diff --git a/src/items/ean/ean.service.ts b/src/items/ean/ean.service.ts
new file mode 100644
index 0000000..e8f0309
--- /dev/null
+++ b/src/items/ean/ean.service.ts
@@ -0,0 +1,33 @@
+import { Controller } from '@nestjs/common';
+import assert from 'assert';
+
+@Controller()
+export class EANService {
+  // https://en.wikipedia.org/wiki/International_Article_Number#Calculation_of_checksum_digit
+  calcChecksum(id: string) {
+    assert(id.length === 12);
+    let sum = id
+      .split('')
+      .map((d) => parseInt(d, 10))
+      .reduce((s, d, i) => s + d * (i % 2 === 0 ? 3 : 1), 10);
+    while (sum >= 10) {
+      sum -= 10;
+    }
+    return sum;
+  }
+
+  isValid(ean: string) {
+    return this.calcChecksum(ean.slice(0, 12)) === parseInt(ean[12], 10);
+  }
+
+  toID(ean: string) {
+    if (!this.isValid(ean)) {
+      throw new Error(`Invalid EAN: "${ean}"`);
+    }
+  }
+
+  fromID(id: string) {
+    assert(id.length === 12);
+    return id + this.calcChecksum(id);
+  }
+}
diff --git a/src/items/items.model.ts b/src/items/items.model.ts
index 29ef4fa..61a6e9e 100644
--- a/src/items/items.model.ts
+++ b/src/items/items.model.ts
@@ -7,6 +7,9 @@ export class ItemModel {
   @Field((type) => ID)
   id: string;
 
+  @Field()
+  ean13: string;
+
   @Field()
   name: string;
 
diff --git a/src/items/items.module.ts b/src/items/items.module.ts
index 8d0ff3a..d2ad63d 100644
--- a/src/items/items.module.ts
+++ b/src/items/items.module.ts
@@ -4,10 +4,11 @@ import { Connection } from 'typeorm';
 import { ItemsResolver } from './items.resolver';
 import { ItemsService } from './items.service';
 import { Item } from './items.entity';
+import { EANService } from './ean/ean.service';
 
 @Module({
   imports: [TypeOrmModule.forFeature([Item])],
-  providers: [ItemsResolver, ItemsService],
+  providers: [ItemsResolver, ItemsService, EANService],
 })
 export class ItemsModule {
   constructor(private connection: Connection) {}
diff --git a/src/items/items.resolver.ts b/src/items/items.resolver.ts
index 8cc21a1..f980371 100644
--- a/src/items/items.resolver.ts
+++ b/src/items/items.resolver.ts
@@ -8,18 +8,24 @@ import {
   Resolver,
 } from '@nestjs/graphql';
 import { NewItemInput } from './dto/new-item.input';
+import { EANService } from './ean/ean.service';
 import { ItemModel } from './items.model';
 import { ItemsService } from './items.service';
 
 @Resolver((of) => ItemModel)
 export class ItemsResolver {
-  constructor(private itemsService: ItemsService) {}
+  constructor(private itemsService: ItemsService, private eans: EANService) {}
 
   @Query((returns) => ItemModel, { nullable: true })
   async item(@Args('id', { type: () => ID }) id: string) {
     return this.itemsService.getItem(id);
   }
 
+  @ResolveField()
+  ean13(@Parent() item: ItemModel) {
+    return this.eans.fromID(item.id);
+  }
+
   @ResolveField()
   async parent(@Parent() item: ItemModel) {
     return this.itemsService.getItemParent(item);