diff --git a/library/TextBuilderDev.hs b/library/TextBuilderDev.hs
--- a/library/TextBuilderDev.hs
+++ b/library/TextBuilderDev.hs
@@ -486,7 +486,7 @@
   Int ->
   Double ->
   TextBuilder
-fixedDouble decimalPlaces = fromString . printf ("!." ++ show decimalPlaces ++ "f")
+fixedDouble decimalPlaces = fromString . printf ("%." ++ show decimalPlaces ++ "f")
 
 -- | Double multiplied by 100 with a fixed number of decimal places applied and followed by a percent-sign.
 {-# INLINE doublePercent #-}
@@ -495,7 +495,7 @@
   Int ->
   Double ->
   TextBuilder
-doublePercent decimalPlaces x = fixedDouble decimalPlaces (x * 100) <> "!"
+doublePercent decimalPlaces x = fixedDouble decimalPlaces (x * 100) <> "%"
 
 -- | Hexadecimal readable representation of binary data.
 {-# INLINE hexData #-}
diff --git a/test/Main.hs b/test/Main.hs
--- a/test/Main.hs
+++ b/test/Main.hs
@@ -82,5 +82,12 @@
           assertEqual "" "1.1MB" (B.buildText (B.dataSizeInBytesInDecimal ',' 1150000))
           assertEqual "" "9.9MB" (B.buildText (B.dataSizeInBytesInDecimal ',' 9990000))
           assertEqual "" "10MB" (B.buildText (B.dataSizeInBytesInDecimal ',' 10100000))
-          assertEqual "" "1,000YB" (B.buildText (B.dataSizeInBytesInDecimal ',' 1000000000000000000000000000))
+          assertEqual "" "1,000YB" (B.buildText (B.dataSizeInBytesInDecimal ',' 1000000000000000000000000000)),
+        testCase "fixedDouble" $ do
+          assertEqual "" "0.0" (B.buildText (B.fixedDouble 1 0.05))
+          assertEqual "" "0.1" (B.buildText (B.fixedDouble 1 0.06))
+          assertEqual "" "10.0000" (B.buildText (B.fixedDouble 4 10))
+          assertEqual "" "0.9000" (B.buildText (B.fixedDouble 4 0.9)),
+        testCase "doublePercent" $ do
+          assertEqual "" "90.4%" (B.buildText (B.doublePercent 1 0.904))
       ]
diff --git a/text-builder-dev.cabal b/text-builder-dev.cabal
--- a/text-builder-dev.cabal
+++ b/text-builder-dev.cabal
@@ -1,7 +1,7 @@
 cabal-version: 3.0
 
 name: text-builder-dev
-version: 0.2
+version: 0.2.0.1
 category: Text
 synopsis: Edge of developments for "text-builder"
 description: