Methodology

Our promise is to show our work. This page explains how every CoverageMath calculator computes its numbers, where the data comes from, and how we keep it current. If you ever find a discrepancy with a primary source, that's a bug — tell us.

Principles

  • Primary sources only. Every formula traces to the IRS, HHS/ASPE, CMS, or the Social Security Administration — not to a competitor's calculator.
  • Data lives in the open. Year-dependent numbers (poverty guidelines, applicable percentages, premium figures, IRMAA brackets) live in JSON data files, each carrying its source_url and the date we last verified it. The calculator code never hard-codes a year's numbers.
  • Estimates, clearly labeled. Where we use an approximation (for example, a state-average benchmark premium instead of a county-exact one), we say so on the page.
  • We cross-check. Flagship outputs are validated against an independent reference (the Kaiser Family Foundation calculator) for the same inputs.

ACA Subsidy Calculator

The premium tax credit (PTC) is computed with the standard HealthCare.gov / IRS method:

  1. Income as a percent of the Federal Poverty Level (FPL). We divide your household's expected Modified Adjusted Gross Income (MAGI) by the FPL for your household size and state group. Note: a coverage year uses the prior year's poverty guidelines — 2026 coverage uses the 2025 HHS guidelines.
  2. Applicable percentage. Your %FPL maps to a "required contribution" percentage on the IRS applicable-percentage schedule, interpolated within each income band. For 2026 this reverts to the pre-ARPA schedule (the enhanced subsidies expired December 31, 2025), per IRS Rev. Proc. 2025-25.
  3. Expected contribution. Your MAGI × the applicable percentage = what you're expected to pay toward the benchmark plan for the year.
  4. Benchmark premium. The second-lowest-cost Silver plan (SLCSP) available to you. We take the SLCSP reference premium for a 21-year-old and scale each household member by the federal Default Standard Age Curve (counting at most the three oldest children under 21). Community-rated states (New York, Vermont) don't vary premiums by age, so the age curve isn't applied there.
  5. Premium tax credit = max(0, benchmark annual premium − expected contribution).
  6. The 400% cliff. In 2026 there is a hard cutoff: above 400% of FPL you receive no premium tax credit. Below 100% FPL, you're generally directed to Medicaid (in expansion states, up to 138% FPL) or fall in the coverage gap (non-expansion states).
  7. Cost-sharing reductions (CSR). Flagged when your income is at or below 250% FPL and you'd pick a Silver plan.

Benchmark-premium caveat: at launch, the SLCSP figures are clearly-labeled state averages. County-exact figures from the CMS public-use files are being added. Always confirm your actual benchmark and credit on HealthCare.gov or your state Marketplace.

Two different MAGIs

"MAGI" means different things in different programs, and using the wrong one gives the wrong answer. We keep both explicit:

  • ACA MAGI = Adjusted Gross Income + tax-exempt interest + excluded foreign income + the non-taxable portion of Social Security benefits.
  • IRMAA MAGI = Adjusted Gross Income + tax-exempt interest. (Social Security is not added back.)

Annual maintenance

By design, a full annual update touches only data files — never the calculator code. Each file carries a source_url and a last_verified date. When the IRS, HHS, CMS, or SSA publishes new figures for the next year, we add them under the new year key and update the verification date. You can always see, on each calculator page, the date the underlying figures were last verified.

Found a discrepancy?

We treat any mismatch with a primary source as a defect. If a number here disagrees with the IRS, HHS, CMS, or SSA, please let us know via the About page so we can correct it.