How we build, verify, and maintain accurate calculators.
Formula Sourcing
Every calculator on ThisCalc is built on established, peer-reviewed mathematical formulas. We source our formulas from authoritative references including:
- Finance: Standard amortization formulas, IRS tax tables and publications, Federal Reserve historical rate data, and widely-used financial modeling equations (e.g., M = P[r(1+r)n]/[(1+r)n−1] for mortgage payment calculations)
- Health: Clinically-validated formulas such as the Mifflin-St Jeor equation for resting energy expenditure (Am J Clin Nutr 1990), WHO BMI classification thresholds, CDC adult BMI guidance, and the Harris-Benedict energy expenditure model
- Math & Science: Textbook formulas for statistics, physics, and engineering drawn from standard references including NIST CODATA fundamental constants and peer-reviewed statistics texts
- Specialized domains: Published research papers and domain-specific standards including Henssge's nomogram for forensic time-of-death estimation, NASA planetary fact sheets and orbital mechanics data, and NWS weather-related calculation references
Each calculator page cites its primary formula and source. Where multiple valid approaches exist, we document which method we use and why. For additional verification, see the Wikipedia article on compound interest or the CFPB Owning a Home resources for home-buying calculators.
Implementation Process
Building a new calculator follows a consistent process:
- Research: Identify the target formula, its assumptions, and its limitations. Cross-reference at least two independent sources.
- Implement: Write the calculation logic in client-side JavaScript. All computation runs in your browser with no server involvement.
- Test edge cases: Verify results against known reference values, boundary conditions (zero inputs, very large numbers, negative values), and common user mistakes.
- Document: Write clear explanations of what the calculator does, what each input means, and how to interpret results. Include the formula, assumptions, and limitations.
- Review: A second review pass checks formula accuracy, user interface clarity, and content quality before publication.
Accuracy Commitments
We take accuracy seriously because people use our calculators to make real decisions about mortgages, retirement planning, health, and more. Our commitments:
- Reproducible results: Every calculation can be verified by hand using the documented formula. There is no hidden logic.
- Transparent assumptions: When a calculator makes simplifying assumptions (e.g., fixed interest rate, no tax adjustments), we state them clearly.
- Rounding disclosure: Financial calculators use standard banker's rounding. We note when rounding may affect results.
- No black boxes: Since all code runs in your browser, you can inspect the calculation logic directly via your browser's developer tools.
Limitations
No online calculator can replace professional advice. Our calculators have inherent limitations:
- Financial calculators provide estimates. Actual loan payments, tax obligations, and investment returns depend on factors beyond any calculator's scope (lender terms, tax law, market conditions).
- Health calculators use population-level formulas that may not apply to every individual. BMI, calorie estimates, and body composition results are screening tools, not diagnoses.
- Specialized calculators (forensics, space physics, survival) are educational tools. They should not be used as the sole basis for professional decisions in those fields.
We include specific limitation disclosures on every calculator page where the results could be misapplied.
Review Cycle
Each calculator displays a "Last reviewed" date. This date reflects when the formula, content, and functionality were last checked for accuracy. We review calculators on a rolling basis, prioritizing those in finance and health categories due to their higher impact on user decisions. See our Editorial Standards for details on our review process.
Last updated: April 2026
