# Confusion regarding factor 2 in uncertainty principle [duplicate]

I have encountered a problem regarding Heisenberg's uncertainty principle. Somewhere I found the expression

$$\Delta (x) \Delta (p) \geq \hbar$$

Whereas in some other places,

$$\Delta (x) \Delta(p) \geq \frac{h}{4\pi}$$

Here the second expression yields different expression than the first one. Why are the two expressions of uncertainty principle different? Which one is the correct form?

• Commented Aug 1, 2016 at 16:54
• This principle is almost never used for exact calculations but rather to estimate orders of magnitude so you can just as well write $\Delta x \Delta o \geq \pi^{0.3} \hbar$ and it will still be valid. Commented Aug 1, 2016 at 22:07
• Possible duplicates: physics.stackexchange.com/q/69604/2451 , physics.stackexchange.com/q/103208/2451 and links therein. Commented Aug 2, 2016 at 9:31

See Ballentine: Quantum mechanics p. 224 (chapter 8): the uncertainty principle applied to position and momentum yields, in 1 dimension: $$\Delta_X \Delta_P \geq \frac{\hbar}{2}=\frac{h}{4 \pi}$$ But the other version is often used when it's mainly about an estimation of magnitude and the factor of $0.5$ does not really matter.
Edit: On Wikipedia it says that the first unproofed heuristically motivated version of the principle given by Heisenberg himself was without the factor of $0.5$. Maybe that is a historical explanation.