A/B Testing Sample Size Calculator

A beautiful, web-based calculator for determining the required sample size for A/B testing experiments. Built with pure HTML, CSS, and JavaScript - no backend required!

🚀 Live Demo

Try it now!

Features

• Real-time Calculations: Results update instantly as you type
• Complete Parameter Control:
  • Baseline Conversion Rate (BCR)
  • Minimum Detectable Effect (MDE) - relative percentage improvement
  • Significance Level (α) - typically 0.05 for 95% confidence
  • Statistical Power (1-β) - typically 0.8 for 80% power
  • Test Type - one-sided or two-sided
  • Traffic Split Ratio - customizable test/control split
• Modern UI: Clean, responsive design with glassmorphism effects
• No Backend Needed: All calculations run in your browser
• Mobile Responsive: Works perfectly on all device sizes

Statistical Methodology

The calculator uses the two-proportion z-test formula to determine sample sizes:

n = 2 × [(Z_α + Z_β)² × p̄(1-p̄)] / δ²

Where:

• Z_α = z-score for significance level (adjusted for one/two-sided)
• Z_β = z-score for statistical power
• p̄ = pooled probability = (p₁ + p₂) / 2
• δ = absolute effect size = |p₂ - p₁|
• p₁ = baseline conversion rate
• p₂ = test conversion rate = p₁ × (1 + MDE/100)

The formula is adjusted for unequal traffic splits using ratio multipliers.

Usage

Local Usage (Simplest)

1. Open the calculator:

   • Just double-click index.html in your file explorer
   • It will open in your default browser and work immediately!

2. Enter your parameters:

   • BCR: Your current conversion rate (e.g., 5%)
   • MDE: Smallest improvement you want to detect (e.g., 30% relative improvement)
   • Alpha: Usually keep at 0.05 (95% confidence)
   • Power: Usually keep at 0.8 (80% power)
   • Test Type: Two-sided for "any change", one-sided for "improvement only"
   • Split Ratio: Adjust if you want unequal traffic distribution

3. View results:

   • Total sample size needed
   • Control group size
   • Test group size

Project Structure

SampleSizeCalculatorforABTesting/
├── index.html      # Main HTML structure
├── styles.css      # Modern CSS styling
├── script.js       # Statistical calculations & UI logic
└── README.md       # This file

Browser Compatibility

Works on all modern browsers:

• Chrome/Edge (v90+)
• Firefox (v88+)
• Safari (v14+)
• Mobile browsers

License

Free to use and modify. No attribution required.

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Built with ❤️ for better A/B testing