Mertens' Theorem says (page 65, An Introduction to Sieve Methods and Their Applications, Cojocaru and Murty):
$$ \displaystyle\prod_{p< x}\left(1-\frac 1p\right)=\frac{e^{-\gamma}}{\log x}\left(1+O\left(\frac{1}{\log x}\right)\right), $$
where $\gamma$ is the Euler-Mascheroni constant.
