# 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 ## Form and deadline master table (tax year 2026 — verify at irs.gov) | 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 1. Project net income for the year (books YTD × remaining months, adjusted for seasonality). 2. 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). 3. Subtract withholding (if any W-2 income). 4. Divide by 4; adjust remaining vouchers if income shifts mid-year. 5. 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.