What is hydrodynamic radius (Rh)?
The hydrodynamic radius is basically how “big” a molecule appears to be when it moves through a fluid, like water. But it’s more than just size—it also captures the molecule’s shape and how it interacts with the surrounding fluid.
Here’s a mental image: Imagine replacing your molecule with a smooth, round sphere that moves through liquid at the same speed. The radius of that imaginary sphere? That’s the hydrodynamic radius.
It might sound a bit abstract, but it’s incredibly useful for understanding what molecules are doing in solution.
Why measure it?
Rh is like a molecular behavior fingerprint. It helps us understand how molecules change, interact, or misbehave in ways that aren’t always obvious from their mass or structure alone.
Let’s break it down:
Tracking Molecular Size
Rh gives a direct readout of how “big” a molecule acts in solution.
For example, a small, neatly folded protein will have a smaller Rh than an unfolded one of the same mass. That’s because the folded version is more compact and moves faster through the fluid.
Monitoring Conformational Changes
When proteins unfold (or refold), they change shape—and that affects how they move.
An unfolded protein exposes more surface area, interacts more with water, and ends up with a larger Rh. So if you’re measuring Rh, you can actually track these conformational changes in real time.
Studying Protein Aggregation
Aggregated molecules act like a much larger particle in solution, resulting in a higher Rh. Measuring Rh over time can help detect aggregation early—even before it’s visible to the eye—making it valuable for protein stability studies.
Quantifying Molecular Interactions
When two molecules bind (e.g., a protein and a drug), the resulting complex is larger and moves more slowly—increasing the Rh. By comparing Rh before and after mixing, you can detect and quantify binding events.
Determine the theoretical hydrodynamic radius of your protein
This converter uses the theoretical relationship between hydrodynamic radius and the molecular weight of a protein to be able to convert between the two. This tool can be used to determine the theoretical hydrodynamic radius of your protein or protein complex before running it on the Fluidity One-M to confirm that the practical results are within an expected size range. Note that this calculator assumes that the folded form is globular and so does not account for the actual folded conformation, which is often less compact than the globular estimate.
For a structure-based prediction you can use our PDB converter.
Input the weight of your protein in kDa into the molecular weight side of the converter, select whether the protein is likely to be folded (globular) or unfolded. It will output the theoretical hydrodynamic radius in nm. This tool can also be used in reverse to give theoretical molecular weight in kDa from hydrodynamic radius in nm.