平均成本法（英语：Dollar Cost Averaging，DCA）又名「懒人理财术」或「定期定额投资法」，为投资学名词，是指在特定间隔期间（例如每月买入一次），买入固定金额的某资产的投资方式。

平均成本法是定时投资策略的一种，以规律机械式的方式进行资金的投入，除了用以累积投资资本，投资人也利用这种方式试图克服单笔投资的困境，将「择时能力」从投资决策因子中排除，以分批投资方式进入市场，降低单一时间点投入的风险，让投资者不必「等」或「猜」所谓适当时机，不需去挑选何时运行进入、何时退出市场的指令，只要选定投资标的、日期与金额，即可开始行动。

Dollar cost averaging (DCA) is an investment strategy for reducing the impact of volatility on large purchases of financial assets such as equities. By dividing the total sum to be invested in the market (e.g. $100,000) into equal amounts put into the market at regular intervals (e.g. $1000 over 100 weeks), DCA reduces the risk of incurring a substantial loss resulting from investing the entire "lump sum" just before a fall in the market. Dollar cost averaging is not always the most profitable way to invest a large sum, but it minimises downside risk.