New on FinoLib · Volume V: Personal Finance
I have just published a new article on FinoLib, my free finance education library: Social Security Tax Wage Base vs Medicare Tax Explained. It is part of Volume V: Personal Finance. It is part 2 of a 2-part guide. Below is a summary of the topic in plain, expert language, so you can follow the idea even before you open the full article.
A closer look
Any row in that table is a reason to rerun the IRS Tax Withholding Estimator with a recent pay stub. Section 9.3 walks through each step of Form W-4 and the safe harbors that decide whether a shortfall costs a penalty.
Under the hood
Under the Hood: Why Social Security Tax Stops and Medicare Tax Does Not
The two FICA taxes fund different promises. Social Security benefits are calculated only on earnings up to the annual wage base, so the 6.2% tax stops there too: $184,500 in 2026. Medicare’s 1.45% has no cap, and a 0.9% Additional Medicare Tax applies to wages above $200,000 for single filers ($250,000 married filing jointly). Employers must begin withholding it once your wages with them pass $200,000 in a year, whatever your filing status. Take a $200,000 salary paid biweekly with no pre-tax premiums: $200,000 ÷ 26 = $7,692.31 a check.
Decision rule
First, contribute at least the percentage that earns the full employer match before taking any extra take-home pay: on this household’s numbers each $1 of take-home given up buys $1.92 of contributions. Then rerun the IRS Tax Withholding Estimator whenever you or a spouse start a second job, your pay changes by a bonus, raise or overtime, or last year’s return ended with a refund larger than one net paycheck or a balance due of $1,000 or more (the level at which the underpayment penalty can start).
The costliest mistake
Two working spouses who leave Step 2 of Form W-4 blank. A married couple earning $75,000 and $60,000 each files a W-4 marked “married filing jointly.” Each employer then subtracts the full $32,200 joint standard deduction and uses the joint brackets as if its paycheck were the household’s only income: ($75,000 − $32,200) taxed at joint rates is $4,640 withheld; ($60,000 − $32,200) is $2,840. Together $7,480. The couple’s actual tax on $135,000 − $32,200 = $102,800 is $12,040, so $4,560 is due in April, above the $1,000 line where an underpayment penalty can apply (9.3).
Edge Cases: When the Standard Answer Changes
The worked example assumes one job, one salary, the same check every two weeks and no state tax. Each of these breaks one of those assumptions: Situation What changes Why Number or rule Two jobs at once Each employer withholds as if its job were the only one Each applies the full standard deduction and the lowest brackets Single filer, $75,000 + $25,000 (no pre-tax deductions): $7,670 + $890 withheld against $13,170 owed, $4,610 short; Step 2 of Form W-4…
Recap
A paycheck moves through four stages: gross pay, pre-tax deductions, taxes withheld, and net pay. On the running $75,000 salary paid biweekly, gross of $2,884.62 loses $173.08 to a 6% traditional 401(k), $100.00 to a cafeteria-plan health premium, $213.02 to FICA and $234.92 to federal income tax, leaving $2,163.60, or 75.0% of gross, in a state with no income tax.
More from the article
■Two jobs at once — Each employer withholds as if its job were the only one — Each applies the full standard deduction and the lowest brackets
■A bonus or commission check — Withheld at a flat rate or by the aggregate method — Supplemental-wage rules
■Overtime — Withheld as wages, FICA included; part of it may be deductible on the return — The 2025–2028 overtime deduction lowers income tax only
■Pay above $184,500 — Social Security withholding stops for the rest of the year — The wage base caps the tax
■A year with 27 biweekly paydays — If your employer spreads salary over 27 checks, each is smaller — Some calendar years hold 27 paydays on a two-week cycle
■Semimonthly pay — 24 checks of $3,125.00 instead of 26 of $2,884.62 — Pay dates fixed to the 15th and month-end
■A wage garnishment — A court order takes part of disposable pay — Consumer Credit Protection Act, Title III
■A state income tax — Another withholding line appears — Most states tax wages
What the full article covers
The full article includes a look under the hood at the mechanics, a clear decision rule, the one mistake that costs people the most, a recap, a reveal-on-click quiz and the official sources behind every figure.
Volume V: Personal Finance
Read the full article on FinoLib
The complete article, Social Security Tax Wage Base vs Medicare Tax Explained, with every worked example, diagram, quiz and source, is free to read in Volume V, Personal Finance on finolib.com.
Explore the Finance Education Series: Vol. II — Capital Markets · Vol. I — Global Economy · Vol. III — Finance Encyclopedia · Vol. IV — Finance Careers · Vol. V — Personal Finance