HealthCalc Pro is a premium, fully functional health and fitness tool built in a single HTML file using pure HTML, CSS, and vanilla JavaScript. It helps users instantly calculate:

BMI (Body Mass Index) with clear category (Underweight, Normal, Overweight, Obese)
Daily calorie needs (maintenance, weight loss, and weight gain) using the scientifically accurate Harris-Benedict formula
Personalized suggestions based on results
Beautiful, modern UI with dark/light mode toggle
100% offline – no internet, no API keys, no external dependencies
Fully responsive – works perfectly on mobile, tablet, and desktop

This tool is extremely practical for anyone interested in fitness, weight management, diet planning, or general health tracking – gym enthusiasts, students, trainers, nutritionists, and everyday people all love it.
You can easily sell this on platforms like Gumroad, Etsy, or CodeCanyon for $19–$49 as a "Premium Single-File Health & Fitness Calculator Template". Buyers get just one file to download, open in any browser, and start using immediately – no setup required!

Key Features & Details

Accurate Calculations:
BMI = weight (kg) ÷ height² (m)
BMR calculated using revised Harris-Benedict equation (different for male/female)
TDEE (Total Daily Energy Expenditure) based on 5 activity levels
Shows calories needed to maintain, lose (~0.5kg/week), or gain weight (~0.5kg/week)

User Input Fields:
Age (years)
Gender (Male/Female)
Weight (kg)
Height (cm)
Activity Level (5 options from sedentary to extra active)

Beautiful Result Display:
Large BMI number with color-coded category
Clear calorie recommendations
Helpful personalized tip based on BMI

Extra Features:
Smooth dark/light theme toggle
Input validation with friendly alert
Results auto-scroll into view
Glassmorphism design with gradient background

Technical Details:
Single HTML file (~250 lines)
Uses Google Fonts (Poppins) – loads from CDN
No frameworks, no libraries, no backend
Works offline after first load
Easy to customize (change colors, add logo, translate, etc.)


Why People Will Buy It

Health and fitness niche is evergreen and highly searchable
Extremely useful daily tool (people check BMI/calories regularly)
One-file simplicity = instant value for buyer
Professional look that feels like a paid app
Great for portfolios, side hustles, or reselling with modifications