About the Force Calculator
This force calculator applies Newton’s second law, F = m × a, and can solve for any one of the three quantities. Enter mass in kilograms, grams, pounds or tonnes and acceleration in m/s², ft/s² or g, and it returns the net force in newtons, kilonewtons, pounds-force and dynes.
Use it for physics and engineering homework, to estimate the force a car or lift motor must produce, to find how hard a seatbelt must hold a passenger in a crash, or to back-calculate mass from a measured force. The result is also expressed as a multiple of the object’s own weight, which is often the easiest way to picture how large a force is.
The force here is the net (resultant) force. If several forces act on an object — for example thrust minus friction and drag — subtract the opposing forces first, or use the answer as the net force the object must experience.
How to use the force calculator
- 1Select whether to solve for force, mass or acceleration.
- 2Enter the two known values.
- 3Choose units for each — kg or lb, m/s² or g, N or lbf.
- 4Read the answer and the conversions to other units.
Formula and method
Newton’s second law says the net force on an object equals its mass multiplied by its acceleration. In SI units a force of one newton accelerates one kilogram at one metre per second squared (1 N = 1 kg·m/s²). The calculator converts all inputs to kilograms, m/s² and newtons, solves the equation, and converts back to your chosen units.
One pound-force (lbf) is the weight of a one-pound mass under standard gravity, 4.448 N. The “multiple of weight” figure divides the force by m × 9.80665 m/s², which is the same as expressing the acceleration in g.
- F
- Net force (N)
- m
- Mass (kg)
- a
- Acceleration (m/s²)
Worked examples
Force to accelerate a 1,200 kg car at 3 m/s²
F = 1,200 kg × 3 m/s² = 3,600 N, about 809 lbf. That is roughly 0.31 of the car’s own weight, matching an acceleration of 0.31 g.
Acceleration of a 70 kg sprinter pushed with 500 N
a = F ÷ m = 500 N ÷ 70 kg = 7.14 m/s². A net 500 N push accelerates the sprinter at about 0.73 g.
Mass from force in pounds-force
200 lbf is 889.6 N. Dividing by 2 m/s² gives a mass of 444.8 kg (about 980.7 lb).
Frequently asked questions
What is Newton’s second law?+
Newton’s second law states that the net force on an object equals its mass times its acceleration (F = ma). A larger force produces a larger acceleration, and a heavier object needs more force for the same acceleration.
What is a newton?+
A newton (N) is the SI unit of force: the force needed to accelerate a 1 kg mass at 1 m/s². An apple weighing about 100 g exerts roughly 1 N on your hand.
How do I convert newtons to pounds-force?+
Divide by 4.44822. For example 1,000 N ≈ 224.8 lbf. To go the other way, multiply pounds-force by 4.44822.
What is the difference between mass and weight?+
Mass is the amount of matter (kg) and does not change with location. Weight is the force gravity exerts on that mass, W = m × g, about 9.81 N per kilogram on Earth.
What is net force?+
Net force is the vector sum of all forces acting on an object. If you push a box with 100 N and friction resists with 30 N, the net force is 70 N and that is what determines the acceleration.