diff --git a/changelog.md b/changelog.md
--- a/changelog.md
+++ b/changelog.md
@@ -1,3 +1,7 @@
+0.8
+
+* Add decimal arithmetic
+
 0.6
 
 * Removed the `Digit` datatype
diff --git a/digit.cabal b/digit.cabal
--- a/digit.cabal
+++ b/digit.cabal
@@ -1,5 +1,5 @@
 name:               digit
-version:            0.7
+version:            0.8
 license:            BSD3
 license-file:       LICENCE
 author:             Queensland Functional Programming Lab <oᴉ˙ldɟb@llǝʞsɐɥ>
@@ -13,7 +13,7 @@
 cabal-version:      >= 1.10
 build-type:         Simple
 extra-source-files: CONTRIBUTORS, changelog.md
-tested-with:        GHC == 8.4.1, GHC == 8.2.2, GHC == 8.0.2, GHC == 7.10.3
+tested-with:        GHC == 8.6.1, GHC == 8.4.3, GHC == 8.2.2, GHC == 8.0.2, GHC == 7.10.3
 description:
   <<http://i.imgur.com/uZnp9ke.png>>
   .
@@ -28,7 +28,7 @@
                     Haskell2010
 
   build-depends:
-                    base               >= 4.7 && < 5
+                    base               >= 4.8 && < 4.13
                     , lens             >= 4.0 && < 5
                     , parsers          >= 0.12.3 && < 0.13
                     , semigroups       >= 0.12 && < 1.0
@@ -87,17 +87,17 @@
                     Haskell2010
 
   build-depends:
-                    base             >=4.8    && <4.12
+                    base             >=4.8    && <4.13
                     , lens           >=4.0    && <5
                     , ansi-wl-pprint >=0.6    && <0.7
-                    , hedgehog       >=0.5    && <0.7
-                    , tasty          >=0.11   && <1.2
+                    , hedgehog       >=0.5    && <1.1
+                    , tasty          >=0.11   && <1.3
                     , tasty-hspec    >=1.1    && <1.2
                     , parsec         >=3.1    && <3.2
                     , parsers        >=0.12.3 && <0.13
                     , pretty         >=1.1    && <1.2
                     , text           >=1.2    && <1.3
-                    , tasty-hedgehog >=0.1    && <0.3
+                    , tasty-hedgehog >=0.1    && <1.1
                     , tasty-hunit    >=0.9    && <0.11
                     , semigroups     >= 0.12 && < 1.0
                     , digit
diff --git a/src/Data/Digit/Integral.hs b/src/Data/Digit/Integral.hs
--- a/src/Data/Digit/Integral.hs
+++ b/src/Data/Digit/Integral.hs
@@ -30,24 +30,30 @@
 , integralHeXaDeCiMaLNoZero
 , integralHeXaDeCiMaL
 , _HeXDigitsIntegral
+, mod10
+, addDecDigit
+, addDecDigit'
 ) where
 
 import           Prelude                (Eq, Integral, error, fst, lookup,
-                                         quotRem, (*), (+), (-), (==), (>=))
+                                         quotRem, (*), (+), (-), (==), (>=), mod, divMod)
 
 import           Control.Applicative    (Applicative)
 import           Control.Category       (id, (.))
 import           Control.Lens           (APrism, Choice, Prism', Review,
-                                         clonePrism, outside, prism', unto,
-                                         ( # ), (.~), (^?!))
+                                         clonePrism, outside, prism', unto, over, _1,
+                                         ( # ), (.~), (^?!), (^?))
 import           Control.Lens.Extras    (is)
 
+import           Data.Bool              (Bool, bool)
 import           Data.Either            (Either (..), either)
 import           Data.Foldable          (find, foldl')
 import           Data.Function          (($),const)
 import           Data.Functor           ((<$>))
+import           Data.Int               (Int)
 import           Data.List.NonEmpty     (NonEmpty)
 import           Data.Maybe             (fromMaybe)
+import           Data.Ord               ((>))
 
 import           Data.Digit.Binary
 import           Data.Digit.Decimal
@@ -505,6 +511,30 @@
 _HeXDigitsIntegral = either (\n -> -(go n) - 1) go
   where
     go = foldl' (\b a -> (integralHeXaDeCiMaL # a) + 16 * b) 0
+
+mod10 ::
+  Integral a =>
+  a
+  -> DecDigit
+mod10 n =
+  let r = n `mod` 10
+  in fromMaybe (mod10 r) (r ^? integralDecimal)
+
+addDecDigit ::
+  DecDigit
+  -> DecDigit
+  -> (Bool, DecDigit)
+addDecDigit a b =
+  let (x, r) =
+        (integralDecimal # a + integralDecimal # b) `divMod` 10
+  in  (x > 0, mod10 (r :: Int))
+
+addDecDigit' ::
+  DecDigit
+  -> DecDigit
+  -> (DecDigit, DecDigit)
+addDecDigit' a b =
+  over _1 (bool x0 x1) (addDecDigit a b)
 
 ---- not exported
 associatePrism ::