). You can get some idea here:

https://taxfoundation.org/2017-tax-brackets/