This paper presents a simple formula for thermal designing of natural-air-cooled electronic equipment casings with standard arrangement of circuit boards and power supplies. The formula meets the requirements as a practical formula, since it represents an air-cooling system in its simplified form with due regard to such factors as the stack effect, air flow resistance, natural convective transfer and so on. The formula was applied to predict the temperature rise in two practically used electronic equipment cabinets with standard arrangements and a modeling case. The predicted temperature rise values, obtained through the formula, slightly differed - within 10 percent - from the actual values based upon experiment results.