diff --git a/src/Spout/Common/Entity/Style/NumberFormat.php b/src/Spout/Common/Entity/Style/NumberFormat.php
index a82fb11..2d9226e 100644
--- a/src/Spout/Common/Entity/Style/NumberFormat.php
+++ b/src/Spout/Common/Entity/Style/NumberFormat.php
@@ -36,16 +36,6 @@ class NumberFormat
         }
     }
 
-    /**
-     * @param int $id
-     * @return NumberFormat
-     */
-    public function setId($id)
-    {
-        $this->id = $id;
-        return $this;
-    }
-
     /**
      * @param bool $commas
      * @return NumberFormat
@@ -56,14 +46,6 @@ class NumberFormat
         return $this;
     }
 
-    /**
-     * @return int|null
-     */
-    public function getId()
-    {
-        return $this->id;
-    }
-
     /**
      * @param int $type
      * @return NumberFormat
diff --git a/src/Spout/Writer/XLSX/Manager/Style/StyleManager.php b/src/Spout/Writer/XLSX/Manager/Style/StyleManager.php
index 5a50476..6de247b 100644
--- a/src/Spout/Writer/XLSX/Manager/Style/StyleManager.php
+++ b/src/Spout/Writer/XLSX/Manager/Style/StyleManager.php
@@ -75,7 +75,8 @@ EOD;
         foreach ($registeredFormats as $styleId) {
             $style = $this->styleRegistry->getStyleFromStyleId($styleId);
             $format = $style->getNumberFormat();
-            $content .= '<numFmt numFmtId="'.$format->getId().'" formatCode="'.$format->getFormatCode().'"/>';
+            $formatId = $this->styleRegistry->getNumberFormatIdForStyleId($styleId);
+            $content .= '<numFmt numFmtId="'.$formatId.'" formatCode="'.$format->getFormatCode().'"/>';
         }
 
         $content .= '</numFmts>';
diff --git a/src/Spout/Writer/XLSX/Manager/Style/StyleRegistry.php b/src/Spout/Writer/XLSX/Manager/Style/StyleRegistry.php
index 4bf1d82..66a13d6 100644
--- a/src/Spout/Writer/XLSX/Manager/Style/StyleRegistry.php
+++ b/src/Spout/Writer/XLSX/Manager/Style/StyleRegistry.php
@@ -121,10 +121,8 @@ class StyleRegistry extends \Box\Spout\Writer\Common\Manager\Style\StyleRegistry
                 $registeredFormatId = $this->styleIdToNumberFormatMappingTable[$registeredStyleId];
                 $this->styleIdToNumberFormatMappingTable[$styleId] = $registeredFormatId;
             } else {
-                $formatId = count($this->registeredNumberFormats);
                 $this->registeredNumberFormats[$serializedFormat] = $styleId;
-                $this->styleIdToNumberFormatMappingTable[$styleId] = $formatId;
-                $format->setId($formatId);
+                $this->styleIdToNumberFormatMappingTable[$styleId] = count($this->registeredNumberFormats);
             }
         } else {
             // The fillId maps a style to a fill declaration