The radicand, $19\text{,}$ is between $16$ and $25\text{,}$ so $\sqrt{19}$ is between $\sqrt{16}$ and $\sqrt{25}\text{,}$ or between $4$ and $5\text{.}$ To be more precise, we notice that $19$ is in the middle between $16$ and $25\text{,}$ but closer to $16\text{.}$ We estimate $\sqrt{19}$ to be about $4.4\text{.}$ We will check our estimate with a calculator: $$\sqrt{19}\approx4.358$$