# Reference — Filing US Business Taxes
# Contents
- Form and deadline master table (tax year 2026)
- Penalty table
- Estimated-tax worksheet pattern
- Payroll filing calendar
- Common elections
- Gotchas
| Filing |
Who |
Due |
Extension (via 7004/4868) |
| Form 1040 + Schedule C + SE |
Sole props, SMLLCs |
April 15 |
Oct 15 (file only) |
| Form 1065 + K-1s |
Partnerships, MMLLCs |
March 15 |
Sept 15 |
| Form 1120-S + K-1s |
S-corps |
March 15 |
Sept 15 |
| Form 1120 |
C-corps (calendar year) |
April 15 |
Oct 15 |
| Form 941 |
Employers, quarterly |
Apr 30 / Jul 31 / Oct 31 / Jan 31 |
— |
| Form 940 (FUTA) |
Employers, annual |
Jan 31 |
— |
| W-2 / W-3 |
Employers |
Jan 31 (to employees and SSA) |
— |
| 1099-NEC |
Payers of contractors ≥$600 |
Jan 31 |
— |
| Estimated taxes (1040-ES / 1120-W) |
Self-employed, corps |
Apr 15 / Jun 15 / Sep 15 / Jan 15 |
— |
Fiscal-year C-corps: return due the 15th day of the 4th month after year-end.
# Penalty table (verify current amounts at irs.gov)
| Failure |
Penalty |
| Late filing (income tax) |
5% of unpaid tax per month, max 25% |
| Late payment |
0.5% per month, max 25% (plus interest) |
| Late 1065/1120-S |
Per-partner/shareholder, per-month flat penalty — applies even with zero tax |
| Missing/late 1099 or W-2 |
Tiered per-form penalty rising with lateness |
| Payroll deposit late |
2–15% of the deposit depending on days late |
Rule of thumb encoded in the skill: filing on time without paying costs 10× less than not filing.
# Estimated-tax worksheet pattern
- Project net income for the year (books YTD × remaining months, adjusted for seasonality).
- Compute income tax + SE tax (15.3% up to the Social Security wage base; 2.9% Medicare above; 0.9% additional Medicare over $200k single).
- Subtract withholding (if any W-2 income).
- Divide by 4; adjust remaining vouchers if income shifts mid-year.
- Safe harbor check: payments ≥ 100% (110% high earners) of last year's total tax avoids penalties regardless of this year's outcome.
# Payroll filing calendar (employer with staff)
| When |
What |
| Each pay run |
Withhold federal income tax + employee FICA; accrue employer FICA |
| Semiweekly/monthly |
Deposit withheld taxes (schedule set by lookback period) |
| Quarterly |
Form 941 |
| Jan 31 |
Form 940, W-2s to employees + SSA, 1099-NECs |
# Common elections
| Election |
Form |
Deadline |
| LLC → S-corp taxation |
Form 2553 |
~2.5 months into the tax year it takes effect |
| LLC → C-corp taxation |
Form 8832 |
Effective date within 75 days back / 12 months forward |
# Gotchas
- K-1 timing: partners cannot file accurate 1040s until the 1065/1120-S is done — a late business return cascades into late personal returns.
- Distributions are not payroll: S-corp distributions without reasonable salary invite reclassification + back payroll taxes + penalties.
- Home-state nexus: hiring a remote employee in another state usually creates payroll and possibly income-tax obligations there.
- Q2 estimated payment is June 15 — only two months after Q1, not three; cash-flow plan for it.
- Schedule M-1/M-2: book-tax differences must reconcile on corporate returns; "plug" numbers get flagged.