A traditional 401(k) deferral comes out before federal income tax. It does not come out before Social Security or Medicare. The stub looks like a raise in the FICA column and a cut in the FIT column. Same gross.
Single, $2,000 biweekly, Step 2 off:
- 0% 401(k): FIT $195.85, FICA $153.00, net $1,651.15
- 6% ($120): FIT $181.45, FICA $153.00, net $1,545.55
- 10% ($200): FIT $171.85, FICA $153.00, net $1,475.15
The $120 you deferred saved $14.40 of FIT this check ($195.85 − $181.45). It saved $0.00 of FICA.
Confirm the deferral cap on the 401(k) room calculator. More tools: United States calculators hub.
Why FICA does not move
IRC § 3121 still treats elective deferrals as FICA wages. Only a § 125 cafeteria premium or an HSA contribution (other pre-tax on this tool) shrinks the FICA base.
| 401(k) % | Deferred | FIT wages | FIT | FICA | Net |
|---|---|---|---|---|---|
| 0% | $0 | $2,000 | $195.85 | $153.00 | $1,651.15 |
| 6% | $120 | $1,880 | $181.45 | $153.00 | $1,545.55 |
| 10% | $200 | $1,800 | $171.85 | $153.00 | $1,475.15 |
6% net = $2,000 − $120 − $181.45 − $153.00 = $1,545.55. Annualized at 26 checks: $40,184.30.
net = gross − 401(k) − FIT(gross − 401(k)) − FICA(gross)
Wrong lever, common mix-ups
- Roth 401(k): after-tax. FIT stays $195.85. FICA stays $153. The deposit just drops by the deferral. Roth vs traditional.
- IRA from the checking account: does not change this stub. IRA room.
- Employer match: not on the employee FICA line. Match first.
No. On $2,000 the Medicare piece stays $29.00 (1.45%). Social Security stays $124.00. Total FICA $153.00.
This $52,000 single check sits in the 12% Pub 15-T band. $120 × 12% = $14.40. The engine is not using the 22% supplemental bonus table.
Then employee SS can drop to $0 on later checks. Medicare still runs. FICA wage-base room.
v1 paycheck math is federal + FICA. No state income tax. No Roth box. Type the percent your plan actually defers.