Quick answer
This calculator uses the speed of light, c = 299,792,458 m/s, to find light travel times and distances. It can also find how fast light moves inside a material such as water or glass, using v = c ÷ n.
Speed of light calculator
How to Use the Speed of Light Calculator
- Choose what you want: travel time, distance, or speed in a material.
- Enter the distance in kilometres, the time in seconds, or the refractive index.
- Press Calculate.
Tip: For huge distances, type numbers in e-notation: 1.496e8 km is the average Earth–Sun distance.
Speed of Light Formulas
| To find | Formula |
|---|---|
| Travel time | t = d ÷ c |
| Distance | d = c × t |
| Speed in a material | v = c ÷ n |
The speed of light in a vacuum is exactly 299,792,458 m/s by definition. Our full article on the speed of light explains how it was measured and why nothing can go faster.
Worked Example
Worked example: sunlight
Question: The Sun is about 1.496 × 10⁸ km from Earth. How long does sunlight take to reach us?
- Choose “Light travel time from distance”.
- t = 1.496 × 10⁸ ÷ 299,792.458 ≈ 499 s.
- 499 ÷ 60 ≈ 8.3 minutes.
Answer: Sunlight takes about 8 minutes 19 seconds to reach Earth.
Light Travel Times Across Space
| Trip | Approximate light time |
|---|---|
| Earth to Moon (384,400 km) | 1.28 s |
| Sun to Earth | 8.3 minutes |
| Sun to Neptune | about 4.2 hours |
| Nearest star, Proxima Centauri | about 4.2 years |
Practise: test yourself with speed of light MCQs with answers for physics exams.
Frequently Asked Questions
What is the exact speed of light?
In a vacuum, light travels at exactly 299,792,458 m/s, which is about 3.0 × 10⁸ m/s or 300,000 km/s.
How fast is light in water?
Water has a refractive index of about 1.33, so light travels at about 2.25 × 10⁸ m/s, roughly 75% of its speed in a vacuum.
What is a light-year?
A light-year is the distance light travels in one year, about 9.46 × 10¹² km. It is a unit of distance, not time.