Mass is the amount of matter in an object, and it also measures how strongly that object resists a change in motion. Its SI unit is the kilogram (kg); one kilogram equals about 2.2046 pounds. Unlike weight, it stays the same on Earth, on the Moon, or in orbit.
People say “weight” every day when they really mean mass. For example, a bag of flour labeled 2 lb (0.907 kg) holds the same amount of flour in your kitchen, on a mountaintop, or on the International Space Station. Instead, what changes from place to place is the pull of gravity on it, and that pull is its weight.
In physics, the idea has two faces. First, it tells you how much stuff an object contains. Second, it tells you how hard you must push to speed the object up or slow it down. Isaac Newton tied the two together in his second law, F = ma, so a heavier truck needs more force than a bicycle to reach the same acceleration.
In short: mass is a fixed property of an object, while weight is a force that depends on where the object is.
The official definition has also changed recently. Since May 20, 2019, the kilogram has rested on a constant of nature instead of a metal cylinder in France. The International Bureau of Weights and Measures (BIPM) now defines it by fixing the Planck constant at exactly 6.62607015 x 10^-34 joule-seconds.
Next, you can convert between units, see how scales and balances work, and learn where everyday measurements go wrong.
Convert Mass Units: Kilograms, Pounds, Ounces and Grams
Enter an amount and pick the unit you have. The converter then uses the exact NIST definitions (1 lb = 453.59237 g and 1 oz = 28.349523125 g), so the results match any lab reference.
Recommended Tools for Measuring Mass
Pick the tool by the size of what you weigh and how fine a reading you need. For example, a kitchen scale with 1 g steps suits cooking. By contrast, a pocket scale with 0.01 g steps handles jewelry, coffee and small lab samples. A triple beam balance compares masses directly, so it reads the same anywhere, and it is still a classroom standard. Finally, a set of calibration weights lets you check any of these against a known value.
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Key Takeaways
- Mass measures the amount of matter in an object and its resistance to acceleration.
- The SI unit is the kilogram; 1 kg is about 2.2046 lb, and 1 lb is exactly 453.59237 g.
- By contrast, weight is a force: weight = mass x gravitational acceleration, measured in newtons.
- On the Moon you would weigh about one-sixth as much, yet your body contains the same matter.
- A balance compares masses directly; a spring or digital scale infers it from force.
- Also, since 2019 the kilogram rests on the Planck constant, not a metal artifact.
- Finally, calibrate scales with certified weights, especially for lab, trade or medical use.
What Mass Means in Physics
Physicists describe two kinds, and experiments show they are equal to extreme precision. First, inertial mass sets how much an object resists acceleration. If you push a shopping cart and a loaded pickup with the same force, the cart speeds up far faster. In short, that difference is inertia.
Gravitational mass, on the other hand, sets how strongly an object pulls on other objects and how strongly they pull on it. Newton’s law of gravitation says the force between two bodies grows with the product of their masses and shrinks with the square of the distance between them. As a result, the Sun holds the planets in orbit, and Earth holds you on the ground.
Because the two values match, all objects fall at the same rate in a vacuum. Galileo’s idea, and the famous hammer-and-feather drop on the Moon during Apollo 15, show this directly. Later, Einstein built general relativity on that equality.
Mass is also a scalar quantity. In other words, it has size but no direction, unlike force or velocity. Moreover, in everyday conditions it is additive: put two 1 kg bags in a box and the contents total 2 kg.
Mass vs Weight: The Difference That Matters
Weight, by contrast, is the gravitational force on an object. Therefore, you find it with W = m x g, where g is the local gravitational acceleration. Standard gravity is defined as exactly 9.80665 m/s^2, so a 70 kg person weighs about 686.5 newtons at sea level.
Take the same person to the Moon, where g is about 1.62 m/s^2. Their weight drops to about 113 newtons, roughly one-sixth of the Earth value. Their body, however, still contains exactly 70 kg of matter. Similarly, on Mars (about 3.71 m/s^2) the weight would be close to 260 newtons.
| Property | Mass | Weight |
|---|---|---|
| What it is | Amount of matter; resistance to acceleration | Force of gravity acting on an object |
| SI unit | kilogram (kg) | newton (N) |
| US customary unit | pound (lb), ounce (oz), slug | pound-force (lbf) |
| Changes with location? | No | Yes (Earth, Moon, altitude) |
| Scalar or vector? | Scalar | Vector (points toward the center of gravity) |
| Typical instrument | Balance | Spring scale or force gauge |
The pound causes the most confusion. In US law the pound is a unit of mass (exactly 0.45359237 kg). Engineers, by contrast, also use the pound-force, which is the weight of a 1 lb object under standard gravity, about 4.448 N. So when a bathroom scale shows “150 lb”, it reports your mass (about 68.04 kg), estimated from the force you press down with.
Mass Units and Conversions
Today, most of the world uses the kilogram and its multiples. However, this is not universal. The United States also uses pounds and ounces, while jewelers, pharmacists and reloaders keep a few special units alive. In addition, every value below comes from an exact legal definition.
| Unit | Symbol | Exact or derived value | Where you see it |
|---|---|---|---|
| Microgram | mcg (µg) | 0.000001 g | Vitamins, medicine |
| Milligram | mg | 0.001 g | Medicine, nutrition labels |
| Grain | gr | 64.79891 mg (exact) | Bullets, arrows, some pills |
| Carat | ct | 200 mg (exact) | Gemstones |
| Gram | g | 0.001 kg | Cooking, lab work |
| Ounce (avoirdupois) | oz | 28.349523125 g (exact) | Food, postage |
| Pound | lb | 453.59237 g (exact) | Body weight, groceries |
| Kilogram | kg | about 2.2046 lb | SI base unit |
| Slug | slug | about 14.594 kg | US engineering |
| Metric ton (tonne) | t | 1,000 kg | Shipping, industry |
For a closer look at the smaller units, see our guide to how much an ounce is and our explainer on what ug means on a label.
How the Kilogram Is Defined Today
From 1889 to 2019, the kilogram was simply the mass of one object: a platinum-iridium cylinder kept near Paris and nicknamed “Le Grand K.” As a result, every scale in the world traced back to it. Over the decades, though, careful comparisons showed its official copies drifting by tens of micrograms relative to it, and nobody could tell which one was “right.”
So in November 2018, the General Conference on Weights and Measures voted to redefine the unit, and the change took effect on May 20, 2019. Now the Planck constant has a fixed value, and labs realize the kilogram with a Kibble balance, which weighs a test object against electromagnetic force. As NIST explains, the unit no longer depends on any single object that can get scratched, dirty or lost. Of course, your kitchen scale did not change at all; only the deepest link in the chain did.
How to Measure Mass With a Balance or Scale
In practice, a good reading depends more on technique than on price. Use these steps for a kitchen, pocket or lab scale.
- Place the scale on a firm, level surface. A wobbly counter or a soft tablecloth skews the load cell.
- Let it warm up. Electronic scales drift for the first minute or two after you switch them on.
- Zero or tare it. Press tare with the empty container on the pan so the display reads 0.
- Check it with a known weight. A calibration weight shows whether the reading is still true.
- Add the sample gently to the center. Otherwise, off-center loads and drops can shift the reading.
- Wait for a stable reading. Most scales show a stability mark; record the value only then.
- Write down the unit. Note g, oz or lb with the number so nobody confuses them later.
Balances vs Scales: Which Tools Measure Mass?
A balance compares an unknown object with known masses. Because gravity pulls on both sides equally, the result stays the same on Earth or on the Moon. For example, a triple beam balance in a school lab works this way.
A scale, by contrast, measures force. A spring scale reads how far a spring stretches, and a digital scale reads the tiny bend of a strain-gauge load cell. The scale then converts force into kilograms or pounds by assuming local gravity. That is why a precise lab scale needs calibration where it sits. Our full guide to the tools used to measure mass compares each type in detail.
| Instrument | What it senses | Typical resolution | Best for |
|---|---|---|---|
| Triple beam balance | Compares against sliding riders | 0.1 g | Classrooms, teaching |
| Kitchen scale | Force on a load cell | 1 g (some 0.1 g) | Cooking, baking, postage |
| Pocket scale | Force on a load cell | 0.01 g | Jewelry, coffee, small samples |
| Analytical balance | Electromagnetic force compensation | 0.0001 g | Chemistry and pharmacy labs |
| Spring scale | Stretch of a spring | Varies widely | Fishing, luggage, classroom force demos |
In physics problems, you can also find it indirectly. For instance, if you know the force and the acceleration, m = F / a. So a net force of 3,000 N that speeds a car up at 3 m/s^2 means the car is 1,000 kg. Likewise, momentum and kinetic energy tie it to speed, as our guide on using mass to find speed shows.
Do and Don’t When Weighing Things
Do
- First, tare the container before adding the sample.
- Check your scale against a known weight now and then.
- Stay within the rated capacity of the scale, because overloads can damage it.
- Also keep the pan clean and dry.
- Record the unit along with the number.
Don’t
- Weigh on a carpet, a towel or an uneven table.
- Read the display before it settles, because it may still drift.
- Confuse ounces of weight with fluid ounces of volume.
- Trust a 0.01 g reading next to a fan or open window.
- Drop heavy objects onto a load cell, since the shock can bend it.
Honest Limits of Everyday Measurements
Admittedly, no scale is perfect. First, every one has a resolution (the smallest step it shows) and an accuracy (how close it is to the true value), and these are not the same. A pocket scale may display 0.01 g yet only be accurate to plus or minus 0.03 g.
Second, a digital scale assumes a value for gravity. Gravity varies by about 0.5 percent between the equator and the poles, so a scale calibrated in one city can read slightly off in another. For cooking, however, that error is irrelevant. In a lab, by contrast, it is one reason technicians recalibrate after moving equipment.
Finally, buoyancy plays a small part. Specifically, air pushes up on every object, so a bulky, low-density item reads very slightly light. As a result, analytical labs correct for this; at home, you never need to.
When to Call a Professional
However, some jobs need certified results. Instead of trusting a consumer scale, use an accredited calibration lab when a scale prices goods for sale, when a lab must meet ISO 17025 or GLP rules, or when a reading supports a legal or safety decision. In addition, state weights-and-measures offices inspect commercial scales under NIST Handbook 44. Similarly, for body measurements and dosing, ask a doctor or pharmacist.
Frequently Asked Questions
What is mass in simple words?
It is how much matter an object contains. It also tells you how hard the object is to speed up, slow down or turn.
What is the SI unit of mass?
The kilogram (kg). Grams, milligrams and metric tons are decimal multiples or fractions of it.
Is mass the same as weight?
No. Weight is the force of gravity on an object and changes with location. The amount of matter stays the same everywhere.
Does my mass change on the Moon?
No. Your body contains the same matter, but your weight falls to about one-sixth of its Earth value because lunar gravity is weaker.
How many pounds are in a kilogram?
One kilogram is about 2.2046 pounds. Going the other way, one pound is exactly 453.59237 grams.
Can mass be negative?
Not for any ordinary object. Every known particle has either zero or positive rest mass.
Do photons have mass?
Photons have zero rest mass. Even so, they still carry energy and momentum, which is why light can push on a solar sail.
What tool measures mass most directly?
A balance, because it compares the object with known masses. Gravity acts on both sides equally, so location does not matter.
Why does a bathroom scale show pounds or kilograms?
It senses the force you apply and then divides by an assumed gravity. So the display reports an estimate of your body mass.
Is mass conserved in chemical reactions?
For practical purposes, yes. The total stays the same before and after a reaction, a rule chemists call the law of conservation of mass.
The Bottom Line on Mass
To sum up, mass is the amount of matter in an object and its resistance to acceleration, measured in kilograms or pounds. Also, it never changes with location. Weight, meanwhile, is the pull of gravity on that matter and changes from Earth to the Moon.
For everyday use, a good digital scale and a calibration weight give you reliable numbers. For conversions, the calculator at the top of this page handles kilograms, grams, pounds, ounces, grains and carats with exact definitions.
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