Methodology & Sources

This page explains how DailyNumeric’s calculators work, which formulas they use, how we check them, and what their limitations are. Every calculator page also states its own formula and a worked example directly — this page collects the fuller picture in one place.

How our calculators are built

Each calculator is implemented as a small, isolated calculation function with no dependency on the page it renders on. Age and date calculators share one calendar-math library so that leap years and month-length differences are handled consistently across every date-based tool. Health and fitness calculators share a second library implementing each named formula once, so the same BMI or BMR calculation, for example, can’t drift between calculators that both use it. Construction calculators likewise share a common library for unit conversion and currency formatting.

Before a calculator ships, we check its output against known reference values — for example, textbook or published worked examples for a given formula — and test edge cases such as zero, negative, or extreme inputs so the calculator shows a clear message instead of a nonsensical result (like NaN or a negative age).

Age, Date & Calendar formulas

These calculators compute exact calendar differences (years, months, weeks, days, hours, minutes, or seconds) between two dates, using the Gregorian calendar and accounting for leap years and varying month lengths. Business/working-day calculators count Monday–Friday by default and do not know about regional public holidays unless you exclude those dates yourself.

Health & Fitness formulas

Our health and fitness calculators are estimates based on established, published formulas — not personalized medical assessments. See each calculator’s own “Formula & methodology” section for its specific formula. The named methods used across this category:

Body Mass Index (BMI): weight (kg) ÷ height (m)², categorized using the thresholds published by the U.S. CDC and the World Health Organization. BMI is a population-level screening measure — it does not directly measure body fat and can be misleading for athletes, older adults, or during pregnancy.
Basal Metabolic Rate (BMR): the Mifflin-St Jeor equation, one of the more widely used predictive BMR formulas in nutrition research. Like all predictive BMR equations, it estimates a population average and can be off for a given individual, particularly at unusual body compositions.
Total Daily Energy Expenditure (TDEE): BMR multiplied by an activity factor (a commonly used set of multipliers ranging from sedentary to extremely active). Activity factors are necessarily approximate.
Ideal body weight: the Devine, Robinson, Miller, and Hamwi formulas — four commonly cited height-based estimates from the clinical/pharmacology literature, shown together so you can see the range rather than a single number presented as fact.
Body fat percentage: the U.S. Navy circumference method (neck, waist, and — for women — hip measurements). This is a field-estimation method, not a diagnostic-grade measurement like DEXA or hydrostatic weighing, and is sensitive to measurement technique.
Body Surface Area (BSA): the Du Bois & Du Bois (1916) and Mosteller (1987) formulas, both long-standing and widely cited in clinical contexts such as drug dosing calculations — shown here for general informational use only, not for clinical dosing decisions.
Lean body mass: the Boer (1984) formula.
Waist-to-height and waist-to-hip ratio: ratio thresholds commonly referenced in cardiometabolic risk research as general screening indicators, not diagnostic cutoffs.
Protein, carbohydrate, fat, and water intake: general ranges commonly cited in sports-nutrition guidance (for example, grams of protein per kilogram of body weight by activity level). These are starting points for a conversation with a dietitian, not individualized prescriptions.

A note on citations: the formulas above are described by name so they can be independently verified against the nutrition/clinical literature. We link directly to official sources (like the CDC) where we have a stable, verifiable link; for the original journal papers behind formulas like Mifflin-St Jeor, Devine, Du Bois, Boer, and the U.S. Navy method, we name the method and year rather than link to a specific paywalled or unverified copy, to avoid citing a source we haven’t confirmed.

Pregnancy & Fertility formulas

Pregnancy calculators estimate dates using standard obstetric dating conventions, not individual medical assessment. The Pregnancy Calculator uses Naegele’s rule: a 40-week (280-day) pregnancy counted from the first day of the last menstrual period, adjusted by the difference between your cycle length and 28 days. When you enter a conception date instead, it adds 266 days. Trimesters are counted as weeks 0–13, 14–27, and 28 onward. A due date is an estimate, and an early ultrasound may change the date your doctor or midwife uses. The Pregnancy Week Calculator measures gestational age from the same 40-week timeline, counts the week you are in as completed weeks plus one, and maps weeks to pregnancy months using a common approximate convention. The Due Date Calculator also supports IVF dating, which adds 266 days to the embryo transfer date and subtracts the embryo’s age at transfer, and ultrasound dating, which subtracts the gestational age measured at the scan from 280 days. Term stages follow the commonly used early term, full term, late term, and post-term definitions.

Construction & Building formulas

Construction calculators produce planning estimates from the values you enter, not quotes or engineering assessments. The Construction Cost Calculator multiplies your total built-up area by the construction cost per unit of area you provide to get a base cost, then adds a contingency and an allowance for design, permits, and other costs, each calculated as a percentage of that base cost. The cost per unit of area is entered by you rather than built into the calculator, because construction rates vary widely by location, materials, labour, and finish level. Area conversions use 1 m² = 10.7639 sq ft. Results do not include land, furnishings, landscaping, or financing unless you include them in your inputs. For a real budget, get itemized quotes from local contractors and consult a qualified architect or engineer.

Limitations that apply to every health calculator

Health and fitness calculator results on DailyNumeric are general estimates for informational and educational purposes. They are not a medical diagnosis, and they are not a substitute for advice from a doctor, registered dietitian, or other qualified professional. Individual results can vary based on factors these formulas don’t capture — body composition, medical conditions, medications, and more. If you’re making decisions about weight, nutrition, or exercise that could affect your health, please talk to a qualified professional.

Questions or corrections

If you believe a calculator’s formula, category thresholds, or an example is incorrect, please contact us — calculation accuracy is the highest priority for this site and we want to fix mistakes quickly.