Every calculator on AllToolsCalculator uses a published, verified formula. This page lists the primary sources and methodologies across categories. For tool-specific details, see the "About this calculator" section on each calculator's page.
Financial calculators
Mortgage & Loan EMI. We use the standard amortisation formula: M = P · (r(1+r)ⁿ) / ((1+r)ⁿ − 1) where M is monthly payment, P is principal, r is the monthly interest rate, and n is the number of monthly payments. This matches the method used by major US banks and the Consumer Financial Protection Bureau.
Compound interest. We use the standard compound-interest formula with optional periodic contributions, compounded at the frequency you specify. Monthly contributions are modelled as an annuity: each contribution compounds from its deposit date until the end of the period.
Tip calculator. Simple percentage-of-bill arithmetic. Split-the-bill divides the total (including tip) by the number of people.
Salary to hourly. Assumes a standard work year based on weeks worked and hours per week, which you set. We do not subtract public holidays automatically because these vary by country and contract type.
Health & fitness calculators
BMI. Body Mass Index is calculated as weight (kg) divided by height (m) squared. Category thresholds follow the World Health Organization adult classification: underweight < 18.5, normal 18.5–24.9, overweight 25–29.9, obese ≥ 30.
Calorie calculator (TDEE). Basal metabolic rate (BMR) is calculated using the Mifflin–St Jeor equation, the most accurate formula for the general population per a 2005 meta-analysis published in the Journal of the American Dietetic Association. TDEE is BMR multiplied by an activity factor (1.2 sedentary through 1.9 very active).
Ideal weight. We present ranges from three established formulas (Hamwi 1964, Devine 1974, Robinson 1983) alongside the WHO BMI-based healthy range. Differences between them highlight that "ideal weight" is a range, not a single number.
Macros. Protein is calculated at 0.8–1.2 g per pound of body weight depending on goal; fat is set as a percentage of total calories; carbohydrates fill the remainder. Protein ratios follow ISSN and ACSM guidance.
Math calculators
Percentage. Three modes: X% of Y (multiplication), percentage change ((new − old) / |old| · 100), and ratio (X / Y · 100).
Average / Statistics. Standard definitions: mean (sum / count), median (middle value or average of two middle values), mode (most frequent value), and range (max − min).
Business calculators
Profit margin. Gross profit = revenue − cost. Margin = gross profit / revenue · 100. Markup = gross profit / cost · 100. We surface both because they are commonly confused.
Break-even. Break-even units = fixed costs / (price − variable cost per unit). Break-even revenue = break-even units × price.
Tax & date calculators
Age calculator. Uses the Gregorian calendar. We account for leap years and varying month lengths. All date arithmetic is performed in your local timezone.
Verification
Each calculator is tested against hand-worked examples and against published reference tables (for example, mortgage amortisation schedules from the CFPB). We re-test the full suite whenever the engine is updated.
Found an error?
We correct bugs quickly. Contact us with the calculator URL, your inputs, the result you saw, and the result you expected.