diff --git a/CHANGELOG.markdown b/CHANGELOG.markdown
index 815e9e7..9362df9 100644
--- a/CHANGELOG.markdown
+++ b/CHANGELOG.markdown
@@ -1,4 +1,4 @@
 0.1.1.0
 -------
 * parse* functions fails if the parser does not consume all of the inputs
-* generate '#product=' and 'sizeproduct=' in header line of OPB/WBO files
+* generate '#product=', 'sizeproduct=' and '#soft=' in header line of OPB/WBO files
diff --git a/src/Data/PseudoBoolean/Builder.hs b/src/Data/PseudoBoolean/Builder.hs
index 5c9a430..3c34827 100644
--- a/src/Data/PseudoBoolean/Builder.hs
+++ b/src/Data/PseudoBoolean/Builder.hs
@@ -60,6 +60,7 @@ wboBuilder wbo = size <> part1 <> part2
     sp = Prelude.sum [IntSet.size tm | tm <- Set.toList p]
     size = fromString (printf "* #variable= %d #constraint= %d" nv nc)
          <> (if np >= 1 then fromString (printf " #product= %d sizeproduct= %d" np sp) else mempty)
+         <> fromString (printf " #soft= %d" (wboNumSoft wbo))
          <> fromString "\n"
     part1 = 
       case wboTopCost wbo of
diff --git a/src/Data/PseudoBoolean/ByteStringBuilder.hs b/src/Data/PseudoBoolean/ByteStringBuilder.hs
index d29c288..1ad68d0 100644
--- a/src/Data/PseudoBoolean/ByteStringBuilder.hs
+++ b/src/Data/PseudoBoolean/ByteStringBuilder.hs
@@ -66,6 +66,7 @@ wboBuilder wbo = size <> part1 <> part2
     sp = Prelude.sum [IntSet.size tm | tm <- Set.toList p]
     size = string7 "* #variable= " <> intDec nv <> string7 " #constraint= " <> intDec nc
          <> (if np >= 1 then string7 " #product= " <> intDec np <> string7 " sizeproduct= " <> intDec sp else mempty)
+         <> string7 " #soft= %d" <> intDec (wboNumSoft wbo)
          <> char7 '\n'
     part1 = 
       case wboTopCost wbo of
diff --git a/src/Data/PseudoBoolean/Types.hs b/src/Data/PseudoBoolean/Types.hs
index c1fce37..785b7bd 100644
--- a/src/Data/PseudoBoolean/Types.hs
+++ b/src/Data/PseudoBoolean/Types.hs
@@ -36,6 +36,7 @@ module Data.PseudoBoolean.Types
   , pbProducts
   , wboComputeNumVars
   , wboProducts
+  , wboNumSoft
   ) where
 
 import GHC.Generics (Generic)
@@ -140,3 +141,6 @@ wboProducts softformula = Set.fromList $ do
   let tm2 = IntSet.fromList tm
   guard $ IntSet.size tm2 > 1
   return tm2
+
+wboNumSoft :: SoftFormula -> Int
+wboNumSoft softformula = length [() | (Just _, _) <- wboConstraints softformula]