Health Guides

56 in-depth, plain-language guides that go beyond the calculators — how the numbers work, what they mean, and how to act on them. Every figure is sourced, the same as our calculators.

Body Mass Index

BMI

H
How BMI Is Calculated
The exact BMI formula, a worked example, and how metric and imperial measurements produce the same result — with the WHO source behind it.
W
What Is a Healthy BMI?
The WHO BMI categories — underweight, normal, overweight, and obese — what each range means, and why a "healthy" BMI is a starting point, not a finish line.
W
Why India Uses Different BMI Cutoffs
The WHO's Asian BMI cutoffs put overweight at 23 and obese at 27.
B
BMI, Muscle, and Why Athletes Read High
BMI can't tell muscle from fat, so lean, muscular people often read "overweight.

Basal Metabolic Rate

BMR

W
What Is BMR, and Why Does It Matter?
Your BMR is the energy your body burns at complete rest.
B
BMR vs TDEE — What's the Difference?
BMR is what you burn at rest; TDEE is what you burn in total.
W
Why BMR Estimates Vary Between People
Two people with identical height, weight, age and sex can have genuinely different resting metabolisms.

Body Fat

Body Fat

H
How Body Fat Percentage Is Calculated
The U.
H
How to Measure Yourself Accurately
Your tape placement drives the result — a 2 cm waist error shifts your body fat estimate by 1.
B
Body Fat vs BMI — Which Should You Trust?
BMI is faster and better validated; body fat says what your weight is made of.
H
How Accurate Is the Navy Body Fat Method?
About ±3–4% against a DEXA scan, better for some bodies than others.

Body Surface Area

Body Surface Area

W
What Is Body Surface Area?
BSA is used clinically for drug dosing, not for assessing health.

Calories Burned

Calories Burned

H
How Calories Burned Is Calculated
One formula, one table of MET values, and your body weight.
H
How Accurate Are Calorie Burn Estimates?
MET values are population averages applied to an individual.

Daily Fiber Target

Daily Fiber Intake

H
How Much Fiber Do You Need?
The official Adequate Intake figures vary by sex and age, and are derived from calorie intake.

Added-Sugar Limit

Daily Sugar Intake

A
Added Sugar Limits Explained
Two accepted framings — the AHA's gram limits and the WHO's percentage of energy.

Fat-Free Mass Index

FFMI

U
Understanding FFMI
FFMI measures muscularity relative to height, unlike BMI.

Healthy Weight Range

Healthy Weight Range

F
Finding Your Healthy Weight Range
A range derived directly from WHO BMI cutoffs, why it's wide, and why the India figures differ from the global ones.

Max Heart Rate

Heart Rate Zones

H
How Heart Rate Zones Work
The five training zones, what each is for, and how they're calculated from your estimated maximum heart rate.
W
Why We Don't Use "220 Minus Age"
The familiar 220 − age rule has known accuracy problems.
H
How Accurate Are Heart Rate Zones?
Individual max heart rate varies by around ±10 bpm from any age formula.

Ideal Weight

Ideal Weight

H
How "Ideal Weight" Is Calculated
The Devine formula estimates ideal body weight from height and sex alone.
I
Ideal Weight vs Healthy Weight Range
One gives a single number from a 1974 dosing formula; the other gives a range from WHO cutoffs.
W
Why "Ideal Weight" Ignores Your Build
Height and sex are the only inputs.

Lean Body Mass

Lean Body Mass

W
What Is Lean Body Mass?
Everything that isn't fat — muscle, bone, organs, water.

Daily Calories

Macro Split

H
How Macro Splits Work
Turning a calorie target into grams of protein, carbs and fat.
H
How Macro Splits Compare to Official Guidelines
The IOM publishes recommended macronutrient ranges.
C
Choosing a Macro Split
Balanced, high protein, low carb or high carb — what each preset suits, and why the split matters less than the calorie total.

One-Rep Max

One-Rep Max

H
How One-Rep Max Is Estimated
Estimate your 1RM from a set you've already done.
E
Epley vs Brzycki — Why Two Formulas?
The two standard 1RM formulas agree at low reps and diverge as reps climb.
H
How Accurate Are 1RM Estimates?
Good at low reps, unreliable at high ones, and different by exercise.

Estimated Ovulation

Ovulation

H
How the Fertile Window Is Estimated
Ovulation is estimated by counting back 14 days from your next period.
H
How Accurate Are Ovulation Predictions?
Calendar-based estimates rest on averages that vary a lot between people and cycles.
I
Irregular Cycles and Ovulation Estimates
Cycle length is the input that moves the answer most.

Pace

Pace

H
How to Calculate Running Pace
Pace is time divided by distance.

Estimated Due Date

Pregnancy Due Date

H
How Due Dates Are Calculated
Due dates are estimated as 280 days from the first day of your last period.
H
How Accurate Is Your Due Date?
Only about 1 in 20 babies arrives on the estimated date.
P
Pregnancy Weeks and Trimesters Explained
Why week 1 starts before conception, how gestational age is counted, and where the ACOG trimester boundaries fall.
W
What If Your Cycle Isn't 28 Days?
Naegele's rule assumes a 28-day cycle.

Gestational Age

Pregnancy Week

T
Tracking Your Pregnancy Week by Week
How your current week is calculated, why it's written as weeks and days, and what the number is useful for.

Recommended Total Gain

Pregnancy Weight Gain

P
Pregnancy Weight Gain Guidelines
The Institute of Medicine's recommended ranges depend on your pre-pregnancy BMI.
W
Why Recommended Gain Varies by Starting BMI
The recommended range narrows and lowers as pre-pregnancy BMI rises.

Daily Protein

Protein Intake

H
How Much Protein Do You Need?
Protein needs scale with your body weight and how you train.
W
Why Your Protein Target Is a Range, Not a Number
Most calculators pick a midpoint and present it as precise.
P
Protein for Strength and Endurance Training
Why training raises protein needs well above the RDA, what the 1.
B
Body Weight or Lean Mass — Which Should You Use?
Protein guidelines are written per kg of total body weight, but lean mass drives the requirement.

Calories Burned

Steps to Calories

H
How Steps Convert to Calories
A rough conversion scaled by body weight, why it's only an approximation, and where the 10,000-step target actually came from.

Daily Energy Expenditure

TDEE

H
How to Calculate Your TDEE
TDEE is your BMR multiplied by an activity factor.
W
Why We Use Mifflin-St Jeor, Not Harris-Benedict
Most calorie calculators quietly pick a BMR equation for you.
C
Choosing Your Activity Level Honestly
The activity multiplier swings your TDEE by over 1,000 calories.
U
Using TDEE to Lose or Gain Weight
How to turn your maintenance calories into a realistic target, why about 500 kcal a day is the usual step, and why bigger deficits rarely work better.

Waist-to-Height Ratio

Waist-to-Height Ratio

W
Waist-to-Height Ratio, Explained
One measurement, one division, and a rule you can remember: keep your waist under half your height.
W
Waist-to-Height Ratio vs BMI
Both are high-confidence screens, but they measure different things.
H
How to Measure Your Waist Properly
Where to place the tape, when to measure, and the common mistakes that quietly change your result.

Daily Water

Water Intake

H
How Much Water Should You Drink?
The "8 glasses" rule has no real basis.
W
Why Water Needs Vary So Much
A body-weight formula can't see sweat, heat, or what you ate.

Every calculator also includes a detailed methodology section — the formula, its source, who it's most and least accurate for, and its limitations. Browse the calculators →