diff --git a/src/Spout/Writer/Common/Helper/CellHelper.php b/src/Spout/Writer/Common/Helper/CellHelper.php
index 50ead93..0bee960 100644
--- a/src/Spout/Writer/Common/Helper/CellHelper.php
+++ b/src/Spout/Writer/Common/Helper/CellHelper.php
@@ -64,6 +64,17 @@ class CellHelper
         return (gettype($value) === 'string' && $value !== '');
     }
 
+    /**
+     * Returns whether the given value looks like a formula
+     *
+     * @param $value
+     * @return bool whether the given value looks like a formula
+     */
+    public static function isFormulaString($value)
+    {
+        return (strpos($value,'=') === 0);
+    }
+
     /**
      * Returns whether the given value is numeric.
      * A numeric value is from type "integer" or "double" ("float" is not returned by gettype).
diff --git a/src/Spout/Writer/XLSX/Internal/Worksheet.php b/src/Spout/Writer/XLSX/Internal/Worksheet.php
index 0bd909d..c636c7c 100644
--- a/src/Spout/Writer/XLSX/Internal/Worksheet.php
+++ b/src/Spout/Writer/XLSX/Internal/Worksheet.php
@@ -212,8 +212,10 @@ EOD;
         $columnIndex = CellHelper::getCellIndexFromColumnIndex($cellNumber);
         $cellXML = '<c r="' . $columnIndex . $rowIndex . '"';
         $cellXML .= ' s="' . $styleId . '"';
-
-        if (CellHelper::isNonEmptyString($cellValue)) {
+        
+        if (CellHelper::isFormulaString($cellValue)) {
+            $cellXML .= '><f>'.substr($cellValue,1).'</f><v>0</v></c>'; // seriously, that's it.
+        } else if (CellHelper::isNonEmptyString($cellValue)) {
             $cellXML .= $this->getCellXMLFragmentForNonEmptyString($cellValue);
         } else if (CellHelper::isBoolean($cellValue)) {
             $cellXML .= ' t="b"><v>' . intval($cellValue) . '</v></c>';