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Methodology · Tax & Depreciation

Tax & depreciation calculator methodology

Reviewed by · Last reviewed .

1. Overview

The tax-and-depreciation category covers four calculators: quarterly estimated tax (Form 1040-ES safe harbor), payroll tax for a first hire (FICA / FUTA / SUTA / workers' comp), R&D tax credit (IRC §41), and self-employment tax (Schedule SE). Tax math is unforgiving — wrong by 0.5% on a six-figure number is a real five-figure mistake — so this category gets the strictest source discipline.

2. Primary sources

  • IRS Publication 505 — Tax Withholding and Estimated Tax. The publication that governs estimated-tax computation for self-employed filers. Used directly by the quarterly-tax calculator. irs.gov/p505
  • IRS Form 1040-ES. The form (and its instructions) defining the worksheet logic used for quarterly estimated-tax payments and the IRC §6654 safe-harbor tests. irs.gov/form-1040-es
  • IRC §6654 — Failure by Individual to Pay Estimated Income Tax. The Internal Revenue Code section defining the safe-harbor tests (90% of current-year tax OR 100% of prior-year tax — 110% if prior-year AGI > $150K).
  • IRC §164(f) — Deduction for One-Half of Self-Employment Taxes. The Code section authorizing the above-the-line deduction the SE and quarterly-tax calculators apply.
  • Schedule SE (Form 1040). The IRS schedule defining the 92.35% net-earnings calculation, the 12.4% Social Security portion (capped at the wage base), and the 2.9% Medicare portion (uncapped). irs.gov/schedule-se
  • Social Security Administration — Contribution and Benefit Base. Source for the annual SS wage base ($176,100 for 2026). Updated annually in October for the following year. ssa.gov/OACT/COLA/cbb
  • IRC §41 + IRS Form 6765. The Code section and form governing the R&D tax credit, including the small-business payroll-tax election (up to $500K).
  • IRS Publication 946 + IRC §179 / §168(k). Sources for the depreciation calculator inputs the equipment financing tool uses. §179 immediate-expensing limits and §168(k) bonus-depreciation phase-down come straight from the publication.
  • U.S. Department of Labor — FUTA / SUTA. Source for the federal unemployment tax (6.0% on the first $7K of wages, with a 5.4% credit for paying state unemployment timely) used in the payroll-tax calculator.

3. Formula derivations

Self-employment tax (Schedule SE). Net earnings from SE = SE income × 92.35%. Social Security portion = 12.4% on net SE earnings up to the SS wage base ($176,100 for 2026), reduced by W-2 Social Security wages already taxed in the year. Medicare portion = 2.9% on all net SE earnings (uncapped). Additional Medicare Tax = 0.9% on combined wages + SE earnings above the filing-status threshold ($200K single/HoH, $250K MFJ, $125K MFS).

Quarterly estimated tax. Annual federal tax = SE tax + federal income tax on taxable income (AGI − standard deduction − other deductions) at 2026 statutory brackets. Above-the-line deduction = ½ of SE tax (IRC §164(f)). Quarterly payment = (annual federal tax − W-2 withholding) ÷ 4. Safe harbor = no penalty when total payments cover the lesser of 90% of current-year tax OR 100% of prior-year tax (110% if prior-year AGI > $150K).

Payroll tax for a first hire. Employer FICA = 6.2% Social Security (capped at SS wage base) + 1.45% Medicare (uncapped). FUTA = 6.0% × first $7K of wages, less 5.4% credit for paying state unemployment timely (effective 0.6%). SUTA varies by state and experience rating; the calculator uses a state-specific input. Workers' comp varies by classification code and state; calculator takes a percentage input. Benefits load is a configurable percentage of base salary.

R&D tax credit (IRC §41). Two paths surfaced: (a) the regular research credit (20% of qualified research expenses above a base amount); (b) the Alternative Simplified Credit (14% of QREs above 50% of average QRE for the prior three years). Small-business payroll-tax election: qualified small businesses (gross receipts < $5M, in first five years of revenue) can elect to apply up to $500K of R&D credit against payroll tax instead of income tax.

4. Edge cases and assumptions

  • State income tax. Not modeled. The quarterly-tax and SE-tax calculators are federal only. Operators in income-tax states (most of them) need to layer state estimated-tax payments on top. State conformity to federal base varies materially.
  • Multi-state nexus. The payroll-tax calculator assumes a single state of employment. Remote employees create state-nexus issues that affect SUTA, workers' comp, and state income-tax withholding.
  • QBI deduction (§199A). Not modeled in v1. The 20% qualified-business-income deduction for pass-through income is materially complex (specified service trade or business limitations, W-2 wage limitations, taxable income thresholds) and warrants its own calculator.
  • R&D base-amount calculation. For taxpayers with a fixed-base percentage (legacy method), the calculator uses the simplified Alternative Simplified Credit method. The regular method is available; we surface it as an explicit option rather than a default.
  • Benefits and pre-tax deductions. The payroll-tax calculator computes employer-side cost. Employee pre-tax deductions (401(k), §125 cafeteria, HSA) reduce the wage base for FICA but not for federal income tax — the calculator does not enumerate these by default.

5. Update protocol

Tax-and-depreciation calculators are reviewed quarterly and updated immediately on these triggers:

  • The IRS publishes the annual inflation adjustments (Rev. Proc., typically October-November for the following tax year).
  • SSA publishes the annual Contribution and Benefit Base for the following year (typically October).
  • Congress passes a bill changing IRC §41, §179, §168(k), §164(f), or §6654 (any of these triggers an immediate review).
  • The IRS issues new guidance affecting the R&D credit, the §6654 safe harbor, or the Form 1040-ES worksheet.
  • A new IRS publication or a material revision to Pub 505, Pub 946, Schedule SE, or Form 1040-ES.

Material errors are documented on the corrections page.

6. Limitations

These calculators are estimates for educational use. They are not tax advice. Tax math has edge cases the calculators don't model: alternative minimum tax (AMT), passive activity loss limitations, basis tracking, multi-state filing, §1411 net investment income tax, FBAR / FATCA reporting, state-and-local conformity exceptions, prior-year carryforwards, and dozens of others. Operators making real-money decisions on these calculators should consult a CPA or enrolled agent. The tools are designed to make you a better-informed buyer of professional tax advice — not a replacement for it.

7. Reviewer

Reviewed by Byron Malone, Founder, Bedrocka Tools. Operator background includes multi-entity tax structuring, payroll setup for first-hire decisions, and runway modeling that depends on getting estimated tax right. Year 2 plan: regulated-domain calculators add credentialed contributors (CPA / EA / tax attorney) who sign the page in parallel with operator review. Read the full bio at /authors/byron-malone.

8. Last reviewed

. Reviewed against IRS Pub 505 (current), Form 1040-ES (TY 2026 instructions), Schedule SE (TY 2026), Form 6765 (current), Pub 946 (TY 2026), and SSA Contribution and Benefit Base (2026 release). Bedrocka Tools follows documented editorial standards.