Rules of thumb for business valuation provide a quick method of estimating the value of small to medium size businesses. Business valuation is the process of estimating what it would cost an independent buyer to purchase the entire business.How to Calculate EBITDA. To quickly value a business, find its total liabilities and subtract them from the total assets. This will give you an idea of its book value. Take the business's assets and total them. Then subtract all the liabilities. Figuring out your business's value isn't just about crunching numbers. Interested in Buying a business, Selling a business, or getting a business valuation? Haggling over the price is usually one of the main hurdles in negotiating a business purchase.