Methodology

How Calc.Cards Computes Things

Last updated 2026-05-22

This page is the master technical reference for how every calculator on Calc.Cards arrives at its number. Each calculator's own page carries the cited primary source in its trust panel; this page collects the per-category methodology and the assumptions we've baked into the implementations.

If you find an error, a stale figure, or a methodology choice you disagree with, please email hello@calc.cards. Verified corrections land on the relevant calculator page and on this document, with the last-updated date moved forward. See the corrections log for the public record of changes we've made.

Finance

Loan-payment math uses the standard fixed-rate amortization formula:M = P × [r(1+r)^n] / [(1+r)^n − 1]where M is the monthly payment, P is the principal, r is the monthly interest rate (annual rate divided by 12), and n is the total number of payments (years × 12). This formula is the same one used by every major mortgage and consumer-loan calculator; the implementation differences across sites are typically in how PMI, taxes, insurance, and fees are layered on top.

On top of P&I, calculators in the Finance category fold in:

• Property taxes — entered annually, billed monthly into escrow. Defaults pull from county-assessor norms (0.5%–2.5% of assessed value annually for most US properties).
• Homeowner's insurance — typical range $1,000–$2,500/yr for a single-family home, varies by location and coverage.
• PMI — required on conventional loans with under 20% down. Calculator uses 0.5%–1.5% of loan amount annually, calibrated against the Freddie Mac PMI rate schedule.
• FHA MIP— separate from PMI; uses HUD's current upfront (1.75%) and annual (0.55%–1.05%) schedule.

Compound-interest and retirement calculators use standard time-value-of-money formulas. Inflation-adjusted projections use a long-run inflation assumption of 2.5–3% by default, configurable per calculator. Retirement projections that assume an investment return default to 7% nominal (rough long-run S&P 500 average) and call out the assumption on the page; this is not a forecast, and we say so.

Primary sources:Consumer Financial Protection Bureau mortgage guidance (consumerfinance.gov), Freddie Mac Primary Mortgage Market Survey (freddiemac.com/pmms), HUD FHA program documentation (hud.gov), IRS Publication 590-A & 590-B for retirement-account contribution and distribution rules, and Social Security Administration replacement-rate tables for retirement projections.

Health

BMI is the standard kg/m² formula. We display the WHO adult categories (underweight <18.5, normal 18.5–24.9, overweight 25–29.9, obese ≥30) on the result, and we flag pediatric users to use the CDC age-and-sex-specific BMI-for-age percentile, not the adult cutoffs.

Basal Metabolic Rate (BMR) uses the Mifflin-St Jeor equation, which replaced the older Harris-Benedict as the clinical default in 2005 and has lower mean error in published meta-analyses:Men: BMR = 10 × wt(kg) + 6.25 × ht(cm) − 5 × age + 5 Women: BMR = 10 × wt(kg) + 6.25 × ht(cm) − 5 × age − 161Total Daily Energy Expenditure (TDEE) multiplies BMR by an activity factor (1.2 sedentary through 1.9 very active). Calorie targets subtract or add ~500 kcal/day from TDEE to target ~1 lb/week of loss or gain — a textbook clinical default that is appropriate for moderate goals and inappropriate for medical weight management.

Body-fat estimators use the US Navy circumference formula (Hodgdon & Beckett) by default. Ideal-weight estimators offer the four published formulas (Devine, Hamwi, Miller, Robinson) and report all four because none is materially better than the others — clinicians typically use Devine for medication-dosing approximations.

Primary sources:WHO BMI classification (who.int/health-topics/obesity), CDC pediatric growth charts (cdc.gov/growthcharts), Mifflin et al. 1990 (American Journal of Clinical Nutrition), Hodgdon & Beckett 1984 US Navy report on circumference-based body-fat estimation, and the original published ideal-weight equations.

What we do not publish: drug-dosage calculators tied to specific medications or patient populations, clinical decision tools that require licensed-professional judgment, and any calculator that would benefit from individual lab values or imaging. Those belong on a clinical platform with appropriate guardrails, not a free public site.

Real Estate

Cap rate is the standard net-operating-income (NOI) divided by purchase price. NOI defaults to gross rental income minus a 30–40% operating-expense ratio when the user doesn't supply detailed expense data, calibrated against published REIT operating benchmarks. The calculator surfaces both the ratio it used and the underlying NOI so users can verify.

Rent-vs-buy uses a discounted-cash-flow comparison over a user-set time horizon (default 7 years, matching the median US homeowner tenure). The buy-side total cost includes mortgage payments, taxes, insurance, maintenance (1–2% of value annually), opportunity cost on the down payment, and a transaction-cost layer (6% of sale price for agent commissions and closing). The rent-side cost includes rent plus the opportunity-cost return on what would have been the down payment. Equity build-up is netted from the buy side.

Primary sources: NAR existing-home sales data (nar.realtor), NAREIT operating-expense benchmarks, BLS rent CPI components for rental-cost growth assumptions.

Construction

Material-volume calculators use the standard geometric formulas: area × thickness for slabs (concrete, asphalt), linear length × unit coverage for fences and edging, and surface area divided by a coverage rate for paint and stain. Concrete defaults assume standard 4-inch slab thickness, 3,000 PSI concrete, and a 5% waste factor. Real job-site waste is often higher; the calculator flags this and recommends checking with the local supplier.

Paint calculators use a coverage rate of 350 sq ft per gallon for one coat on a primed, smooth surface. Real coverage drops on textured walls, porous substrates, and dark-over-light color changes; the calculator surfaces a coverage-rate input so users can override based on the manufacturer's spec.

What we do not publish: calculators that would require local code knowledge to be safe (load-bearing structural calculations, electrical wire-sizing for installations, etc.). Those calculators should be performed by, or verified by, a licensed contractor or engineer.

Primary sources: Portland Cement Association guidance (cement.org), Sherwin-Williams and Benjamin Moore coverage specifications, USDA Forest Service lumber-grading tables for board-foot calculations.

Math & Conversion

Pure-math calculators (percentage, fraction, ratio, square root, quadratic formula, trigonometry) use the textbook formulas without modification. We surface intermediate steps in the results panel so students can verify their own work; the calculator is meant to be a check, not a substitute for understanding the math.

Unit converters use the SI definitions and NIST-published conversion factors. Where a unit has historical regional variants (the US customary gallon vs. the imperial gallon, for example), the calculator surfaces both and the user picks. Conversion factors are carried to enough significant figures that round-trip error is below 0.01% for any plausible input.

Primary sources: NIST Special Publication 811 for unit definitions and conversion factors, BIPM SI Brochure for the formal definitions of the base SI units.

Cooking

Recipe-scaling calculators do straight proportional arithmetic with a few overrides built in: leavening agents (baking powder, baking soda, yeast) don't scale linearly above 2x, and the calculator warns the user when their target scale enters that regime. Baking temperature and time also don't scale linearly with pan size; the calculator estimates a corrected time based on standard surface-area-to-volume rules but recommends visual doneness checks rather than blind reliance on the number.

Primary sources:America's Test Kitchen published baking-scale guidance, Modernist Cuisine's ratio-driven cooking volumes.

Time

Time-zone conversion uses the IANA time-zone database (tzdata), which is the same source that powers macOS, iOS, Linux, and the major browsers' built-in timezone handling. The IANA database is updated several times a year to reflect new daylight-saving rules; we sync our deployment to the latest stable tzdata release on every meaningful update.

Primary sources: IANA time-zone database (iana.org/time-zones).

A note on AI in our drafting

We use AI assistance during the article-drafting phase the same way a newsroom uses a research assistant: to pull together candidate phrasings, surface adjacent questions, and propose article structure. The math, the formulas, and the cited numbers come from the primary sources listed above — not from any model. Every calculator page is reviewed by a human editor on the Wilcoe team before publication, with the formula verified against the cited source and at least one worked example carried through end-to-end.

If you find a discrepancy between what an AI tool tells you and what a Calc.Cards calculator returns, our number is what we're willing to defend. Email hello@calc.cards if it looks wrong and we'll investigate.

When this page updates

This document is reviewed quarterly and updated when:

• A primary source publishes a new edition of the underlying methodology
• An IRS, HUD, FHA, or similar agency updates a cited threshold or limit
• A reader-submitted correction is verified and published
• The site adds a new category whose methodology needs documenting

See the corrections log for the running record of changes. The editorial process behind every change is on the editorial team page.