Español Calculate now

Paycheck diagnostic · tax year 2026 · engine 2026.2.1

Why did my paycheck change?

Enter the paycheck you understood and the one that surprised you. The calculator changes one thing at a time and reports what each change was worth, so the pieces add up to the whole difference — no leftover, no hand-waving.

Two paychecks, side by side

Change what moved · inputs stay in your browser

Previous paycheck

The check you understood.

Current paycheck

The check that surprised you.

Annual tax liability is estimated and spread evenly across pay periods; employer withholding tables can differ. Nothing you type leaves your browser.

Line by line: previous vs current

LinePreviousCurrentChange
Gross pay (annual) $72,000 $76,500 +$4,500
401(k) deferral $2,880 $6,120 +$3,240
Pre-tax §125 deductions $2,470 $3,770 +$1,300
Federal income tax $5,833 $5,824 −$9
Social Security $4,311 $4,509 +$198
Medicare $1,008 $1,055 +$46
Additional Medicare $0 $0 $0
State income tax $1,051 $1,049 −$1
Take-home pay (annual) $54,447 $54,173 −$275
Take-home pay per check $2,094.13 $2,083.56 −$10.57

Taxes are estimated for tax year 2026; 401(k) and §125 lines are money you kept or spent before tax, not tax. Social Security stops at the $184,500 wage base and Additional Medicare withholding starts at $200,000 of wages.

How the attribution works

1. One change at a time

The calculator starts from the previous paycheck and applies the changed inputs in a fixed order — gross pay, pay frequency, filing status, state, 401(k) percentage, pre-tax §125 — running the same tax engine after each step. The difference a step produces is that cause's share.

2. The shares must add up

Because each step starts where the previous one finished, the shares chain together and land exactly on the current paycheck. The panel prints the unexplained remainder; it is $0.00 by design and a test fails the build if it ever is not.

3. Order is fixed so the answer is reproducible

When two changes interact — a raise that also pushes wages past the Social Security wage base, or a move combined with a bigger deferral — the interaction lands on whichever cause is applied later. A different order would split the same total differently, so the order is fixed and published. Thresholds the engine crossed are listed separately instead of being buried inside one line.

Important limits

  • These are planning estimates: the annual liability is spread evenly across pay periods rather than following the IRS Publication 15-T tables your employer uses each check.
  • Local city and county income taxes, state disability and paid-leave premiums, garnishments, post-tax deductions and imputed income are not modelled.
  • Bonuses, commissions and supplemental wage withholding are not modelled; a one-off check will not match.
  • Credits, itemised deductions and mid-year W-4 changes are not modelled.
  • Results are estimates for planning — not tax, legal or financial advice.

Frequently asked questions

Why is my paycheck smaller after a raise?

Usually because something else changed in the same pay period. A larger 401(k) percentage takes a bigger slice of a bigger salary, a new health plan raises the pre-tax premium, and open-enrolment changes often land on the same check as the raise. This tool applies one change at a time so you can see which one outweighed the raise. A raise by itself never lowers take-home pay: only the income inside a higher bracket is taxed at the higher rate.

How is each cause measured?

The calculator starts from your previous paycheck, changes exactly one input, runs the same tax engine again, and records the difference that step produced. It repeats this in a fixed order — gross pay, pay frequency, filing status, state, 401(k), pre-tax deductions — until the scenario matches your current paycheck. Because the steps chain, the individual amounts add up to the total change to the cent.

Does the order of the steps matter?

Yes, when two changes interact. If a raise also pushes wages past the Social Security wage base, the saving lands on whichever cause is applied later in the walk. The order is fixed and published so the same pair of paychecks always produces the same breakdown, and the interaction is called out separately in the engine notes rather than hidden inside one line.

Is a bigger 401(k) contribution a pay cut?

No. A 401(k) deferral lowers the amount deposited in your bank account, but the money is still yours — it moved into your retirement account before federal income tax. The breakdown labels it as a transfer, not as tax. Note that 401(k) deferrals still pay Social Security and Medicare tax.

Why does switching pay frequency change my check but not my year?

Frequency changes how the same estimated annual take-home is sliced. Biweekly pay means 26 checks a year and semi-monthly means 24, so each semi-monthly check is larger while the annual total is identical, and the calculator shows a per-check change with a zero annual change for that cause. The one exception is a pre-tax §125 deduction: it comes out of every check, so fewer checks a year means less deducted over the year. When that happens the calculator says so on the frequency line instead of claiming the year did not move.

Does this replace my pay stub?

No. These are planning estimates for tax year 2026: the annual liability is spread evenly across pay periods, while your employer withholds using the IRS Publication 15-T tables, your Form W-4 entries and any local taxes. Use this to understand the direction and rough size of a change, then check the figure against your stub.

Sources behind every figure

Engine 2026.2.1, tax year 2026, reviewed September 5, 2026. The full parameter set, the changelog and the known limitations are published on the methodology page. Estimates only — not tax, legal or financial advice.