Are tax talbes permissive or mandatory?

The way the statute is written, it uses percentage only, not tables. My guess is the tables are for people who aren't up to using percentages. If you use the schedules, it should be fine.

The 1040 instructions are pretty blunt that "If your taxable income is less than $100,000, you must use ..." so it would be surprising if the IRS computer checks don't validate the return following the same directive. At most, the tax difference could be $12. Whether the IRS computers will consider that 'error' de minimis and accept your return as filed or whether they automatically 'correct' the return (and reduce your refund if one is due) is hard to predict.

Sec. 3(a) of the Internal Revenue Code prescribes the use of the tax tables for anyone who does not itemize deductions. Sec. 3(b) delegates to the Sec'y of the Treasury the authority to prescribe the use of the tax tables for those who itemize. The Sec'y has so authorized.

Therefore, the answer to the question "Is using the tax tables in fact a legal requirement?" is Yes with two exceptions:

1. Someone filing a short period return due to a change in the annual accounting period and
2. Estates and trusts.

Not fine. See my reply to the OP or read Section 3 of the IRC.

The statute trumps any conflicting regulations, and the regulations trump any conflicting information in IRS publications. Under the statute the amount of tax owed is determined strictly by the bracket and percentage. I frequently use the schedules rather than the tables, and have never heard a peep about it from the IRS.

Thanks for that. You're right and I was wrong. Use the tax tables.

Thanks, Alan! That's not the answer I was expecting, so I'm glad I asked.

I do wonder about the rationale for that legal requirement, but then there are plenty of other laws that don't have any (obvious) rationale.

No special insight, but I can think of three reasons: (1) Equal treatment of taxable incomes. This has a nice principled basis and means that you get the same result from the same taxable income. (2) Simplification for taxpayers. You don't need to compute that tax two different ways to see which is more advantageous. Especially for what will be minor dollar amounts. (3) Ease of IRS administration. It's much easier to check the numbers if there is only one way of doing the calculation. It doesn't need to be done both ways to figure out if the value is valid.

I figure #3 is the most likely.