Formula used
How to use this calculator
- Select a computation mode that matches your data source.
- Choose RPM or rad/s, then enter the angular speed.
- Fill in the mode-specific inputs shown on the form.
- Enter η, ρ, and v̄ if you want buoyancy or s20,w correction.
- Press Compute to display results above the form.
- Use the export buttons to save results for lab notes.
Example data table
| Species | Approx. mass (kDa) | Typical s (S) | Notes |
|---|---|---|---|
| Lysozyme | 14 | ~1.8 | Compact globular protein in aqueous buffer. |
| Bovine serum albumin | 66 | ~4.3 | Common reference; shows moderate hydration. |
| IgG antibody | 150 | ~6.5–7.5 | Flexible shape; s depends on glycosylation and buffer. |
| Ribosome (bacterial 70S) | ~2500 | 70 | S values are historical labels, not additive masses. |
Sedimentation coefficient: professional notes
1) What the coefficient represents
The sedimentation coefficient s links a particle’s radial velocity to centrifugal acceleration. It is defined as s = v/(ω²r) and has units of seconds. In practice, it condenses size, shape, hydration, and buoyancy into a single transport parameter. One Svedberg (1 S) equals 10-13 s, so most macromolecules fall in a convenient numerical range.
2) Typical values and scale
Compact proteins often lie between about 2–10 S, while large assemblies can be tens of S. For reference, bovine serum albumin is commonly reported around 4–5 S, many IgG antibodies near 6–8 S, and bacterial ribosomes are labeled 70S. These labels are historical and are not additive across subunits, because friction and buoyancy do not scale linearly with mass.
3) Velocity mode and boundary tracking
In sedimentation velocity experiments, you may estimate v directly at radius r, or infer it from boundary movement between r1 and r2 over time t. The boundary approach uses v ≈ (r2 − r1)/t and an average radius r̄ for the denominator. At 45,000 RPM, ω is roughly 4.7×103 rad/s, so small changes in ω strongly affect s through ω².
4) Molecular parameters and the friction term
The molecular form s = M(1 − ρv̄)/(NAf) highlights competing factors. Higher molar mass M increases s, while higher friction f decreases it. For a sphere, Stokes friction is f0 = 6π η a, using viscosity η and hydrodynamic radius a. A shape factor (f/f0) above 1.0 models elongation or flexibility that increases drag.
5) Buoyancy: density and partial specific volume
The buoyancy factor (1 − ρv̄) reduces effective mass in solution. Typical proteins have v̄ ≈ 0.70–0.75 mL/g, while aqueous buffers have densities near 1.00 g/mL. If ρv̄ approaches 1, buoyancy nearly cancels the driving force and the calculation becomes unstable. Measuring density and using a realistic v̄ improves reliability.
6) Temperature and viscosity effects
Viscosity strongly depends on temperature and solvent composition. Water at 20°C has η20,w ≈ 1.002 mPa·s and density ρ20,w ≈ 0.9982 g/mL. More viscous solvents lower sedimentation velocities and therefore lower s. If your experiment was run at a different temperature, standardization helps comparisons across instruments and studies.
7) Standardization to s20,w
A common reporting convention is s20,w, the coefficient corrected to water at 20°C. The correction scales s by the viscosity ratio and by a buoyancy ratio: s20,w = s·(η/η20,w)·(1 − ρ20,wv̄)/(1 − ρv̄). This tool computes that correction when η, ρ, and v̄ are supplied.
8) Data quality checks for robust results
Use consistent SI units: r in meters, t in seconds, and η in Pa·s. Confirm that radii are positive and that the boundary movement direction matches centrifugation geometry. When using boundary movement, avoid very short t that amplifies timing error. When using molecular mode, ensure a realistic a and shape factor, because friction dominates s for similarly sized species.
FAQs
1) What does 1 S mean in seconds?
One Svedberg equals 10-13 seconds. It is a convenient scaling of the sedimentation coefficient so macromolecules report as single-digit or double-digit numbers rather than very small seconds.
2) Why is the ribosome called 70S?
“70S” is a measured sedimentation coefficient label, not a mass unit. Friction, hydration, and buoyancy influence s, so 30S and 50S subunits do not add to 80S.
3) Which mode should I use: velocity or boundary?
Use velocity mode when you already know v at radius r. Use boundary mode when you track a boundary shift from r1 to r2 over time and want the calculator to estimate v for you.
4) Why does viscosity reduce the sedimentation coefficient?
Higher viscosity increases hydrodynamic drag, which lowers the sedimentation velocity for the same centrifugal field. In the molecular form, viscosity appears in the friction coefficient f, which sits in the denominator.
5) What is a typical partial specific volume for proteins?
Many proteins fall around 0.70–0.75 mL/g. The exact value depends on composition and hydration. Using a realistic v̄ improves the buoyancy term (1 − ρv̄) and the s20,w correction.
6) When should I report s20,w instead of s?
Report s20,w when comparing results across different temperatures, buffers, or labs. It normalizes viscosity and buoyancy to water at 20°C, reducing condition-dependent variation in reported coefficients.
7) Why is my computed s negative or unrealistic?
Negative values typically indicate inconsistent inputs, such as r2 < r1 with positive time, or an incorrect sign convention for boundary movement. Unrealistic values often arise from wrong units or an ω value not matching RPM.