diff --git a/.babelrc b/.babelrc
index ab7bedb..8fae5fa 100644
--- a/.babelrc
+++ b/.babelrc
@@ -1,5 +1,8 @@
 {
   "presets": [
     ["env", { "loose": true }]
+  ],
+  "plugins": [
+    "transform-object-rest-spread"
   ]
 }
diff --git a/README.md b/README.md
index 64b6a1d..f701ce9 100644
--- a/README.md
+++ b/README.md
@@ -13,7 +13,7 @@ const ComplexityLimitRule = createComplexityLimitRule(1000);
 // Then use this rule with validate().
 ```
 
-You can also provide custom costs for scalars and objects, and a custom cost factor for lists.
+You can provide a configuration object with custom costs for scalars and objects as `scalarCost` and `objectCost` respectively, and a custom cost factor for lists as `listFactor`.
 
 ```js
 const ComplexityLimitRule = createComplexityLimitRule(1000, {
@@ -23,6 +23,19 @@ const ComplexityLimitRule = createComplexityLimitRule(1000, {
 });
 ```
 
+The configuration object also supports an `onCost` callback for logging query costs and a `formatErrorMessage` callback for customizing error messages. `onCost` will be called for every query with its cost. `formatErrorMessage` will be called with the cost whenever a query exceeds the complexity limit, and should return a string containing the error message.
+
+```js
+const ComplexityLimitRule = createComplexityLimitRule(1000, {
+  onCost: (cost) => {
+    console.log('query cost:', cost);
+  },
+  formatErrorMessage: cost => (
+    `query with cost ${cost} exceeds complexity limit`
+  ),
+});
+```
+
 [build-badge]: https://img.shields.io/travis/4Catalyzer/graphql-validation-complexity/master.svg
 [build]: https://travis-ci.org/4Catalyzer/graphql-validation-complexity
 
diff --git a/package-lock.json b/package-lock.json
index adb0c5b..8ff9184 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -338,6 +338,12 @@
       "integrity": "sha1-nufoM3KQ2pUoggGmpX9BcDF4MN4=",
       "dev": true
     },
+    "babel-plugin-syntax-object-rest-spread": {
+      "version": "6.13.0",
+      "resolved": "https://registry.npmjs.org/babel-plugin-syntax-object-rest-spread/-/babel-plugin-syntax-object-rest-spread-6.13.0.tgz",
+      "integrity": "sha1-/WU28rzhODb/o6VFjEkDpZe7O/U=",
+      "dev": true
+    },
     "babel-plugin-syntax-trailing-function-commas": {
       "version": "6.22.0",
       "resolved": "https://registry.npmjs.org/babel-plugin-syntax-trailing-function-commas/-/babel-plugin-syntax-trailing-function-commas-6.22.0.tgz",
@@ -488,6 +494,12 @@
       "integrity": "sha1-KrDJx/MJj6SJB3cruBP+QejeOg4=",
       "dev": true
     },
+    "babel-plugin-transform-object-rest-spread": {
+      "version": "6.23.0",
+      "resolved": "https://registry.npmjs.org/babel-plugin-transform-object-rest-spread/-/babel-plugin-transform-object-rest-spread-6.23.0.tgz",
+      "integrity": "sha1-h11ryb52HFiirj/u5dxIldjH+SE=",
+      "dev": true
+    },
     "babel-plugin-transform-regenerator": {
       "version": "6.24.1",
       "resolved": "https://registry.npmjs.org/babel-plugin-transform-regenerator/-/babel-plugin-transform-regenerator-6.24.1.tgz",
diff --git a/package.json b/package.json
index 5959e00..eada68a 100644
--- a/package.json
+++ b/package.json
@@ -41,6 +41,7 @@
     "babel-cli": "^6.24.1",
     "babel-eslint": "^7.2.3",
     "babel-jest": "^20.0.3",
+    "babel-plugin-transform-object-rest-spread": "^6.23.0",
     "babel-preset-env": "^1.5.2",
     "codecov": "^2.2.0",
     "eslint": "^3.19.0",
diff --git a/src/index.js b/src/index.js
index d5327d6..c26aef4 100644
--- a/src/index.js
+++ b/src/index.js
@@ -130,7 +130,20 @@ export class ComplexityVisitor {
   }
 }
 
-export function createComplexityLimitRule(maxCost, options = {}) {
+function complexityLimitExceededErrorMessage() {
+  // By default, don't respond with the cost to avoid leaking information about
+  // the cost scheme to a potentially malicious client.
+  return 'query exceeds complexity limit';
+}
+
+export function createComplexityLimitRule(
+  maxCost,
+  {
+    onCost,
+    formatErrorMessage = complexityLimitExceededErrorMessage,
+    ...options
+  } = {},
+) {
   return function ComplexityLimit(context) {
     const visitor = new ComplexityVisitor(context, options);
 
@@ -149,9 +162,15 @@ export function createComplexityLimitRule(maxCost, options = {}) {
         }
 
         if (node.kind === 'Document') {
-          if (visitor.getCost() > maxCost) {
+          const cost = visitor.getCost();
+
+          if (onCost) {
+            onCost(cost);
+          }
+
+          if (cost > maxCost) {
             context.reportError(new GraphQLError(
-              'query exceeds complexity limit', [node],
+              formatErrorMessage(cost), [node],
             ));
           }
         }
diff --git a/test/createComplexityLimitRule.test.js b/test/createComplexityLimitRule.test.js
index 1c52f21..0835d32 100644
--- a/test/createComplexityLimitRule.test.js
+++ b/test/createComplexityLimitRule.test.js
@@ -15,10 +15,11 @@ describe('createComplexityLimitRule', () => {
     `);
 
     const errors = validate(schema, ast, [createComplexityLimitRule(9)]);
+
     expect(errors).toHaveLength(0);
   });
 
-  it('should not report error on an invalid query', () => {
+  it('should report error on an invalid query', () => {
     const ast = parse(`
       query {
         list {
@@ -28,9 +29,69 @@ describe('createComplexityLimitRule', () => {
     `);
 
     const errors = validate(schema, ast, [createComplexityLimitRule(9)]);
+
     expect(errors).toHaveLength(1);
     expect(errors[0]).toMatchObject({
       message: 'query exceeds complexity limit',
     });
   });
+
+  it('should call onCost with complexity score', () => {
+    const ast = parse(`
+      query {
+        item {
+          name
+        }
+      }
+    `);
+
+    const onCostSpy = jest.fn();
+
+    const errors = validate(schema, ast, [
+      createComplexityLimitRule(9, { onCost: onCostSpy }),
+    ]);
+
+    expect(errors).toHaveLength(0);
+    expect(onCostSpy).toHaveBeenCalledWith(1);
+  });
+
+  it('should call onCost with cost when there are errors', () => {
+    const ast = parse(`
+      query {
+        list {
+          name
+        }
+      }
+    `);
+
+    const onCostSpy = jest.fn();
+
+    const errors = validate(schema, ast, [
+      createComplexityLimitRule(9, { onCost: onCostSpy }),
+    ]);
+
+    expect(errors).toHaveLength(1);
+    expect(onCostSpy).toHaveBeenCalledWith(10);
+  });
+
+  it('should call formatErrorMessage with cost', () => {
+    const ast = parse(`
+      query {
+        list {
+          name
+        }
+      }
+    `);
+
+    const errors = validate(schema, ast, [
+      createComplexityLimitRule(9, {
+        formatErrorMessage: cost => `custom error, cost ${cost}`,
+      }),
+    ]);
+
+    expect(errors).toHaveLength(1);
+    expect(errors[0]).toMatchObject({
+      message: 'custom error, cost 10',
+    });
+  });
 });