BMI Calculator & Health Advisor

Rahul Vijayraghavan
20th March 2026

The Problem

Why does BMI matter?

  • Over 1 billion people worldwide live with obesity (WHO, 2022)
  • BMI is the standard first-line screening tool used by clinicians globally
  • Most people have no quick, accessible way to check their BMI and understand what to do next

What this app solves:

A single, jargon-free web tool that: 1. Accepts weight & height in Metric or Imperial units 2. Calculates BMI instantly 3. Explains the result with personalised, actionable advice 4. Requires zero medical knowledge to use

No sign-up. No downloads. Results in seconds.

How It Works — The Maths

BMI is computed with one simple formula. The R code below demonstrates it live — the same logic runs inside the Shiny server.R:

# Example: person weighing 80 kg, 175 cm tall
weight_kg <- 80
height_m  <- 175 / 100          # convert cm → m

bmi_value <- round(weight_kg / height_m^2, 1)
cat("BMI =", bmi_value)

BMI = 26.1

# WHO classification
classify_bmi <- function(bmi) {
  if      (bmi < 18.5) "Underweight"
  else if (bmi < 25.0) "Normal weight"
  else if (bmi < 30.0) "Overweight"
  else                 "Obese"
}

cat("\nCategory:", classify_bmi(bmi_value))


Category: Overweight

The app handles imperial conversion automatically:

  • Weight: lbs × 0.4536 = kg
  • Height: inches × 0.0254 = m

App Features

Input widgets (sidebar)

Widget Purpose
Radio buttons Switch between Metric / Imperial
Weight slider Select body weight (30–200 kg or 66–440 lbs)
Height slider Select height (100–250 cm or 39–98 in)
Checkbox Show / hide BMI reference table

Reactive outputs (main panel)

  • BMI number — large, instantly updated
  • Category label — colour-coded: blue / green / orange / red
  • Healthy weight range — calculated for your height
  • Personalised advice — plain-English guidance per category
  • Visual BMI gauge — colour-band scale with your position marked
  • Reference table — all WHO classes at a glance

Try It!

The app is live on shinyapps.io:

https://rahulvijay97.shinyapps.io/bmi-calculator/

Source code on GitHub:

https://github.com/rahulvijay007/BMI-Shiny-App

  • ui.R — layout, widgets, inline documentation
  • server.R — reactive BMI logic, colour coding, advice engine

Grading checklist met:

  • Multiple input widgets (radio, sliders, checkbox)
  • Server-side reactive calculations
  • Colour-coded reactive HTML output
  • Complete user documentation embedded in the app
  • 5-slide deck with live R code (previous slide)