What Is Weight?
In physics, weight is the force exerted on a mass by gravity. It is not the same as mass. Mass is a fixed property of matter, while weight changes depending on the strength of gravity in a given location. On Earth, a 70 kg person weighs about 686 Newtons. On the Moon, that same person weighs only about 113 Newtons because lunar gravity is about one-sixth of Earth's.
Weight is measured in Newtons (N) in the SI system, or in pound-force (lbf) in the imperial system. In everyday language, "weight" and "mass" are used interchangeably, but this calculator uses the strict scientific definition.
What This Calculator Does
This calculator solves the weight formula for any one of its three variables. You can calculate:
- Weight (N): from mass and gravitational acceleration
- Mass (kg): from weight and gravitational acceleration
- Gravitational acceleration (m/s²): from weight and mass
Gravity presets for Earth, Moon, Mars, Jupiter, and the Sun are provided for quick selection.
How the Calculation Works
Weight (W) = Mass (m) × Gravitational Acceleration (g)
Mass (m) = Weight (W) / Gravitational Acceleration (g)
Gravity (g) = Weight (W) / Mass (m)
Earth's standard gravitational acceleration is 9.80665 m/s² (commonly rounded to 9.81 or 9.807). The result in Newtons can be converted to pound-force (1 lbf = 4.44822 N) or kilogram-force (1 kgf = 9.807 N).
How to Use the Calculator
- Select the variable you want to solve for using the tabs
- Enter the two known values
- Use a gravity preset or enter a custom gravitational acceleration
- The result appears instantly with additional unit conversions for weight
Example Calculations
Example 1: Weight of a 70 kg person on Earth
W = 70 kg × 9.807 m/s² = 686.49 N. This equals about 154.3 lbf or 70 kgf (which is why kgf is sometimes called "kilogram-weight").
Example 2: Weight on the Moon
The same 70 kg person on the Moon: W = 70 × 1.620 = 113.4 N, which is about 25.5 lbf. The astronaut feels much lighter but has the same mass.
Real-World Scenarios
Structural Engineering
Engineers must calculate the weight (force) that structures must support. A floor beam must hold the weight of people, furniture, and the floor itself. Mass alone is insufficient for structural load calculations.
Space Exploration
Mission planners calculate how much fuel is needed to land on other planets based on local gravity. Equipment that weighs 1,000 N on Earth weighs only 377 N on Mars, changing propulsion requirements significantly.
Physics Education
The weight formula is one of the first equations taught in physics. Understanding the difference between mass and weight is fundamental to understanding Newton's laws of motion.
Why This Calculation Matters
Weight as a force drives many critical calculations in engineering and science. Bridges, elevators, aircraft, and spacecraft all require precise weight calculations to function safely. Misunderstanding weight versus mass is a common source of error in applied physics.
Common Mistakes to Avoid
- Treating mass and weight as identical: A 10 kg object weighs 98.07 N on Earth. Using 10 as the weight in Newtons would give wrong results
- Using the wrong gravity value: Earth's gravity at sea level is 9.807 m/s², but it varies slightly by latitude and altitude. The standard value 9.80665 m/s² is used for most calculations
- Confusing lbf and lb: Pound-force (lbf) is a unit of force. Pound (lb) is a unit of mass in everyday use. In the imperial system, an object with a mass of 1 lb exerts a weight of approximately 1 lbf on Earth due to how the system is defined