Concentrated water–alkaline mixtures of sodium and potassium borohydrides and borates are used as fuel and hydrogen sources in hydrogen power engineering, including low-temperature fuel cells. The performance of such mixtures is determined by their physicochemical properties. An algorithm to calculate the density of mixed solutions of the five-component (Na, K)BH4 + (Na, K)BO2 + (Na, K)OH + H2O system based on density quasi-additivity is proposed. The density–concentration–temperature dependences for aqueous KOH, NaOH, KBO2, NaBO2, NaBH4, and KBH4 solutions of any composition in a temperature range of (10 to 50) °C and in the whole concentration range are described mathematically. The technique and algorithm of calculation have been verified by comparison with measured properties.