diff --git a/app/lib/code_insee.rb b/app/lib/code_insee.rb
new file mode 100644
index 000000000..358af2279
--- /dev/null
+++ b/app/lib/code_insee.rb
@@ -0,0 +1,14 @@
+class CodeInsee
+  def initialize(code_insee)
+    @code_insee = code_insee
+  end
+
+  def to_departement
+    dept = @code_insee.strip.first(2)
+    if dept < "97"
+      dept
+    else
+      @code_insee.strip.first(3)
+    end
+  end
+end
diff --git a/spec/lib/code_insee_spec.rb b/spec/lib/code_insee_spec.rb
new file mode 100644
index 000000000..817aac94d
--- /dev/null
+++ b/spec/lib/code_insee_spec.rb
@@ -0,0 +1,9 @@
+describe CodeInsee do
+  it 'converts to departement' do
+    expect(CodeInsee.new('75002').to_departement).to eq('75')
+    expect(CodeInsee.new('2B033').to_departement).to eq('2B')
+    expect(CodeInsee.new('01345').to_departement).to eq('01')
+    expect(CodeInsee.new('97100').to_departement).to eq('971')
+    expect(CodeInsee.new('98735').to_departement).to eq('987')
+  end
+end