diff --git a/src/Spout/Common/Entity/Style/Style.php b/src/Spout/Common/Entity/Style/Style.php
index 261e9bf..d544007 100644
--- a/src/Spout/Common/Entity/Style/Style.php
+++ b/src/Spout/Common/Entity/Style/Style.php
@@ -72,6 +72,9 @@ class Style
     private $hasSetBackgroundColor = false;
 
     private $numberFormat;
+    /** @var bool Whether border properties should be applied */
+    private $shouldApplyNumberFormat = false;
+
 
     /**
      * @return int|null
@@ -117,7 +120,7 @@ class Style
      */
     public function getNumberFormat()
     {
-        return $this->numberFormat ?? null;
+        return $this->numberFormat;
     }
 
     /**
@@ -126,6 +129,7 @@ class Style
      */
     public function setNumberFormat(NumberFormat $format)
     {
+        $this->shouldApplyNumberFormat = true;
         $this->numberFormat = $format;
 
         return $this;
@@ -138,6 +142,13 @@ class Style
     {
         return $this->shouldApplyBorder;
     }
+    /**
+     * @return bool
+     */
+    public function shouldApplyNumberFormat()
+    {
+        return $this->shouldApplyNumberFormat;
+    }
 
     /**
      * @return bool
diff --git a/src/Spout/Writer/Common/Manager/Style/StyleRegistry.php b/src/Spout/Writer/Common/Manager/Style/StyleRegistry.php
index d33b3f2..d2f5bc3 100644
--- a/src/Spout/Writer/Common/Manager/Style/StyleRegistry.php
+++ b/src/Spout/Writer/Common/Manager/Style/StyleRegistry.php
@@ -2,7 +2,6 @@
 
 namespace Box\Spout\Writer\Common\Manager\Style;
 
-use Box\Spout\Common\Entity\Style\NumberFormat;
 use Box\Spout\Common\Entity\Style\Style;
 
 /**
@@ -38,11 +37,6 @@ class StyleRegistry
      */
     public function registerStyle(Style $style)
     {
-        $format = $style->getNumberFormat();
-        if (!empty($format)) {
-            $registeredFormat = $this->registerNumberFormat($format);
-            $style->setNumberFormat($registeredFormat);
-        }
         $serializedStyle = $this->serialize($style);
 
         if (!$this->hasStyleAlreadyBeenRegistered($style)) {
@@ -56,18 +50,6 @@ class StyleRegistry
         return $this->getStyleFromSerializedStyle($serializedStyle);
     }
 
-    public function registerNumberFormat(NumberFormat $format)
-    {
-        $serializedFormat = $this->serializeFormat($format);
-        if (!$this->hasFormatAlreadyBeenRegistered($format)) {
-            $nextFormatId = count($this->serializedNumberFormatToFormatIdMappingTable);
-            $format->setId($nextFormatId);
-
-            $this->serializedNumberFormatToFormatIdMappingTable[$serializedFormat] = $nextFormatId;
-            $this->numberFormats[$nextFormatId] = $format;
-        }
-        return $this->getFormatFromSerializedFormat($serializedFormat);
-    }
 
     /**
      * Returns whether the given style has already been registered.
@@ -83,19 +65,6 @@ class StyleRegistry
         return isset($this->serializedStyleToStyleIdMappingTable[$serializedStyle]);
     }
 
-    /**
-     * Returns whether the given number format has already been registered.
-     *
-     * @param NumberFormat $format
-     * @return bool
-     */
-    protected function hasFormatAlreadyBeenRegistered(NumberFormat $format)
-    {
-        $serializedFormat = $this->serializeFormat($format);
-
-        // Using isset here because it is way faster than array_key_exists...
-        return isset($this->serializedNumberFormatToFormatIdMappingTable[$serializedFormat]);
-    }
 
     /**
      * Returns the registered style associated to the given serialization.
@@ -110,19 +79,6 @@ class StyleRegistry
         return $this->styleIdToStyleMappingTable[$styleId];
     }
 
-    /**
-     * Returns the registered number format associated to the given serialization.
-     *
-     * @param string $serializedFormat The serialized number format from which the actual format should be fetched from
-     * @return NumberFormat
-     */
-    protected function getFormatFromSerializedFormat($serializedFormat)
-    {
-        $formatId = $this->serializedNumberFormatToFormatIdMappingTable[$serializedFormat];
-
-        return $this->numberFormats[$formatId];
-    }
-
     /**
      * @return Style[] List of registered styles
      */
@@ -131,14 +87,6 @@ class StyleRegistry
         return array_values($this->styleIdToStyleMappingTable);
     }
 
-    /**
-     * @return NumberFormat[] List of registered number formats
-     */
-    public function getRegisteredNumberFormats()
-    {
-        return array_values($this->numberFormats);
-    }
-
     /**
      * @param int $styleId
      * @return Style
@@ -168,25 +116,4 @@ class StyleRegistry
 
         return $serializedStyle;
     }
-
-    /**
-     * Serializes the number format for future comparison with other formats.
-     * The ID is excluded from the comparison, as we only care about
-     * actual number format properties.
-     *
-     * @param Style $style
-     * @return string The serialized style
-     */
-    public function serializeFormat(NumberFormat $format)
-    {
-        // In order to be able to properly compare style, set static ID value
-        $currentId = $format->getId();
-        $format->setId(0);
-
-        $serializedFormat = serialize($format);
-
-        $format->setId($currentId);
-
-        return $serializedFormat;
-    }
 }
diff --git a/src/Spout/Writer/XLSX/Manager/Style/StyleManager.php b/src/Spout/Writer/XLSX/Manager/Style/StyleManager.php
index 83b26a9..f8365c9 100644
--- a/src/Spout/Writer/XLSX/Manager/Style/StyleManager.php
+++ b/src/Spout/Writer/XLSX/Manager/Style/StyleManager.php
@@ -3,6 +3,7 @@
 namespace Box\Spout\Writer\XLSX\Manager\Style;
 
 use Box\Spout\Common\Entity\Style\Color;
+use Box\Spout\Common\Entity\Style\NumberFormat;
 use Box\Spout\Common\Entity\Style\Style;
 use Box\Spout\Writer\XLSX\Helper\BorderHelper;
 
@@ -70,6 +71,7 @@ EOD;
         }
 
         $content = '<numFmts count="'.count($registeredFormats).'">';
+        /** @var NumberFormat $format */
         foreach ($registeredFormats as $format) {
 
             $content .= '<numFmt numFmtId="'.$format->getId().'" formatCode="'.$format->getFormatCode().'"/>';
diff --git a/src/Spout/Writer/XLSX/Manager/Style/StyleRegistry.php b/src/Spout/Writer/XLSX/Manager/Style/StyleRegistry.php
index abd89f1..9b61acd 100644
--- a/src/Spout/Writer/XLSX/Manager/Style/StyleRegistry.php
+++ b/src/Spout/Writer/XLSX/Manager/Style/StyleRegistry.php
@@ -20,6 +20,10 @@ class StyleRegistry extends \Box\Spout\Writer\Common\Manager\Style\StyleRegistry
      */
     protected $styleIdToFillMappingTable = [];
 
+    protected $registeredNumberFormats = [];
+
+    protected $styleIdToNumberFormatMappingTable = [];
+
     /**
      * Excel preserves two default fills with index 0 and 1
      * Since Excel is the dominant vendor - we play along here
@@ -49,6 +53,7 @@ class StyleRegistry extends \Box\Spout\Writer\Common\Manager\Style\StyleRegistry
         $registeredStyle = parent::registerStyle($style);
         $this->registerFill($registeredStyle);
         $this->registerBorder($registeredStyle);
+        $this->registerNumberFormat($registeredStyle);
 
         return $registeredStyle;
     }
@@ -96,6 +101,47 @@ class StyleRegistry extends \Box\Spout\Writer\Common\Manager\Style\StyleRegistry
             null;
     }
 
+    /**
+     * Register a number format definition
+     *
+     * @param Style $style
+     */
+    private function registerNumberFormat(Style $style)
+    {
+        $styleId = $style->getId();
+
+        if ($style->shouldApplyNumberFormat()) {
+            $format = $style->getNumberFormat();
+            $serializedFormat = serialize($format);
+            $isNumberFormatRegistered = isset($this->registeredNumberFormats[$serializedFormat]);
+
+            // We need to track the already registered background definitions
+            if ($isNumberFormatRegistered) {
+                $registeredStyleId = $this->registeredFills[$serializedFormat];
+                $registeredFormatId = $this->styleIdToNumberFormatMappingTable[$registeredStyleId];
+                $this->styleIdToNumberFormatMappingTable[$styleId] = $registeredFormatId;
+            } else {
+                $this->registeredFills[$serializedFormat] = $styleId;
+                $this->styleIdToNumberFormatMappingTable[$styleId] = $this->fillIndex++;
+            }
+        } else {
+            // The fillId maps a style to a fill declaration
+            // When there is no background color definition - we default to 0
+            $this->styleIdToNumberFormatMappingTable[$styleId] = 0;
+        }
+    }
+
+    /**
+     * @param int $styleId
+     * @return int|null Number Format ID associated to the given style ID
+     */
+    public function getNumberFormatIdForStyleId($styleId)
+    {
+        return (isset($this->styleIdToNumberFormatMappingTable[$styleId])) ?
+            $this->styleIdToNumberFormatMappingTable[$styleId] :
+            null;
+    }
+
     /**
      * Register a border definition
      *
@@ -151,4 +197,11 @@ class StyleRegistry extends \Box\Spout\Writer\Common\Manager\Style\StyleRegistry
     {
         return $this->registeredBorders;
     }
+    /**
+     * @return array
+     */
+    public function getRegisteredNumberFormats()
+    {
+        return $this->registeredNumberFormats;
+    }
 }