托賓q理論（英語：Tobin’s q theory）為美國經濟學家詹姆士·托賓於1969年提出的投資理論，而托賓q比率（英語：Tobin’s q）是以企業的股票市場價值和企業的帳面淨值之間的比較。
公式
The denominator is the price of that capital if it were purchased today.
Example: ABC company has 3 per stock. Tobin’s q?
=\frac{1\ million * \ 3}{40\ million}=0.75<1$
If Tobin’s q > 1, Managers should buy more capital
If Tobin’s q < 1,
Managers should stop buying more capital. They should not replace old capital
It seems that the Tobin’s q theory is unlike the old ones like Consumption according to Keynes, but they have close relationship, recall that Tobin’s q depends on Current and future expected profits from capital So when exceeds the cost of capital, so it earns the profit （边际产量大于边际成本）, vice versa.
The decision of investment is not only depended by the current policy, but also the expected future.