I'm writing a business plan and would like to determine the beta values for similar companies. The problem is that almost all are private. Should I just use the beta values of the nearest public companies, or is there a way to estimate beta for private companies? Could I use their history of previous valuations and compare this to the market, for example?

If you cannot find a similar company to yours, you should try to look for companies that are affected by the same external factors and changes in the economy.

For example, if you are a car manufacturer, and you cannot find a similar car producer, then maybe you could find a listed tire manufacturer in the same size as yours.

I am not sure the example was very good, but the reply to this question is, that look for company's that are affected by the same economy movements as your company.

VALUING PRIVATE COMPANIES AND DIVISIONS
SSRN-The Private Company Discount by Atulya Sarin, John Koeplin, Alan Shapiro
and look at
Principles of private firm valuation by Stanley J. Feldman

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