Example 1
A family car of 1400 kg at 13.9 m/s (about 50 km/h) in town. That is the energy your brakes swallow before every crosswalk.
Kinetic energy calculator
Enter mass and speed. The calculator computes energy of motion Ek = ½ m v². 1000 kg at 20 m/s is 200,000 J, the store brakes have to put away at 72 km/h.
Result
Mass and Speed (m/s). Kinetic energy shows up here.
How it works
Kinetic energy grows with speed squared: Ek = ½ m v². A 1000 kg car at 20 m/s is 200,000 J. At 40 m/s the same mass is 800,000 J, four times more, not two. Brakes have to put that energy away, which is why stopping distance grows faster than the speedometer. 2000 kg at 20 m/s is 400,000 J: twice the mass, twice Ek.
The first field is mass in kilograms, or pounds after the US switch. The second is speed in the labeled unit, usually m/s. 72 km/h is 20 m/s, not 72 in the field. 50 km/h is about 13.9 m/s. The result is in joules. A comma and a period are the same v: 13,9 and 13.9.
Zero kilograms gives zero joules: nothing is moving. Negative mass will not run. Zero speed is a body at rest, Ek = 0. Both fields need a number before 200,000 J can appear. The switch changes the mass label and does not convert a number you already typed.
Work W = F·s is on the neighbouring page: there you push with a force over a distance and get the same joules from another formula. Braking distance is on decelerated motion. Rotational energy ½ I ω² is yet another calculator. Here it stays ½ m v² from two fields.
Twice the v at the same mass is four times Ek. That is why 1400 kg at 13.9 m/s and the same car at 33.3 m/s are very different stores to brake away, even though the dial only moves from 50 to 120 km/h.
Type 1000 and 20, click Calculate, and match 200,000 J. Then 1000 and 40: 800,000 J. The header symbol does not pedal the speed.
Braking at this speed: decelerated motion. Force work: W = F·s.
How to use
- In the first field enter mass. In metric that is kilograms: 1000 for a light car, 80 for a person. After the US switch the label may say pounds.
- In the second field enter speed in m/s, as on the label. 20 m/s is 72 km/h. Do not type 72 when you mean kilometers per hour. 50 km/h is about 13.9 m/s.
- Click Calculate. The calculator computes ½ m v². 1000 kg and 20 m/s give 200,000 J. At 40 m/s the same mass is 800,000 J. 2000 kg at 20 m/s is 400,000 J.
- Zero mass gives 0 J. v is squared, so twice as fast is four times the energy, not two. Negative mass will not run.
- Want work from force and distance? Open W = F·s. Braking distance is on decelerated motion. Here it stays energy of motion.
Formula
Ek = ½ · m · v²
Mass and speed from the fields. Result in joules. Doubling v multiplies Ek by four.
Ek, m, and v at 1000 kg and 20 m/s
Energy of motion in this calculator grows with v squared. 1000 kg at 20 m/s is 200,000 J; at 40 m/s the same mass is 800,000 J.
- Ek
- Kinetic energy in joules: Ek = ½ m v². 2000 kg at 20 m/s is 400,000 J: twice the mass, twice Ek.
- m
- Mass in kilograms. Zero kg gives 0 J. A minus is rejected.
- v
- Speed, usually m/s. 72 km/h is 20 m/s, not 72 in the field. 50 km/h is about 13.9 m/s.
- J
- The unit of Ek. Brakes have to put those joules away; stopping distance is another calculator.
Real-life examples
Example 2
The same 1400 kg car at 20.1 m/s (72 km/h). The dial looks only a bit higher, but the v² term is already at work.
Example 3
A 1400 kg car cruising at 33.5 m/s (121 km/h). A highway-sized energy reserve your brakes hope never to meet at once.
Example 4
Rider and bike together at 85.0 kg, rolling at 6.7 m/s (24 km/h). That is the motion you scrub off before every junction.
Example 5
A jogger of 70.0 kg at an easy 3.3 m/s. A small energy budget, absorbed by knees and ankles stride after stride.
Example 6
A match ball of 0.43 kg flying at 25 m/s after a clean strike. That is why keepers punch rather than catch.
Example 7
A motorcycle of 200 kg at 25 m/s (about 90 km/h). Less mass than a car, yet the kilojoules stack up fast.
Example 8
E-scooter plus rider at 90.0 kg, doing 5.6 m/s (roughly 20 km/h). Modest joules, but the pavement is unforgiving.
Example 9
A toy-model light-rail train of 50000 kg at 20 m/s. That is why rail stopping distances run to hundreds of meters.
Example 10
A kid on a bike, 30.0 kg all in, at a gentle 3 m/s. Barely any joules, and the helmet stays on anyway.
Example 11
An e-scooter with rider at 95.0 kg total, running 6.7 m/s (24 km/h). Speed caps exist for exactly this much energy.
Example 12
A basketball of 0.60 kg zipped across the court at 8 m/s. That is the sting your palms feel on a hard chest pass.
Frequently asked questions
How many joules is 1000 kg at 20 m/s?
Ek = 0.5 × 1000 × 400 = 200,000 J. At 40 m/s you get 800,000 J, four times more. 2000 kg at 20 m/s is 400,000 J.
Which units do I type?
Mass in kg, or lb after the switch. Speed in m/s on the label. 72 km/h = 20 m/s. 50 km/h ≈ 13.9 m/s. Result in J.
Where is work W = F·s?
On the work page. Here you compute energy from speed, there work from force and distance. Same joules, different formula.
Can I type 72 km/h as 72?
The field wants m/s unless the label says otherwise. 72 km/h is 20 m/s. 72 in the field is 72 m/s, over 250 km/h.
Why is 40 m/s not twice 20 m/s in joules?
Because v is squared. Twice the speed is four times Ek. Brakes feel the square, not the dial.
Does a comma in 13.9 work?
Yes. 13,9 and 13.9 are the same speed. The calculator does not require a period.
Where is braking distance?
On the decelerated-motion page. Here it stays energy, not stopping distance. Impulse and momentum live elsewhere.
Can mass be negative?
No. Zero kg gives 0 J. A minus is rejected. Zero speed is a body at rest, also 0 J.
Is twice the mass twice Ek?
Yes, at the same v. 2000 kg at 20 m/s is 400,000 J. With v it is different: the square.
How is Ek different from ½ I ω²?
Here translation, ½ m v². Rotational energy is a sibling card, with moment of inertia and ω.
Knowledge sources
Kinetic energy in this calculator is ½mv². Units follow SI; NIST SP 330 and BIPM define the joule.
Page updated in 2026.