Calculator Methodology & Sources
Transparency is central to CalcFlo. Every calculator on this site is built on well-established formulas from peer-reviewed research, government agencies, and industry standards. This page documents the primary methodology and sources behind our most-used calculators.
Last reviewed: July 2026
How We Build and Verify Each Calculator
We take the view that a calculator is only as trustworthy as the source you can check it against. So rather than asking you to trust our expertise, we tell you which formula we used and where it comes from — every entry on this page names a specific publication, agency, or paper you can look up independently.
Each calculator goes through the same four steps before it goes live:
- Source the formula. We start from a primary source — a government publication, a standards body, or a peer-reviewed paper — not from another calculator website. Where a field has competing formulas (BMR is a good example, with Mifflin-St Jeor and Harris-Benedict both in common use), we say which one we implemented and why.
- Cross-check against a second source. Formulas get transcribed wrongly surprisingly often, including by reputable sites. We verify the arithmetic against at least one independent reference before shipping.
- Validate against a known-good tool. Where an official calculator exists — the SSA Retirement Estimator, Taiwan Power’s billing calculator, a tax authority’s own tool — we run test cases through both and compare outputs.
- Re-check when the rules change. Formulas are stable; the numbers plugged into them are not. Tax brackets, contribution limits, and utility rates all move, and a calculator running last year’s figures is worse than no calculator at all.
What We Publish Under
Content on CalcFlo is produced and reviewed by the CalcFlo editorial team rather than credited to individual named authors. We think a verifiable source list is a more useful signal than a byline, so that is what we invest in. Every page carries a “Last Updated” date, and this page carries the review date for the methodology itself.
Financial Calculators
Mortgage Calculator
Formula: Standard amortization formula — M = P × [r(1+r)n] / [(1+r)n − 1], where P = principal, r = monthly interest rate, n = total payments.
Sources:
- Consumer Financial Protection Bureau (CFPB) — Mortgage Key Terms
- Federal Reserve Board — Regulation Z (Truth in Lending)
Compound Interest Calculator
Formula: A = P(1 + r/n)nt + PMT × [((1 + r/n)nt − 1) / (r/n)], with optional regular contributions.
Sources:
- SEC Investor Education — Compound Interest
- Brealey, Myers & Allen — Principles of Corporate Finance (McGraw-Hill)
Retirement Calculator
Formula: Future value projections with inflation adjustment. Withdrawal analysis based on the 4% safe withdrawal rate.
Sources:
- Bengen, W.P. (1994). “Determining Withdrawal Rates Using Historical Data.” Journal of Financial Planning.
- Trinity Study (Cooley, Hubbard, Walz) — updated through 2020 data.
- Social Security Administration — Retirement Estimator methodology.
Loan & Debt Calculators
Formula: Loan calculators (auto, personal, student, EMI) use the same amortization formula as the mortgage calculator, adapted for different terms. Debt payoff calculators model minimum-payment snowball and avalanche strategies.
Sources:
- Federal Reserve — Consumer Credit G.19 Report
- CFPB — Paying Off Credit Card Debt
Health & Fitness Calculators
BMI Calculator
Formula: BMI = weight (kg) / height (m)2. Categories follow the WHO classification.
Sources:
- World Health Organization — BMI Classification (2000, updated 2023).
- CDC — About Adult BMI.
Calorie & BMR Calculator
Formula: Mifflin-St Jeor equation (1990). Males: 10×weight + 6.25×height − 5×age + 5. Females: 10×weight + 6.25×height − 5×age − 161. TDEE = BMR × activity factor.
Sources:
- Mifflin, M.D., et al. (1990). “A new predictive equation for resting energy expenditure in healthy individuals.” American Journal of Clinical Nutrition, 51(2), 241-247.
- Dietary Guidelines for Americans, 2020-2025 (USDA/HHS).
Body Fat Calculator
Formula: U.S. Navy circumference method. Males: 86.010 × log10(waist − neck) − 70.041 × log10(height) + 36.76. Females include hip measurement.
Sources:
- Hodgdon, J.A. & Beckett, M.B. (1984). “Prediction of percent body fat for U.S. Navy men and women from body circumferences and height.” Naval Health Research Center Report No. 84-29.
- American Council on Exercise — Body Fat Percentage Chart.
Blood Pressure Calculator
Categories: Based on AHA/ACC 2017 guidelines.
Sources:
- Whelton, P.K., et al. (2018). “2017 ACC/AHA Guideline for Prevention, Detection, Evaluation, and Management of High Blood Pressure in Adults.” Hypertension, 71(6).
Math & Conversion Calculators
Percentage, Fraction, and Algebra Tools
Methods: Standard arithmetic operations and algebraic identities. Results are cross-verified against Wolfram Alpha and NIST reference data.
Unit Converters
Sources: NIST — Guide for the Use of the International System of Units (SI), SP 811. BIPM — SI Brochure (9th edition, 2019).
Taiwan & Regional Calculators
A significant part of CalcFlo covers Taiwan-specific calculations — electricity bills, income tax, labor and health insurance, vehicle licence tax, property tax. These differ from the general calculators in an important way: the formula is trivial, but the rate tables are the entire product, and they are revised by the issuing authority on their own schedule.
For these pages we work directly from the responsible agency’s current published tables:
- Electricity (ι»θ²») — Taiwan Power Company published residential tariff schedule, including the summer / non-summer split (June 1 – September 30) and the progressive tier boundaries.
- Income tax (ηΆεζεΎη¨ ) — Ministry of Finance published brackets, standard deductions, and exemption amounts for the applicable tax year.
- Labor & health insurance (εε₯δΏ) — Bureau of Labor Insurance and National Health Insurance Administration insured-salary classification tables and contribution split ratios.
- Labor pension (ει) — Bureau of Labor Insurance, employer contribution rules under the Labor Pension Act.
- Vehicle licence tax (ηη §η¨ ) and property tax (ζΏε±η¨ ) — Ministry of Finance national schedules, with local government variations noted where they apply.
Where a rate table has a known effective date, the calculator page states which year’s figures it is running. If you are using one of these tools close to a rate revision, check the stated date before relying on the result.
Update Policy
Different calculators go stale at different speeds, so we do not treat them all the same:
- Rate- and law-driven tools (tax, insurance, utilities, contribution limits) are updated when the issuing authority publishes a revision. These are the pages where being current actually matters.
- Formula-driven tools (mortgage amortization, compound interest, BMI, unit conversion) rest on stable mathematics and change only if the underlying standard is revised — for instance a guideline body reclassifying a health threshold.
- Reference figures inside explanatory content (typical interest rate ranges, cost benchmarks) are reviewed periodically and are labelled as indicative rather than authoritative.
Limitations — What These Tools Cannot Do
Being straightforward about the boundaries matters more than claiming completeness:
- These are estimates, not determinations. A tax calculator cannot see your full situation; a lender’s underwriting will not match our amortization output to the cent; a health calculator does not know your medical history.
- Population-level formulas do not describe every individual. BMI does not distinguish muscle from fat and is a poor fit for athletes. BMR equations are derived from population averages and can be off by 10–15% for a given person.
- We do not model every local variation. Municipal surcharges, employer-specific benefit rules, and transitional provisions after a law change are often outside what a general calculator can represent.
- For anything binding — a filing, a contract, a medical decision — verify against the official source or a qualified professional. Use our result to understand the shape of the problem, not as the final number.
How to Cite a CalcFlo Calculator
If you reference CalcFlo in academic work, consumer education, or reporting, use the following formats:
APA 7th Edition
MLA 9th Edition
Chicago / Turabian
Replace [Calculator Name] with the tool’s title (e.g., “Mortgage Calculator”) and [page] with the URL slug.
Corrections & Feedback
If you believe any formula or source is incorrect, please contact us at admin@calcflo.com. We take accuracy seriously and will investigate all reports promptly.