Pre-Tax vs Post-Tax Deductions: An Employer's Guide

Three weeks after our first employee started, she came to me with her pay stub and a question I could not answer. She had elected a $300 health premium and a $150 Roth contribution. Both were $150 and $300. Both came out of her paycheck. So why, she wanted to know, did one of them seem to cost her less than the other?

It did cost her less. About $104 less per month, in fact. And I did not know why, which is a bad look about twenty days into someone's employment. I told her I would find out, and then I spent an evening learning something I should have known before I ever ran payroll.

This guide covers what that difference actually is: what pre-tax and post-tax deductions are, which is which, how each one moves through the paycheck, the FICA wrinkle that almost nobody explains correctly, the fact that pre-tax deductions save you money and not just the employee, the plan document most small companies do not have and legally need, and what you have to keep on file. Written for the person setting up the deductions, not the person receiving them.

Pre-Tax vs Post-Tax: The Short Answer

Pre-tax deductions are subtracted from gross pay before taxes are calculated. Post-tax deductions are subtracted after. That single ordering difference is the whole thing, and everything else follows from it.

Because a pre-tax deduction reduces the wage figure that taxes are computed on, it reduces the tax. Because a post-tax deduction is taken from money that has already been taxed, it does not. Same dollar amount leaving the paycheck; different effect on take-home pay.

That was the answer to my employee's question. Her $300 pre-tax health premium reduced her taxable wages by $300, so she never paid tax on it. Her $150 Roth contribution came out of money she had already paid tax on. The health premium effectively cost her about $196. The Roth cost her the full $150 and then some, because she paid tax on it first.

What Is a Pre-Tax Deduction?

The practical effect for the employee is that a dollar contributed pre-tax costs them less than a dollar of take-home pay. At a combined 34.65 percent rate (22 percent federal, 7.65 percent FICA, 5 percent state), a $300 pre-tax contribution costs them about $196 in reduced take-home pay. They got $300 of benefit for $196 of paycheck.

The catch, and it is the part most guides skip: not every pre-tax deduction reduces every tax. I will come back to that.

What Is Post-Tax?

Post-tax deductions fall into two very different groups, and it is worth keeping them apart in your head because they create completely different obligations for you as the employer.

Chosen post-tax deductions. The employee elected this: a Roth 401(k), or a disability policy they deliberately want to pay post-tax so the eventual benefit arrives tax-free. Your job is to offer the option and record the election.

Mandatory post-tax deductions. Garnishments and child support orders. The employee did not choose these and neither did you. A court did. Your job is to comply, apply the correct legal limits, and not get it wrong.

Lumping these together is how employers end up treating a court order as if it were an optional benefit election, which is a mistake with legal consequences. The garnishment guide covers the mandatory side in depth.

The Key Differences Between Pre-Tax and Post-Tax

The last two rows are where the tradeoff actually lives. Pre-tax means the tax is deferred, not eliminated: a traditional 401(k) is taxed when the employee withdraws it in retirement. Post-tax means the tax is paid now, and in a Roth, the money and its growth come out tax-free later.

Which is better for any given employee is a personal finance question that depends on their current bracket versus their expected future bracket. You should not be answering it. Offer both, explain the mechanics clearly, and let them decide.

How the Paycheck Order Works

Here is the sequence, with real numbers. This is the picture I wish someone had shown me before I hired anyone.

Read it top to bottom and the logic is unavoidable. The pre-tax deduction changes the number that taxes are computed on. The post-tax deduction does not. Everything else about pre-tax versus post-tax is downstream of that one fact.

This diagram is also the single best tool for explaining deductions to an employee, which is why I will come back to it. The pay stub guide covers how this maps onto what employees actually see.

Which Deductions Are Which

Two entries deserve a flag. Group term life above $50,000 stops being a clean pre-tax benefit and becomes imputed income, which is taxable to the employee even though no cash changed hands. And wage advances require a signed written authorization, with several states imposing stricter requirements than federal law. Deducting a repayment without one is how a routine favor becomes a wage claim. The imputed income guide covers the first case.

The FICA Nuance Nobody Explains

Not all pre-tax deductions are pre-tax for the same taxes, and this is where most guides quietly go wrong.

Section 125 deductions (health premiums, HSA, FSA) reduce wages for federal income tax and FICA. Both employee and employer save.

Traditional 401(k) deferrals reduce wages for federal income tax but not for FICA. Social Security and Medicare tax are withheld on the full amount, including whatever the employee deferred into the plan.

Why this matters in practice: an employee who sees "pre-tax" next to their 401(k) and assumes it means no tax at all will be confused when Social Security and Medicare still come out of it. That is not an error in your payroll. That is how it works, and it is worth saying out loud during onboarding.

Pre-Tax Deductions Save the Employer Money Too

This is the section that does not appear in most articles on this topic, and it is the one that most directly concerns you.

Because Section 125 deductions reduce FICA-taxable wages, they reduce your matching FICA obligation as well as the employee's. You pay 7.65 percent on wages. Lower the wages, lower the 7.65 percent.

Note the asymmetry: this saving applies to Section 125 deductions, not to traditional 401(k) deferrals, because those do not reduce FICA wages. So if you are choosing where to focus, the cafeteria plan is where the employer-side money is.

Section 125: The Document You Probably Do Not Have

To deduct employee health premium contributions on a pre-tax basis, you need a written Section 125 cafeteria plan document, adopted before the plan year it covers. Not a policy in your handbook. An actual plan document.

This is the most common compliance gap I see at small companies, and it is invisible until it is not. Employers deduct premiums pre-tax because their payroll system offers the option and it seems obviously correct, without ever adopting the plan document that authorizes it. The IRS sets out the rules for cafeteria plans and fringe benefit treatment in Publication 15-B, and their cafeteria plan FAQ is the plainest summary of what a plan has to include.

What goes wrong if you skip it: the deductions can be recharacterized as taxable wages, which means back taxes for both parties, a W-2 that was wrong, and the FICA saving you thought you had turns out to be a liability instead. Fixing this before anyone asks is dramatically cheaper than fixing it after.

Limits are per the IRS and change annually. Whatever page you are reading this on, verify against the current IRS figures before you communicate them to employees.

The New Roth Catch-Up Mandate for High Earners

Starting in 2026, under SECURE 2.0, employees age 50 and over whose prior-year FICA wages exceeded $150,000 must make all catch-up contributions on a Roth basis. They no longer have the option of a pre-tax catch-up. This is not a suggestion; it is a mandate.

The threshold is based on Box 3 of the prior year's W-2, meaning Social Security wages, and it is now indexed for inflation.

For a small business, this is a concrete, near-term action item that most owners have not registered: it turns a pre-tax election into a post-tax obligation for a specific group of employees, and it requires a plan feature you may not have.

Garnishments: The Post-Tax Deduction With Legal Limits

Garnishments are post-tax, they are mandatory, and getting the arithmetic wrong exposes you rather than the employee. They come out of disposable earnings, which is what remains after legally required deductions such as taxes.

The critical detail: voluntary deductions like health premiums and 401(k) contributions are not subtracted when calculating disposable earnings. Only legally required deductions are. So an employee with large benefit elections does not get a smaller garnishment because of them.

Per the Department of Labor, for ordinary consumer debts the CCPA caps the weekly garnishment at the lesser of 25 percent of disposable earnings, or the amount by which disposable earnings exceed 30 times the federal minimum wage. At the current $7.25 federal minimum, that floor is $217.50 per week. Child support and alimony orders allow substantially more, up to 50 or 60 percent, with an additional 5 percent for arrears over 12 weeks.

Ordinary consumer debts only. State law may be stricter, and where it is, the smaller garnishment controls. Child support, bankruptcy orders, and tax levies operate under different rules entirely.

What Employers Must Document

The payroll system calculates the deductions. It does not create the paper trail that makes them defensible, and that paper trail is your job.

The first item on that list is the one that catches people. Everything else is good practice. The Section 125 document is a legal prerequisite, and it has to exist before the plan year, not be reconstructed afterward when someone asks.

How to Explain This to Employees

Most deduction questions are not disputes. They are confusion, and confusion costs you a meeting every time it happens. It is far cheaper to prevent.

Where this actually breaks down at a small company is not the explanation. It is that the plan document is in someone's email, the election form is in a drawer, and the person who ran the onboarding conversation left. When an employee asks in month nine what they elected, nobody can find it.

That gap is what I built FirstHR to close. Employee profiles that carry the benefit elections, document management with e-signature so the Section 125 plan document and signed authorizations live where the employee record lives, and onboarding workflows that make the deduction walkthrough a step rather than a thing someone remembers to do. FirstHR does not calculate your payroll. It holds the documentation and the communication around it, which is the part that actually goes missing. The HR document management guide covers what else belongs in that system.

Common Pre-Tax and Post-Tax Deduction Mistakes

The pattern: the errors that cost real money are the ones nobody asks about. Nobody will ever ask whether you have a Section 125 document. They will just quietly take home less than they should, and you will quietly pay more FICA than you need to, for years. The payroll deductions guide covers the full deduction landscape.

Frequently Asked Questions