diff --git a/ChangeLog.md b/ChangeLog.md
--- a/ChangeLog.md
+++ b/ChangeLog.md
@@ -1,5 +1,9 @@
 # Changelog for geomancy
 
+## 0.3.1.0
+
+- Fixed Block.sizeOf40 for Packed 3-vectors (was 16, now 12).
+
 ## 0.3.0.1
 
 - Added Lift instances for data.
diff --git a/geomancy.cabal b/geomancy.cabal
--- a/geomancy.cabal
+++ b/geomancy.cabal
@@ -5,7 +5,7 @@
 -- see: https://github.com/sol/hpack
 
 name:           geomancy
-version:        0.3.0.1
+version:        0.3.1.0
 synopsis:       Vectors and matrix manipulation
 description:    Sometimes it is unavoidable you have to do stuff on CPU.
                 Let's at least do it faster.
@@ -91,7 +91,7 @@
       Paths_geomancy
   hs-source-dirs:
       test
-  ghc-options: -Wall -threaded -rtsopts -with-rtsopts=-N -with-rtsopts=-qn1 -with-rtsopts=-A128m
+  ghc-options: -Wall -threaded -rtsopts "-with-rtsopts=-N -qn1 -A128m"
   build-depends:
       base >=4.7 && <5
     , deepseq
@@ -108,7 +108,7 @@
       Paths_geomancy
   hs-source-dirs:
       bench
-  ghc-options: -Wall -threaded -rtsopts -with-rtsopts=-N -with-rtsopts=-qn1 -with-rtsopts=-A16m
+  ghc-options: -Wall -O2 -threaded -rtsopts "-with-rtsopts=-N1 -A128m -s"
   build-depends:
       base >=4.7 && <5
     , criterion
diff --git a/src/Geomancy/IVec3.hs b/src/Geomancy/IVec3.hs
--- a/src/Geomancy/IVec3.hs
+++ b/src/Geomancy/IVec3.hs
@@ -253,7 +253,7 @@
 
 instance Block Packed where
   type PackedSize Packed = 12
-  alignment140 _  = 16
+  alignment140 _  = 12
   sizeOf140 _     = 16
   alignment430    = alignment140
   sizeOf430       = sizeOf140
diff --git a/src/Geomancy/UVec3.hs b/src/Geomancy/UVec3.hs
--- a/src/Geomancy/UVec3.hs
+++ b/src/Geomancy/UVec3.hs
@@ -267,7 +267,7 @@
 instance Block Packed where
   type PackedSize Packed = 12
   alignment140 _  = 16
-  sizeOf140 _     = 16
+  sizeOf140 _     = 12
   alignment430    = alignment140
   sizeOf430       = sizeOf140
   isStruct _      = False
diff --git a/src/Geomancy/Vec3.hs b/src/Geomancy/Vec3.hs
--- a/src/Geomancy/Vec3.hs
+++ b/src/Geomancy/Vec3.hs
@@ -435,7 +435,7 @@
 instance Block Packed where
   type PackedSize Packed = 12
   alignment140 _  = 16
-  sizeOf140 _     = 16
+  sizeOf140 _     = 12
   alignment430    = alignment140
   sizeOf430       = sizeOf140
   isStruct _      = False