Under current economic conditions, the role of macroeconomic factors, such as inflation, trade openness, and economic uncertainty, has become increasingly important in shaping private investment. This study employs a fuzzy regression model to examine the effects of these variables on private investment in Iran over the period 1979–2025. One of the main reasons for adopting this approach is its flexibility in capturing uncertainty by estimating the left- and right-hand fuzzy spreads for each variable. The upper-bound results at a membership degree of 0.9 indicate that among the determinants of private investment, inflation, economic growth, trade openness and domestic credit have the largest effects, with coefficients of 3.91, 1.76, 1.45, and 0.31, respectively. Inflation can affect private investment through several channels. Higher inflation may influence firms’ performance and profitability by reducing purchasing power and increasing production costs; however, the direction and magnitude of this effect depend on factors such as firms’ pricing power, their ability to pass higher costs on to consumers, and inflation expectations. Under certain conditions, firms may even maintain or improve profitability by adjusting prices in line with inflationary pressures. At the same time, central banks may raise interest rates to contain inflation, thereby increasing financing costs and potentially reducing incentives for new investment. Therefore, policymakers should regularly assess the effects of economic and trade policies on private investment and implement appropriate adjustments when necessary.