diff --git a/hailbytes-vuln-calculator.js b/hailbytes-vuln-calculator.js
index 1d2c2aa..dedec17 100644
--- a/hailbytes-vuln-calculator.js
+++ b/hailbytes-vuln-calculator.js
@@ -550,8 +550,7 @@ const STYLES = `
 
 // ─── Template ─────────────────────────────────────────────────────────────────
 
-const TMPL = document.createElement('template');
-TMPL.innerHTML = `<style>${STYLES}</style>
+const TEMPLATE_HTML = `<style>${STYLES}</style>
 <div class="header">
   <h1><span class="icon">🔍</span> Vulnerability Scanner Infrastructure Calculator <span class="hailbytes-badge">HailBytes</span></h1>
   <p>Size your scanning infrastructure — estimate VM requirements, cloud costs, and ROI entirely in the browser</p>
@@ -642,9 +641,23 @@ TMPL.innerHTML = `<style>${STYLES}</style>
   </div>
 </div>`;
 
+// Only touch the DOM when it exists. This keeps the module importable in
+// DOM-free environments (Node, SSR, Next.js server components) so the pure
+// `calculate` export can be used without a browser or DOM shim.
+const TMPL = typeof document !== 'undefined'
+  ? document.createElement('template')
+  : null;
+if (TMPL) TMPL.innerHTML = TEMPLATE_HTML;
+
 // ─── Web Component ────────────────────────────────────────────────────────────
 
-class HailbytesVulnCalculator extends HTMLElement {
+// Fall back to a plain base class off-DOM so the class declaration itself
+// doesn't throw when `HTMLElement` is undefined. The element is only ever
+// instantiated by the browser's custom-element machinery, so the DOM methods
+// used in the constructor are never reached off-DOM.
+const ComponentBase = typeof HTMLElement !== 'undefined' ? HTMLElement : class {};
+
+class HailbytesVulnCalculator extends ComponentBase {
   static get observedAttributes() { return ['theme', 'branding']; }
 
   constructor() {
@@ -856,7 +869,10 @@ class HailbytesVulnCalculator extends HTMLElement {
   }
 }
 
-customElements.define('hailbytes-vuln-calculator', HailbytesVulnCalculator);
+// Register the custom element only in a browser context.
+if (typeof customElements !== 'undefined') {
+  customElements.define('hailbytes-vuln-calculator', HailbytesVulnCalculator);
+}
 
 export default HailbytesVulnCalculator;
 export { HailbytesVulnCalculator, calculateResources as calculate };
diff --git a/test/dom-free.test.mjs b/test/dom-free.test.mjs
new file mode 100644
index 0000000..1401c8e
--- /dev/null
+++ b/test/dom-free.test.mjs
@@ -0,0 +1,35 @@
+// Regression test: the module must be importable in a DOM-free environment
+// (Node, SSR, Next.js server components) so the pure `calculate` export works
+// without a browser or a DOM shim. NOTE: this file intentionally does NOT set
+// up any `document`/`HTMLElement`/`customElements` globals — `node --test` runs
+// each test file in its own process, so importing here exercises the real
+// off-DOM code path.
+
+import { test } from 'node:test';
+import assert from 'node:assert/strict';
+
+test('module imports without a DOM and exposes calculate()', async () => {
+  assert.equal(typeof globalThis.document, 'undefined');
+  assert.equal(typeof globalThis.HTMLElement, 'undefined');
+  assert.equal(typeof globalThis.customElements, 'undefined');
+
+  const mod = await import('../hailbytes-vuln-calculator.js');
+  assert.equal(typeof mod.calculate, 'function');
+});
+
+test('calculate() runs off-DOM and returns a sizing result', async () => {
+  const { calculate } = await import('../hailbytes-vuln-calculator.js');
+
+  const result = calculate({
+    target_hosts: 1000,
+    scan_intensity: 'medium',
+    scan_frequency: 'weekly',
+    scan_window: 8,
+    scanning_tools: ['openvas'],
+    compliance_needs: [],
+  });
+
+  assert.ok(result.vm_resources.cpu_cores >= 2);
+  assert.ok(result.costs.total_monthly_aws > 0);
+  assert.ok(Array.isArray(result.recommendations));
+});