diff --git a/bench/Main.hs b/bench/Main.hs
index 41d619e..9e285bf 100644
--- a/bench/Main.hs
+++ b/bench/Main.hs
@@ -20,7 +20,8 @@ makeBenchW64s = do
   let files = ("data/bench/" ++) <$> (".word64s" `isSuffixOf`) `filter` entries
   benchmarks <- forM files $ \file -> return
     [ env (IO.mmapFromForeignRegion file) $ \(v :: DVS.Vector Word64) -> bgroup "blah" $ mempty
-      <> [bench ("DVS.size" <> file) (whnf (PV.fromList (HW.length v)) (DVS.toList v))]
+      <> [bench ("DVS.fromList"  <> file) (whnf (PV.fromList                10) (DVS.toList v))]
+      <> [bench ("DVS.fromListN" <> file) (whnf (PV.fromListN (HW.length v) 10) (DVS.toList v))]
     ]
   return (join benchmarks)
 
diff --git a/src/HaskellWorks/Data/PackedVector/PackedVector64.hs b/src/HaskellWorks/Data/PackedVector/PackedVector64.hs
index 2afbfb0..b8723ce 100644
--- a/src/HaskellWorks/Data/PackedVector/PackedVector64.hs
+++ b/src/HaskellWorks/Data/PackedVector/PackedVector64.hs
@@ -5,6 +5,7 @@ module HaskellWorks.Data.PackedVector.PackedVector64
   ( PackedVector64(..)
   , empty
   , fromList
+  , fromListN
   , toList
   ) where
 
@@ -65,11 +66,17 @@ instance AtIndex PackedVector64 where
   {-# INLINE atIndex #-}
 
 fromList :: Count -> [Word64] -> PackedVector64
-fromList wl ws =
-  PackedVector64
-  { swBuffer    = DVS.fromList (packBits wl ws)
+fromList wordLength ws = PackedVector64
+  { swBuffer    = DVS.fromList (packBits wordLength ws)
+  , swBufferLen = fromIntegral (length ws)
+  , swBitSize   = fromIntegral wordLength
+  }
+
+fromListN :: Count -> Count -> [Word64] -> PackedVector64
+fromListN vectorSize wordLength ws = PackedVector64
+  { swBuffer    = DVS.fromListN (fromIntegral vectorSize) (packBits wordLength ws)
   , swBufferLen = fromIntegral (length ws)
-  , swBitSize   = fromIntegral wl
+  , swBitSize   = fromIntegral wordLength
   }
 
 toList :: PackedVector64 -> [Word64]