Methodology

Every constant, its source, and the date it was checked.

Last updated 2026-09-01

This page is generated from the same file the calculators execute. It is not a description of what they do — it is a listing of the values they use. If a number changes in the engine, it changes here, and there is no way for the two to disagree.

How a figure gets here

A tax constant cannot be a bare number in this codebase. Each one carries its label, its source URL, the date it was verified and who verified it, and the build fails without them. That constraint is what makes this page possible.

Tax year 2026, United States federal

Notes on specific figures

Self-employment tax rate

12.4% Social Security + 2.9% Medicare. Statutory, not inflation-adjusted.

Portion of net profit subject to SE tax

IRC 1402(a)(12): net earnings reduced by 7.65% (half of 15.3%) before the rate applies.

Social Security wage base

2026 contribution and benefit base. Medicare has no ceiling.

Additional Medicare Tax rate

Applies to earnings above the filing-status threshold. Not deductible.

Section 199A minimum deduction

OBBBA sec. 70105, new for tax years beginning after 2025-12-31.

Minimum QBI required to claim the deduction

OBBBA sec. 70105, new for tax years beginning after 2025-12-31.

AGI above which the 110% safe harbor applies

$75,000 if married filing separately.

Standard mileage rate, 1 Jan - 30 Jun 2026

The IRS split the 2026 year into two rates; this applies to the first half only.

Standard mileage rate, 1 Jul - 31 Dec 2026

Up from 72.5 cents in the first half of 2026 and 70 cents for all of 2025.

Maximum section 179 deduction

Sits on the $2,500,000 base set by OBBBA section 70301 for years after 2024, then inflation-adjusted.

Section 179 phase-out threshold

The deduction is reduced dollar for dollar by property placed in service above this, not below zero.

De minimis safe harbor, per invoice or item

For a taxpayer with no applicable financial statement. Deduct rather than capitalise; this is the limit that actually applies to a solo operator, not the section 179 ceiling.

401(k) elective deferral limit, section 402(g)

Up from $23,500 in 2025. Applies across all plans in aggregate, not per plan.

Age-50 catch-up contribution, section 414(v)

Higher limit of $11,250 applies for those aged 60 to 63 under SECURE 2.0.

Catch-up contribution, ages 60 to 63

SECURE 2.0 super catch-up. Replaces the $8,000 figure, it does not stack with it.

Overall defined contribution limit, section 415(c)

The ceiling on employee deferrals plus employer contributions combined, per plan. Also the SEP-IRA ceiling.

Annual compensation limit, section 401(a)(17)

Earnings above this are ignored when computing a percentage-of-compensation contribution.

SEP-IRA minimum compensation, section 408(k)(2)(C)

Below this, an employee need not be covered by the plan.

SIMPLE IRA elective deferral limit, section 408(p)(2)(E)

Up from $16,500 in 2025.

Roth IRA contribution phase-out begins, single or head of household

Modified AGI. Fully eliminated at $168,000.

Roth IRA contribution phase-out begins, married filing jointly

Fully eliminated at $252,000.

SIMPLE plan catch-up contribution, age 50

Separate from and smaller than the $8,000 401(k) catch-up.

SIMPLE plan catch-up, ages 60 to 63

SECURE 2.0 higher band, replacing the $4,000 figure for those four years.

SIMPLE employer match, maximum

Against 20% for a SEP or the employer side of a solo 401(k). The gap is the reason a SIMPLE rarely suits a one-person business.

Traditional and Roth IRA contribution limit, section 219(b)(5)(A)

Up from $7,000 in 2025. Separate from and in addition to a workplace plan.

Underpayment interest rate, individuals

Q4 2026. RESET QUARTERLY — federal short-term rate plus 3 points, compounded daily. Was 6% in Q2 2026 and 7% through 2025.

Payment deadlines

What the calculators do not do

• State income tax is mostly not modelled. The nine states with no income tax return zero, which is exact. Every other state returns a federal-only figure with a visible notice. We would rather tell you the number is incomplete than hand you a wrong one.
• The QBI deduction assumes no employees and no qualified property.If you pay W-2 wages, the wage limitation applies and needs a preparer.
• Credits, itemised deductions and carryovers are not included.These are estimates for planning, not a return.

How the arithmetic is tested

The engine is covered by golden tests that assert intermediate lines rather than only final answers — a total that happens to match through two compensating errors is worse than a visible failure. The four federal rate tables are cross-checked against the cumulative figures the IRS publishes directly ("$5,800 plus 22% of the excess over $50,400"), which is a second independent derivation rather than the engine agreeing with itself.

Coverage includes every filing status, the Social Security wage-base boundary, the QBI phase-in boundary, a zero-income case, a loss year, and the W-2-plus-1099 case. Boundaries are where tax engines break.

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