diff --git a/aviation-units.cabal b/aviation-units.cabal
--- a/aviation-units.cabal
+++ b/aviation-units.cabal
@@ -1,5 +1,5 @@
 name:               aviation-units
-version:            0.0.2
+version:            0.0.3
 license:            BSD3
 license-file:       LICENCE
 author:             Tony Morris <ʇǝu˙sıɹɹoɯʇ@ןןǝʞsɐɥ>, Joshua Morris <ɯoɔ˙lᴉɐɯƃ@llǝʞsɐɥ+sᴉɹɹoɯuɥoɾɐnɥsoɾ>
@@ -47,6 +47,7 @@
                     Data.Aviation.Units.Inches
                     Data.Aviation.Units.Kilograms
                     Data.Aviation.Units.Litres
+                    Data.Aviation.Units.Millimetres
                     Data.Aviation.Units.Pounds
                     Data.Aviation.Units.Thouinches
                     Data.Aviation.Units.USGallons
diff --git a/changelog b/changelog
--- a/changelog
+++ b/changelog
@@ -1,3 +1,7 @@
+0.0.3
+
+* Add `Millimetres`
+
 0.0.2
 
 * Update GHC version
diff --git a/src/Data/Aviation/Units.hs b/src/Data/Aviation/Units.hs
--- a/src/Data/Aviation/Units.hs
+++ b/src/Data/Aviation/Units.hs
@@ -9,6 +9,7 @@
 import Data.Aviation.Units.Inches as A
 import Data.Aviation.Units.Kilograms as A
 import Data.Aviation.Units.Litres as A
+import Data.Aviation.Units.Millimetres as A
 import Data.Aviation.Units.Pounds as A
 import Data.Aviation.Units.Thouinches as A
 import Data.Aviation.Units.USGallons as A
diff --git a/src/Data/Aviation/Units/Millimetres.hs b/src/Data/Aviation/Units/Millimetres.hs
new file mode 100644
--- /dev/null
+++ b/src/Data/Aviation/Units/Millimetres.hs
@@ -0,0 +1,18 @@
+{-# LANGUAGE NoImplicitPrelude #-}
+{-# LANGUAGE FlexibleInstances #-}
+
+module Data.Aviation.Units.Millimetres(
+  Millimetres(millimetres)
+) where
+
+import Control.Category(id)
+import Control.Lens(Iso')
+import Prelude(Rational)
+
+class Millimetres a where
+  millimetres ::
+    Iso' Rational a
+
+instance Millimetres Rational where
+  millimetres =
+    id