Star Size Calculator
Enter the star's luminosity in multiples of the Sun's and its surface temperature in kelvin. The calculator works out the radius and diameter, the spectral class and how the star compares with the Sun and the planets' orbits.
- Star radius
- Diameter
- Spectral class
Luminosity is in multiples of the Sun's. Temperature is the surface temperature in kelvin.
A star's size follows from how bright and how hot it is. R = √L × (5,772 K ÷ T)², in Sun radii. The Sun, at 1 L☉ and 5,772 K, comes out as 1. A cool red star at 3,500 K that is 50,000 times as bright as the Sun is about 608 times its radius, a red supergiant.
Star size chart
Examples. Search for a star type.
| Star type | Luminosity | Temperature (K) | Radius |
|---|---|---|---|
| Red dwarf | 0.002 | 3100 | 0.16 |
| Sun | 1 | 5772 | 1 |
| White main sequence | 25 | 9600 | 1.81 |
| Blue main sequence | 20000 | 25000 | 7.5 |
| Red giant | 100 | 4300 | 18 |
| Red supergiant | 50000 | 3500 | 608 |
Radius in Sun radii.
Brightness and size
3,500 K red star. Slide the luminosity.
The star is drawn beside the Sun.
How to find a star's size
Light to radius. Step through it.
Luminosity
× Sun.
Hot blue stars can be bright while small. Cool red stars must be huge to be bright.
Dwarf, giant or supergiant
Star sizes. Compare them.
Small.
Swollen.
Huge.
Most stars in the galaxy are red dwarfs, too faint to see with the naked eye.
Before you compare
Star checks. Tick each item.
Tick each item as you go.
Use the bolometric luminosity, which counts all the light, not just the visible part.
What you get
Star sizing. Each tile explains one result.
× Sun.
km.
Colour.
× Sun.
Comparison.
The Sun's radius is 696,340 km, about 109 times Earth's.
Frequently Asked Questions
No question matches that search.
How do I calculate the size of a star?
Use the Stefan-Boltzmann law: radius equals the square root of luminosity times the Sun's temperature over the star's, squared.
Why are red giants so big?
They are cool, so they need a huge surface to give out so much light.
How big is the Sun?
About six hundred and ninety-six thousand kilometres in radius.
What is the biggest known star?
Some red supergiants are over a thousand times the Sun's radius.
What does spectral class mean?
A letter for surface temperature, from hot O and B stars to cool K and M stars.
Is a brighter star always bigger?
No. A hotter star can be bright while small.
What is luminosity?
The total power a star gives out, often in multiples of the Sun's.
See how big a star really is
Combine brightness and temperature and the true size of any star comes into view, from dwarfs to supergiants.
More size calculators
Each one has its own calculator, worked examples and answers to common questions.