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, so why had the $300 deduction taken noticeably less than $300 out of her take-home pay? Your employees will ask you the same thing, and this guide answers it.

The premium did take 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 starts with what that difference actually is: what pre-tax and post-tax deductions are, which is which, and how each one moves through the paycheck. It is written for the person setting up the deductions, not the person receiving them.

From there the guide covers the Social Security and Medicare (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.

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. So the Roth cost her the full $150, while the health premium cost her about $196 of take-home pay, well under its sticker price.

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.

There is a catch, though: 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. Keep them apart in your head, because each one creates a different obligation for you as the employer.

Chosen post-tax deductions are the ones the employee elected: 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 are 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 Key Differences Between Pre-Tax and Post-Tax

The row about retirement money is where the tradeoff actually lives. There, 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

Every paycheck runs in the same order: gross pay, then pre-tax deductions, then taxes, then after-tax deductions, and finally net pay. Here is that sequence with worked numbers, 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.

Which Deductions Are Which

Pre-tax covers health, dental, and vision premiums under a Section 125 plan, traditional 401(k) and 403(b) deferrals, HSA and FSA contributions, commuter benefits, and group term life premiums paid through a Section 125 plan. Post-tax covers Roth 401(k), garnishments and child support, union dues, insurance the employee elects to pay after tax, charitable payroll giving, and repayment of a wage advance.

Two entries deserve a flag. Group term life coverage above $50,000 stops being a clean pre-tax benefit: its IRS table cost becomes imputed income, which the employee owes tax on without ever receiving it as cash.

Wage advances are the other one. They 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 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 for 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.

Four of those rows answer questions I get asked constantly. A 403(b) deferral works exactly like a traditional 401(k): it reduces income tax but not FICA. Union dues are post-tax. An employee stock purchase plan is funded post-tax, and IRS Topic 427 puts the taxable event at the sale of the shares rather than the purchase. Child support is post-tax.

Are Employer Contributions Taxable to the Employee?

Mostly no. The money you put in on top of wages (the health premiums you cover, your 401(k) match, your HSA contribution) is generally not taxable income to the employee in the year you contribute it, and most of it never touches the payroll tax base at all.

The match is the one that surprises people, because it behaves differently from the employee's own deferral. Their deferral escapes income tax but still pays FICA. Your match escapes both, per the IRS withholding chart for retirement plan contributions. The employee pays income tax on it decades later, when the money comes out of the plan.

Your HSA contribution works the same way. So does a salary reduction the employee routes into their HSA through the cafeteria plan, because IRS Publication 969 treats that as an employer contribution too. Both land in Box 12 under code W, which is why that box generates questions every January.

The exception worth knowing is group term life. Coverage up to $50,000 is a clean tax-free benefit. The cost of coverage above that line becomes imputed income: you add it to wages and withhold Social Security and Medicare on it, even though the employee never sees a dollar of it. Under IRS Publication 15-B, withholding federal income tax on that amount is optional.

Pre-Tax Deductions Save the Employer Money Too

Pre-tax deductions made through a cafeteria plan lower your payroll taxes, not only the employee's. Most articles on this topic leave that out, yet it is the part that concerns you most directly.

Because Section 125 deductions reduce FICA-taxable wages, they reduce your matching FICA obligation as well as the employee's. Per IRS Topic 751, 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 its cafeteria plan FAQ is the plainest summary of what a plan has to include.

What goes wrong if you skip it? Under the proposed Section 125 regulations, a plan with no written document is not a cafeteria plan, so the deductions can be recharacterized as taxable wages, meaning they are treated as ordinary pay that should have been taxed all along.

Recharacterized deductions mean back taxes for both parties and 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.

Several of the most common pre-tax deductions, retirement and health accounts among them, also have annual dollar limits.

The retirement figures come from the IRS announcement of the 2026 contribution limits, the health savings account amounts from Revenue Procedure 2025-19, and the health FSA salary reduction limit from Revenue Procedure 2025-32. Nearly all of them are indexed and move annually, so verify the current numbers before you communicate them to employees.

The HSA figures for 2027 are already published. Revenue Procedure 2026-24 sets the 2027 contribution limits at $4,500 for self-only coverage and $9,000 for family coverage. The $1,000 catch-up at age 55 is fixed in the statute rather than indexed, so it does not change.

The New Roth Catch-Up Mandate for High Earners

Starting in 2026, under the retirement law known as 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. Catch-up contributions are the extra deferrals allowed above the regular limit. For these employees, a pre-tax catch-up is no longer an option, and the Roth treatment is mandatory.

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. The IRS guidance on catch-up contributions states the 2026 figure and confirms the start year.

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 Consumer Credit Protection Act (CCPA) caps the weekly garnishment at the lesser of two amounts: 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 protected 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.

The table covers 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.

The classification is the part that belongs here: a garnishment is post-tax, and no benefit election an employee makes shrinks the base it is measured against. Everything past that point (the answer you owe the court, and the per-period record of what you withheld and remitted) is a separate obligation with its own deadlines, and it is worked through in the guide to garnishment orders.

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. The Section 125 document is a legal prerequisite, and it has to exist before the plan year, not be reconstructed afterward when someone asks.

The workbook below is how you find out where you actually stand. One row per deduction your payroll runs, recording how it is configured today rather than how it should be configured, plus a tab for elections and one for the plan documents. Working through it once usually surfaces at least one deduction classified on the wrong basis or one authorization nobody can produce.

How to Explain This to Employees

Explain deductions before the first paycheck arrives, not after the questions start. 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.

The Total Pretax Contributions Line on a Pay Stub

Most stubs carry a line called total pretax contributions, and it is simply the sum of every pre-tax deduction taken that period. The figure beneath it is what the employee's income taxes were actually calculated on. Health premium, HSA, FSA, and traditional 401(k) all roll into that one number.

The same phrase turns up on retirement plan statements meaning something narrower: only the deferrals made on a pre-tax basis, with Roth money reported separately. When an employee asks why the two numbers disagree, that is the reason. Establish which document they are holding before you try to reconcile anything.

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 keep everything about a person in one place, and document management with e-signature puts the Section 125 plan document, the signed election forms, and the deduction authorizations where the employee record lives. Nobody has to remember which inbox or drawer they went into.

Onboarding workflows make the deduction walkthrough a step every new hire gets, 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.

Common Pre-Tax and Post-Tax Deduction Mistakes

The pattern: the errors that cost real money are the ones nobody asks about. No employee will ever ask whether you have a Section 125 document. If you are running premiums post-tax without one, your employees quietly take home less than they should, and you quietly pay more FICA than you need to, for years.

The deduction audit workbook in the documentation section is the place to start. One pass through it tells you whether each deduction sits on the right side of the tax line and whether the paper behind it exists.

Frequently Asked Questions