Estimate star size from measurable observables with clear unit handling fast today. Compare methods, see intermediate values, and download reports for notes anywhere easily.
| Star | L (L☉) | T (K) | Radius (R☉) from L,T | M (M☉) | g (m/s²) | Radius (R☉) from M,g |
|---|---|---|---|---|---|---|
| Sun | 1.00 | 5772 | 1.00 | 1.00 | 274 | 1.00 |
| Cool giant (illustrative) | 500 | 3500 | ~21 | 2.0 | 5 | ~105 |
| Hot dwarf (illustrative) | 0.10 | 12000 | ~0.07 | 0.6 | 2000 | ~0.20 |
Stellar radius connects directly to luminosity, temperature, density, and evolutionary state. A change from 1 R☉ to 10 R☉ increases surface area by 100×, strongly affecting flux and spectral energy distribution. Radius also helps constrain habitable zone distances and transit depths in exoplanet studies.
Main-sequence stars commonly range from about 0.1–10 R☉. Red giants and supergiants can exceed 50–1000 R☉, while compact remnants are far smaller: white dwarfs are near Earth-sized (~0.01 R☉). Comparing results in solar radii makes these regimes easier to interpret.
The Stefan–Boltzmann relation uses L = 4πR²σT⁴. Because temperature appears as T⁴, a 10% temperature error can drive ~20% radius error at fixed luminosity. For the Sun (T≈5772 K, L=1 L☉), the method returns R≈1 R☉, serving as a quick validation check.
Surface gravity follows g = GM/R². With measured log(g) and an estimated mass, radius drops out cleanly. For solar values (M=1 M☉, g≈274 m/s²), you recover ~1 R☉. Low g values (a few m/s²) often indicate evolved, inflated stars.
Interferometry can provide angular diameters in milliarcseconds, combined with distances from parallax. Using the small-angle approximation R ≈ (θ/2)·d, tiny angles are essential; the approximation is excellent when θ is far below one degree. This method can be very direct when θ and d are well constrained.
The calculator converts solar units to SI internally, ensuring consistent physics. Pay special attention to cgs gravity (cm/s²) versus SI (m/s²): 1 m/s² equals 100 cm/s². For angular inputs, mas and arcseconds are converted to radians before computing linear size.
When multiple observables exist, compare radii from different methods. Agreement within uncertainties boosts confidence, while large discrepancies can indicate inconsistent inputs, extinction-corrected luminosity issues, temperature scale offsets, or surface gravity systematics. Record intermediate values to audit your assumptions.
Start with the method best supported by your dataset, then run at least one independent estimate if possible. Choose a reasonable significant-digit setting to match measurement precision, export CSV for lab notebooks, and attach the PDF summary to reports. Consistent documentation is as important as the final number.
Use Stefan–Boltzmann when you trust luminosity and effective temperature. Use gravity when you have mass and surface gravity. Use angular diameter when you have θ and distance, often from interferometry plus parallax.
In the Stefan–Boltzmann relation, luminosity scales with T⁴ at fixed radius. Solving for radius makes R scale as 1/T² for fixed L, so modest temperature uncertainty can noticeably change the radius estimate.
Convert log(g) to g in cm/s² first: g = 10^{log(g)}. Then enter that value and select the cgs gravity unit. The calculator converts to m/s² internally for the radius computation.
It is excellent when θ is very small, which is typical for stars. Milliarcsecond and arcsecond diameters are firmly in the small-angle regime. If θ approaches degrees, use exact trigonometry instead of the approximation.
Yes. Select W for luminosity and kg for mass, then enter SI values. The output still includes solar radii for interpretation. This is useful when working with model outputs or simulation data in SI units.
Differences usually come from inconsistent inputs or uncertainties: extinction-corrected luminosity, temperature scales, mass estimates, or surface gravity systematics. Compare intermediate values, verify units, and report uncertainties rather than a single exact value.
It is a reliable calculator for quick checks, teaching, and reproducible reporting with exports. For publication-grade results, propagate uncertainties, document data sources, and prefer peer-reviewed calibrations for temperature, luminosity, and gravity.
Use accurate inputs, and your stellar radius improves greatly.
Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.