Methodology

General principles

All calculations run locally in your browser using JavaScript's built-in Number arithmetic (IEEE-754 double precision). Inputs are not transmitted, logged, or stored. None of these tools should be used as a substitute for advice from a qualified professional — they are educational estimators that make standard, well-documented assumptions.

When a tool requires a choice between competing formulas (for example, body fat estimation), we document which method we use and why. When standards differ (for example, BMI categories), we follow the most widely cited authority (typically the World Health Organization for health metrics, and the relevant statute or industry convention for finance).

Mortgage calculator

The principal-and-interest portion of the monthly payment uses the standard fixed-rate amortization formula:

M = P × (r(1+r)ⁿ) / ((1+r)ⁿ − 1)

Where P is the loan principal (home price minus down payment), r is the monthly interest rate (annual APR ÷ 12), and n is the total number of monthly payments (years × 12). Property tax and homeowner's insurance are added on top as monthly amounts (annual figure ÷ 12). PMI is included while the loan-to-value ratio is above 80% and uses the formula (principal × pmiRate) ÷ 12 per month; it is dropped when LTV ≤ 80%, matching standard lender practice. The amortization table is computed iteratively month-by-month and rolled up into yearly principal/interest totals.

Limitations: the calculator assumes a fixed rate for the full term; ARMs, interest-only loans, balloon payments, and bi-weekly schedules are not modelled. Closing costs and HOA dues are not included. Always run final numbers past a licensed mortgage lender before signing anything.

Auto loan calculator

Uses the same fixed-rate amortization formula as the mortgage tool. Sales tax (if entered) is added to the loan amount before amortization. Trade-in value is subtracted from the price before tax is computed, matching how most US dealerships structure the deal. There is no PMI equivalent; the calculator returns monthly P&I and total interest paid.

Compound interest calculator

The closed-form future-value formula for compound interest with regular contributions is:

FV = P(1 + r/n)^nt + PMT × [((1 + r/n)^nt − 1) / (r/n)]

Where P is the starting principal, r is the annual rate (as a decimal), n is the number of compounding periods per year, t is the number of years, and PMT is the per-period contribution. Contributions are assumed to be made at the end of each compounding period (ordinary annuity). Inflation, taxes on dividends or capital gains, and fees are not modelled — adjust your expected real return downward if you want a real-terms figure.

BMI calculator

Body Mass Index is calculated as:

• Metric: BMI = weight (kg) ÷ height (m)²
• Imperial: BMI = (weight (lb) × 703) ÷ height (in)²

Categories follow the World Health Organization's adult classification: under 18.5 = underweight, 18.5–24.9 = healthy weight, 25.0–29.9 = overweight, 30.0–34.9 = obesity class I, 35.0–39.9 = class II, 40+ = class III. The healthy weight range shown is the band that corresponds to a BMI of 18.5–24.9 at the user's height.

Limitations: BMI is a screening metric, not a diagnosis. It can overestimate body fat in muscular individuals and underestimate it in older adults with reduced lean mass. It is not validated for use in children, pregnancy, or some ethnic groups in the same way. Treat it as one data point among several (waist circumference, body fat %, clinical assessment).

TDEE calculator

Resting metabolic rate is computed using the Mifflin–St Jeor equation, which the American Dietetic Association recommends as the most accurate predictive equation for non-athlete adults (Frankenfield et al., 2005):

• Men: BMR = (10 × weight kg) + (6.25 × height cm) − (5 × age) + 5
• Women: BMR = (10 × weight kg) + (6.25 × height cm) − (5 × age) − 161

Total Daily Energy Expenditure is BMR multiplied by an activity factor: 1.2 (sedentary), 1.375 (light), 1.55 (moderate), 1.725 (very active), 1.9 (athlete). These multipliers are the canonical Harris–Benedict activity levels and are the most widely used in fitness and nutrition literature.

Limitations: predictive equations have a typical error of ±10% versus indirect calorimetry. Adjust based on real-world results over 2–4 weeks if the predicted TDEE doesn't match your weight trend.

Body fat calculator

Uses the US Navy method (Hodgdon & Beckett, 1984), which is widely used because it requires only a tape measure:

• Men: %BF = 86.010 × log₁₀(waist − neck) − 70.041 × log₁₀(height) + 36.76
• Women: %BF = 163.205 × log₁₀(waist + hip − neck) − 97.684 × log₁₀(height) − 78.387

Measurements are in inches; if you enter metric, the tool converts. The Navy formula has a typical standard error of about ±3.5% versus hydrostatic weighing. For higher accuracy use DXA or hydrostatic weighing.

Pregnancy due date calculator

Three input methods are supported, all derived from the standard 280-day (40-week) gestation:

• LMP (Naegele's rule): due date = first day of last menstrual period + 280 days. Assumes a 28-day cycle; the calculator adjusts the start date if you specify a different cycle length.
• Conception date: due date = conception date + 266 days.
• IVF transfer: due date = transfer date + 263 days for a 3-day embryo, or + 261 days for a 5-day blastocyst.

Limitations: due-date estimators are accurate to roughly ±1 week. Only about 4% of births occur on the predicted date; the majority are within 2 weeks before or after. A first-trimester ultrasound is the gold standard for dating and should always supersede an LMP-based estimate when available.

Updates and corrections

This page lists the formulas as currently implemented in the calculators. If we change a formula or update a category boundary, we'll bump the "Last updated" date here and on the affected calculator pages. Spotted an error? Email [email protected] and we'll review.