My net pay dropped by exactly the amount of a 401(k) increase. Why?

Pre-tax 401(k) reduces gross before tax, but federal tax savings recoup only the marginal-rate portion of the contribution. The rest comes out of net. The math: $100 401(k) increase reduces net by ~$78 (at 22 percent bracket).

How big a difference between paychecks should I worry about?

Small variations (under 5 percent) are usually rounding, partial periods, or differential changes. Larger swings deserve investigation.

How do I diff two pay stubs quickly?

Side by side on screen or paper. Compare the lines in the order they appear: gross, pre-tax, federal tax, FICA, state tax, post-tax, net.

Paycheck Comparison (Why Two Paychecks Differ)

Two paychecks, different totals. The diff almost always comes from one or two specific lines. Here is how to compare them and what each kind of change usually means.

PayslipIQ provides educational information and estimated calculations only. It does not provide tax, legal, financial, accounting, employment, benefits, or payroll advice. PayslipIQ is not a CPA firm, law firm, financial advisor, payroll provider, or tax authority. Always verify your paycheck, deductions, withholdings, and tax position with your employer's payroll department, a qualified CPA, the IRS, your state tax authority, or another appropriately qualified professional. Calculations are estimates; your actual paycheck may differ based on factors specific to your employer, location, benefits elections, and personal tax situation.

How to compare

Open both pay stubs side by side. Compare these in order:

Gross. If different, you worked different hours, received a bonus, or had a partial pay period.
Pre-tax deductions. Different here means a benefits change, 401(k) percentage update, or HSA election shift.
Federal income tax. Different despite same gross means W-4 changed, supplemental wages were paid, or annualization of a big check shifted you to a different bracket projection.
Social Security. Stops mid-year when you cross the wage base. Restarts January 1.
Medicare. Adds 0.9 percent above $200k single in a calendar year (per employer).
State and local tax. Changes if work-state moved, a benefits change altered taxable wages, or state rates updated.
Post-tax deductions. Roth 401(k), garnishments, after-tax insurance.
Net pay. The bottom line.

Common patterns

Net dropped, federal withholding rose, gross unchanged

Probably a W-4 change increased withholding. Or you started year and federal tables refreshed.

Net rose, Social Security disappeared

You crossed the SS wage base. The tax stops for the rest of the calendar year.

Net dropped, pre-tax deductions rose

You bumped 401(k) or started HSA. More to retirement savings, less to bank.

Net dropped, post-tax garnishment line appeared

A garnishment started. You should have received notice from a court or creditor before this kicked in.

Use the comparison tool

Paste your two paychecks into the Pay Stub Explainer for a guided diff. Currently in private beta, request access via contact.

Frequently asked questions

My net pay dropped by exactly the amount of a 401(k) increase. Why?: Pre-tax 401(k) reduces gross before tax, but federal tax savings recoup only the marginal-rate portion of the contribution. The rest comes out of net. The math: $100 401(k) increase reduces net by ~$78 (at 22 percent bracket).
How big a difference between paychecks should I worry about?: Small variations (under 5 percent) are usually rounding, partial periods, or differential changes. Larger swings deserve investigation.
How do I diff two pay stubs quickly?: Side by side on screen or paper. Compare the lines in the order they appear: gross, pre-tax, federal tax, FICA, state tax, post-tax, net.