I have product called wood. i want to let the user add their length in inches by which it has to calculate price to show the user how much it costs. I have no idea to achive this.Thanks in advance.
You should create a custom field and a product variation for each length. This will allow you to set a specific price, depending on your length.
I came across the same issue for a customer and we decided to develop a set of modules to make this a straight forward process and to also have a feature to update the product price in the product view dynamically when the customer is entering one or more dimensions.
Please have a look at https://drupal.org/project/commerce_product_dimensions a file any issues or questions there in the issue queue.