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Formatting number input with Rails, Bootstrap, SimpleForm and autoNumeric.js

Often you want to format your number inputs in a pretty and proper way for your users. Using Bootstrap and SimpleForm within your Rails application is a good way to start, but how will you deal with number formatting within your input field for decimal and thousands separators?

autoNumeric.js is an awesome jQuery plugin which solves almost all of your number formatting struggles. This blog post will show you how you can use autoNumeric in combination with Bootstrap and SimpleForm to easily get your number input formatting done. Bootstrap, SimpleForm and jQuery will be a prerequisite of your Rails application.

  1. To use autoNumeric in your Rails application, please follow the installation guideline at
  2. Next, we want to create our custom Currency input type within SimpleForm, so that we can use it within our forms. The currency input type will show an € sign in front of the field and it will automatically format the input while typing. To do so, create a new folder app/inputs and add the file currency_input.rb. Add the following code:
    class CurrencyInput < SimpleForm::Inputs::StringInput
      def input(wrapper_options)
        input_options = input_html_options.merge({data: {autonumeric: true}})
        merged_input_options = merge_wrapper_options(input_options, wrapper_options)
        template.content_tag(:div, class: 'input-group') do
          template.concat span_currency_sign
          template.concat @builder.text_field(attribute_name, merged_input_options)
      def span_currency_sign
        template.content_tag(:span, class: 'input-group-addon') do
          template.concat '€'.html_safe

    This code adds the autonumeric data option to your input field and also adds the € sign within an HTML span in front of your input field. By using the class input-group, Bootstrap will render the input field in a proper way and on a single line.

  3. Restart your server.
  4. Head over to your view file which holds your form (by utilizing the simple_form_for helper) and add the following line:
    = f.input :acquisition_cost, as: :currency
  5. Thats it! Open your form and you will see a nicely formatted input field with a leading € sign and your input will be augmented with decimal and thousands separators while typing.