If you are using standard company it is reliable, But may varies 10-20 mmhg

They are not as reliable as a traditional Mercury based apparatus. That said, it is not in accurate. It can vary by 20-30 mm Hg as such. It is convenient and helps in assessing your change from baseline BP, especially when regulatory measures. Your body’s blood pressure can vary between readings naturally. You will have to see how significant the variation is ;