diff --git a/src/lib/credits.js b/src/lib/credits.js
new file mode 100644
index 0000000..270308e
--- /dev/null
+++ b/src/lib/credits.js
@@ -0,0 +1,36 @@
+"use strict";
+
+function assertPositiveInt(name, value) {
+  if (!Number.isInteger(value) || value <= 0) {
+    throw new Error(`${name} must be a positive integer`);
+  }
+}
+
+function validateCreditConfig(config) {
+  assertPositiveInt("baseCredits", config.baseCredits);
+  assertPositiveInt("includedChars", config.includedChars);
+  assertPositiveInt("stepChars", config.stepChars);
+  assertPositiveInt("stepCredits", config.stepCredits);
+  assertPositiveInt("maxCharsPerArticle", config.maxCharsPerArticle);
+}
+
+function calculateCredits(charCount, config) {
+  validateCreditConfig(config);
+
+  if (!Number.isInteger(charCount) || charCount <= 0) {
+    throw new Error("charCount must be a positive integer");
+  }
+
+  if (charCount > config.maxCharsPerArticle) {
+    throw new Error("article_too_long");
+  }
+
+  const overage = Math.max(0, charCount - config.includedChars);
+  const extraSteps = overage === 0 ? 0 : Math.ceil(overage / config.stepChars);
+  return config.baseCredits + (extraSteps * config.stepCredits);
+}
+
+module.exports = {
+  calculateCredits,
+  validateCreditConfig,
+};
diff --git a/test/credits.test.js b/test/credits.test.js
new file mode 100644
index 0000000..4d29a49
--- /dev/null
+++ b/test/credits.test.js
@@ -0,0 +1,35 @@
+"use strict";
+
+const test = require("node:test");
+const assert = require("node:assert/strict");
+const { calculateCredits } = require("../src/lib/credits");
+
+const config = {
+  baseCredits: 1,
+  includedChars: 25000,
+  stepChars: 10000,
+  stepCredits: 1,
+  maxCharsPerArticle: 120000,
+};
+
+test("charges base credit up to included chars", () => {
+  assert.equal(calculateCredits(1, config), 1);
+  assert.equal(calculateCredits(25000, config), 1);
+});
+
+test("charges one extra credit after crossing step boundary", () => {
+  assert.equal(calculateCredits(25001, config), 2);
+  assert.equal(calculateCredits(35000, config), 2);
+});
+
+test("charges multiple steps correctly", () => {
+  assert.equal(calculateCredits(45001, config), 4);
+});
+
+test("rejects oversized articles", () => {
+  assert.throws(() => calculateCredits(120001, config), /article_too_long/);
+});
+
+test("rejects non-positive charCount", () => {
+  assert.throws(() => calculateCredits(0, config), /charCount must be a positive integer/);
+});