We have a useful Pressure Calculator on our website in which you determine the contribution your tubing ID will bring to the overall back pressure in your system.
This can be handy when you are setting up an HPLC system and want to know what ID tubing would be best to use. The lower the tubing ID, the lower the extra-column volume and better the chromatography. This smaller ID will give you higher efficiency peaks (due to less dispersion) but will also increase your system operating pressure. According to the formula, pressure is a function of tubing ID, tubing length, viscosity of the liquid, and flow rate.