How much third world debt?

The developing countries are even more in debt than previously thought.

A survey by the Paris-based Organization for Economic Cooperation and Development estimates that these developing countries owe $626 billion in middle- and long-term external debt, up from $530 billion at the end of last year. The latter figure is more comprehensive than the $465 billion cited previously by the World Bank for the end of 1981.

The study reckons that between mid-1982 and the end of 1983, developing countries may have to reschedule into longer-term debt some $15 billion of short-term debt they owe commercial banks.

The bulk of this rescheduled money will be owed by Latin American nations.

The OECD survey cautions, however, there is considerable uncertainty as to the size of shortfalls in current debt-service payments by certain debtors in Latin America. In fact, the study was probably put together before the seriousness of Brazil's debt problems became evident.

Other facts noted by the survey include:

* Excluding short-term debt, during 1979-81 renegotiated debt service to private banks represented on average less than 3 percent of total debt service due by the developing countries and about 6 percent of debt-service obligations due to banks. Depending on the outcome of current negotiations with several large debtors, and in particular Argentina and Mexico, these ratios may increase this year.

* Particularly during the past four or five years, debt-service payments have increased rapidly. They have reached an estimated $131 billion this year, with interest payments accounting for $60 billion and amortization payments for $71 billion.

* The annual nominal interest costs on fixed-interest debt (medium and long term) rose ''moderately'' between 1972 and 1982, from 5 to 7.9 percent. On floating-interest debt - essentially Eurocurrency debt - interest increased ''sharply,'' from 7.8 percent to 17.5 percent. The OECD study stated a marked decline is expected in 1983 because of a declining London Inter-Bank Offer Rate.

The average interest rate on all types of outstanding debt climbed to 11.3 percent this year.

How much developing countries owe their creditors Total debt at year-end and annual repayments, including interest charges (Billions of dollars) 1971 '78 '80 '81* 82+ Industrial countries and capital markets Total 71 268 361 407 476 Each year 9.2 51.7 75.8 95.9 115.2 Bank loans as percentage of industrial Country loans Total 15.5 39.2 42.9 44.2 44.1 Each year n.a. 47.6 50.3 51.7 48.6 Multilateral organizations, such as World Bank, Asian Development Bank, etc. Total 10 40 56 65 76 Each year 0.9 3.3 4.8 5.6 6.7 Communist countries Total 6 12 15 17 20 Each year 0.6 1. 1. 2.1 2.5 OPEC 0.0 13 18 23 29 Each year 0.0 1.0 1.9 2.4 3.0 Other less developed countries Total 2 6 8 10 -13 Each year 0.1 0. 1.0 1.3 1.9 Other Total 1 6 7 8 12 Each year 11 59 86.9 109.3 131.3 * Preliminary + Estimate Source: Organization for Economic Cooperation and Development