"SDR Valuation", International Monetary Fund website: "The currency value of the SDR is determined by summing the values in U.S. dollars, based on market exchange rates, of a basket of major currencies (the U.S. dollar, Euro, Japanese yen, and pound sterling). The SDR currency value is calculated daily (except on IMF holidays or whenever the IMF is closed for business) and the valuation basket is reviewed and adjusted every five years."
Eichengreen, Barry (May 2005). Sterling's Past, Dollar's Future: Historical Perspectives on Reserve Currency Competition (Report). National Bureau of Economic Research (NBER). SSRN723305.