When a provision of the tax code says one thing will be taxed and another thing will not, taxpayers are encouraged to contort their lives to do more of the untaxed thing and less of the taxed thing, paying lawyers and accountants to keep the records along the way.

“In my case, I have lawyers all over the place, I have accounting firms all over the place,” Donald Trump said in August. Like all of his opponents, Mr. Trump says he would simplify the tax code by lowering tax rates and cutting the number of deductions and preferences offered. Unlike his opponents, he says his simplification would be so effective as to “put H&R Block out of business, knock ’em out, put ’em out of business.”

But the candidates have missed the mark in a few ways. One is by conflating tax simplicity with the number of tax rates imposed by the I.R.S.

“Simplify the income tax system to just three individual income tax rates, instead of the current six,” urges Gov. Chris Christie’s tax plan, although the actual number of tax brackets is seven. But having too many tax brackets is not why it’s hard to do taxes.

Once you know what your taxable income is, applying the right tax rates is simple arithmetic that tax software will do for you. If you make less than $100,000, you don’t even need to do arithmetic; you can read your tax liability on the convenient tax tables published by the I.R.S. What make taxes complex are all the rules that determine your taxable income in the first place.

The candidates’ plans would simplify the taxable income calculation, but they have been hesitant to make this calculation too simple for a reason: That would require taking away tax deductions, and people tend to like the deductions they take.

Most Republican candidates, including Mr. Trump, have said their plans to attack tax preferences would not touch two of the biggest and most popular ones: deductions for home mortgage interest and charitable giving.