diff --git a/grades.js b/grades.js
new file mode 100644
index 0000000..e4ddcf8
--- /dev/null
+++ b/grades.js
@@ -0,0 +1,120 @@
+/**
+ * Performance Grading System
+ * Calculate letter grades (A-F) for various performance metrics
+ * Based on WebPageTest and Lighthouse scoring thresholds
+ */
+
+const GRADE_THRESHOLDS = {
+    fcp: { A: 1800, B: 3000, C: 4500, D: 6000, F: Infinity },
+    lcp: { A: 2500, B: 4000, C: 6000, D: 8000, F: Infinity },
+    cls: { A: 0.1, B: 0.25, C: 0.5, D: 0.75, F: Infinity },
+    tbt: { A: 200, B: 600, C: 1200, D: 2000, F: Infinity },
+    si: { A: 3400, B: 5800, C: 8500, D: 11000, F: Infinity },
+    tti: { A: 3800, B: 7300, C: 10000, D: 13000, F: Infinity },
+    ttfb: { A: 800, B: 1800, C: 3000, D: 5000, F: Infinity }
+};
+
+const GRADE_COLORS = {
+    'A': '#4CAF50',
+    'B': '#8BC34A',
+    'C': '#FFC107',
+    'D': '#FF9800',
+    'F': '#F44336'
+};
+
+function calculateGrade(value, metricKey) {
+    const thresholds = GRADE_THRESHOLDS[metricKey];
+    if (!thresholds || value === undefined || value === null) {
+        return 'N/A';
+    }
+    
+    if (value <= thresholds.A) return 'A';
+    if (value <= thresholds.B) return 'B';
+    if (value <= thresholds.C) return 'C';
+    if (value <= thresholds.D) return 'D';
+    return 'F';
+}
+
+function getGradeColor(grade) {
+    return GRADE_COLORS[grade] || '#999';
+}
+
+function calculateAllGrades(metrics) {
+    return {
+        fcp: {
+            grade: calculateGrade(metrics.fcp, 'fcp'),
+            value: metrics.fcp,
+            label: 'First Contentful Paint'
+        },
+        lcp: {
+            grade: calculateGrade(metrics.lcp, 'lcp'),
+            value: metrics.lcp,
+            label: 'Largest Contentful Paint'
+        },
+        cls: {
+            grade: calculateGrade(metrics.cls, 'cls'),
+            value: metrics.cls,
+            label: 'Cumulative Layout Shift'
+        },
+        tbt: {
+            grade: calculateGrade(metrics.tbt, 'tbt'),
+            value: metrics.tbt,
+            label: 'Total Blocking Time'
+        },
+        si: {
+            grade: calculateGrade(metrics.si, 'si'),
+            value: metrics.si,
+            label: 'Speed Index'
+        },
+        tti: {
+            grade: calculateGrade(metrics.tti, 'tti'),
+            value: metrics.tti,
+            label: 'Time to Interactive'
+        }
+    };
+}
+
+function renderGradeBadge(grade, size = 'medium') {
+    const color = getGradeColor(grade);
+    const sizeClass = size === 'large' ? 'grade-large' : size === 'small' ? 'grade-small' : 'grade-medium';
+    
+    return `
+        <div class="grade-badge ${sizeClass}" style="background: ${color}">
+            ${grade}
+        </div>
+    `;
+}
+
+function renderGradesSection(grades) {
+    let html = '<div class="grades-container">';
+    
+    Object.entries(grades).forEach(([key, data]) => {
+        const valueFormatted = key === 'cls' 
+            ? data.value?.toFixed(3) 
+            : data.value ? `${Math.round(data.value)}ms` : 'N/A';
+            
+        html += `
+            <div class="grade-item">
+                ${renderGradeBadge(data.grade, 'large')}
+                <div class="grade-info">
+                    <div class="grade-label">${data.label}</div>
+                    <div class="grade-value">${valueFormatted}</div>
+                </div>
+            </div>
+        `;
+    });
+    
+    html += '</div>';
+    return html;
+}
+
+// Export for use in main.js
+if (typeof module !== 'undefined' && module.exports) {
+    module.exports = {
+        calculateGrade,
+        calculateAllGrades,
+        renderGradeBadge,
+        renderGradesSection,
+        getGradeColor
+    };
+}