How to create Customer Custom Attribute in Magento 2

Here we will learn, How to create Customer Custom attribute in Magento 2

We can achieve this by using the following methods:

1st Method: Using InstallData file
Create the file Webkul\CustomAttribute\Setup\InstallData.php

<?php
namespace Webkul\CustomAttribute\Setup;

use Magento\Customer\Setup\CustomerSetupFactory;
use Magento\Customer\Model\Customer;
use Magento\Eav\Model\Entity\Attribute\Set as AttributeSet;
use Magento\Eav\Model\Entity\Attribute\SetFactory as AttributeSetFactory;
use Magento\Framework\Setup\InstallDataInterface;
use Magento\Framework\Setup\ModuleContextInterface;
use Magento\Framework\Setup\ModuleDataSetupInterface;

class InstallData implements InstallDataInterface
{
    /**
     * @var CustomerSetupFactory
     */
    protected $customerSetupFactory;
    
    /**
     * @var AttributeSetFactory
     */
    private $attributeSetFactory;
    
    /**
     * @param CustomerSetupFactory $customerSetupFactory
     * @param AttributeSetFactory $attributeSetFactory
     */
    public function __construct(
        CustomerSetupFactory $customerSetupFactory,
        AttributeSetFactory $attributeSetFactory
    ) {
        $this->customerSetupFactory = $customerSetupFactory;
        $this->attributeSetFactory = $attributeSetFactory;
    }
 
    public function install(ModuleDataSetupInterface $setup, ModuleContextInterface $context)
    {
        /** @var CustomerSetup $customerSetup */
        $customerSetup = $this->customerSetupFactory->create(['setup' => $setup]);

        $customerEntity = $customerSetup->getEavConfig()->getEntityType('customer');
        $attributeSetId = $customerEntity->getDefaultAttributeSetId();

        /** @var $attributeSet AttributeSet */
        $attributeSet = $this->attributeSetFactory->create();
        $attributeGroupId = $attributeSet->getDefaultGroupId($attributeSetId);

        $customerSetup->addAttribute(Customer::ENTITY, 'custom_cust_attribute', [
            'type' => 'varchar',
            'label' => 'Custom Attribute',
            'input' => 'text',
            'required' => false,
            'visible' => true,
            'user_defined' => true,
            'position' => 999,
            'system' => 0,
        ]);

        $attribute = $customerSetup->getEavConfig()->getAttribute(Customer::ENTITY, 'custom_cust_attribute')
        ->addData([
            'attribute_set_id' => $attributeSetId,
            'attribute_group_id' => $attributeGroupId,
            'used_in_forms' => ['adminhtml_customer'],//you can use other forms also ['adminhtml_customer_address', 'customer_account_edit', 'customer_address_edit', 'customer_register_address', 'customer_account_create']
        ]);

        $attribute->save();
    }
}

InstallData conforms to InstallDataInterface , which requires the implementation of the install method that accepts two parameters of type ModuleDataSetupInterface and ModuleContextInterface .
Using the addAttribute method on the instance of Magento\Customer\Setup\CustomerSetupFactory, we are instructing Magento to add a number of attributes.
Note: The module’s module.xml file must have setup_version to create customer custom attribute using InstallData. To know steps to create module in Magento2,  you can check our another blog here.

2nd Method: Using Data Patch
In this method, we need to create a data patch file as CreateAttributes.php in our custom module Setup folder. Note: You can also use the different file name and class name.
complete path: app/code/Webkul/CustomAttribute/Setup/Patch/Data/CreateAttributes.php

To know more about data and schema patches, click here.

<?php
namespace Webkul\CustomAttribute\Setup\Patch\Data;

use Magento\Framework\Setup\Patch\DataPatchInterface;
use Magento\Customer\Setup\CustomerSetupFactory;
use Magento\Customer\Model\Customer;
use Magento\Eav\Model\Entity\Attribute\SetFactory as AttributeSetFactory;
use Magento\Framework\Setup\ModuleDataSetupInterface;

class CreateAttributes implements DataPatchInterface
{
    /**
     * @var ModuleDataSetupInterface
     */
    private $moduleDataSetup;

    /**
     * @var CustomerSetupFactory
     */
    protected $customerSetupFactory;

    /**
     * @var AttributeSetFactory
     */
    private $attributeSetFactory;

    /**
     * @param ModuleDataSetupInterface $moduleDataSetup
     * @param CustomerSetupFactory $customerSetupFactory
     * @param AttributeSetFactory $attributeSetFactory
     */
    public function __construct(
        ModuleDataSetupInterface $moduleDataSetup,
        CustomerSetupFactory $customerSetupFactory,
        AttributeSetFactory $attributeSetFactory
    ) {
        $this->moduleDataSetup = $moduleDataSetup;
        $this->customerSetupFactory = $customerSetupFactory;
        $this->attributeSetFactory = $attributeSetFactory;
    }

    /**
     * Add eav attributes
     */
    public function apply()
    {
        /** @var CustomerSetup $customerSetup */
        $customerSetup = $this->customerSetupFactory->create(['setup' => $this->moduleDataSetup]);

        $customerEntity = $customerSetup->getEavConfig()->getEntityType('customer');
        $attributeSetId = $customerEntity->getDefaultAttributeSetId();

        /** @var $attributeSet AttributeSet */
        $attributeSet = $this->attributeSetFactory->create();
        $attributeGroupId = $attributeSet->getDefaultGroupId($attributeSetId);

        $customerSetup->addAttribute(Customer::ENTITY, 'custom_customer_attribute', [
            'type' => 'varchar',
            'label' => 'Custom Attribute',
            'input' => 'text',
            'required' => false,
            'visible' => true,
            'user_defined' => true,
            'position' => 999,
            'system' => 0,
        ]);

        $attribute = $customerSetup->getEavConfig()->getAttribute(Customer::ENTITY, 'custom_customer_attribute')
        ->addData([
            'attribute_set_id' => $attributeSetId,
            'attribute_group_id' => $attributeGroupId,
            'used_in_forms' => ['adminhtml_customer'],//you can use other forms also ['adminhtml_customer_address', 'customer_account_edit', 'customer_address_edit', 'customer_register_address', 'customer_account_create']
        ]);

        $attribute->save();
    }

    /**
     * Get dependencies
     */
    public static function getDependencies()
    {
        return [];
    }

    /**
     * Get Aliases
     */
    public function getAliases()
    {
        return [];
    }
}

After adding these files, run the following command in your magento2 root directory through terminal.

php bin/magento setup:upgrade

Hope it will help you. Thank you.