In all measured indicators the numerator is the debt service annual amount in current US dollars. Its exact "official" definition runs as follows:
Total debt service is the sum of principal repayments and interest actually paid in foreign currency, goods, or services on long-term debt, interest paid on short-term debt, and repayments (repurchases and charges) to the IMF. Data are in current U.S. dollars.
Source : World Bank, Global Development Finance.
The debt service / education budget refers to the ratio between the debt service annual amount in current US dollars and the public spending on education during the same year. Public spending on education is defined as follows:
Education budget (% of GDP) consists of public spending on public education plus subsidies to private education at the primary, secondary, and tertiary levels. (Source : United Nations Educational, Scientific, and Cultural Organization and World Bank and OECD GDP estimates.)
DS / SE = Total debt service (current USD) / [(Public spending on education (% of GDP) / 100) • GDP (current USD)]
A ratio DS/EB = 0,457 means that the debt service represents 45,7% of the education budget. A ratio DS/EB = 3,487 means that the expenses due to debt service is 3,487 times more important than the education budget.
Mode of interpretation : DS/EB measures the burden of the debt service in relation to the country’s public spending on education every year. It shows to what extent debt service prevents spending on education.