Look up the country's national debt in an online reference, such as the CIA World Factbook (see References), which maintains an annual list of all countries ranked by the size of their debt relative to their GDP.
Determine the actual dollar amount of debt, if you are starting with a GDP percentage, by multiplying the percentage by the country's GDP. For example, the most indebted country in the world in 2009 was Zimbabwe, with a debt of approximately 304.3 percent of GDP. Zimbabwe's GDP was $332.1 million US, which multiplied by 3.043 yields a debt of $1.01 billion US.
Calculate the percentage of GDP from a real dollar amount by dividing a national debt by GDP. For example, Japan's 2009 national debt was $7.955 trillion. Divide this by its GDP of $4.14 trillion for a rate of 192.1 percent, the second highest in the world.