What Are the Units Used to Measure Weight and Mass?

The metric base unit for mass is the kilogram. The US customary base unit for weight is the pound.

Both measure roughly the same everyday concept — “how heavy is it.” Technically, mass and weight aren’t identical, and the units reflect that distinction. Most people use them interchangeably without ever running into a problem, though.

Below is the full unit breakdown for both systems, and the actual physics difference between mass and weight. That difference almost never matters — unless you’re doing something unusual, like designing spacecraft.

Metric mass units

The kilogram is the metric unit for mass everything else scales from. Grams and milligrams are fractions of it; the metric ton is a multiple.

That’s what “what is the base metric unit for mass” is really asking. The kilogram is the one official SI base unit, and every other mass unit in the metric system gets defined relative to it.

US customary weight units

A US “short ton” (2,000 lbs) differs from a UK “long ton” (2,240 lbs). Both differ from a metric ton (2,204.6 lbs) too. All three get casually called “a ton,” but they aren’t interchangeable.

Reading a weight figure labeled just “ton” with no country attached? Confirm which one is meant. That matters most for freight, construction materials, and vehicle weight ratings.

Converting between the two systems

Worked example: a 5 kg bag of flour weighs 5 × 2.20462, or about 11.02 pounds.

Reverse example: a 150-pound adult weighs 150 × 0.453592, or about 68.04 kilograms.

Mass vs. weight: the actual difference

Mass is the amount of matter in an object. It doesn’t change no matter where the object sits. Weight is the force gravity exerts on that mass, and it changes with gravity’s local strength.

An object weighs about one-sixth as much on the Moon as it does on Earth. Its mass — the actual amount of stuff it’s made of — stays exactly the same in both places.

Technically, a kilogram is a unit of mass, while a “kilogram-force” (used in some older engineering contexts) is a unit of weight.

For everyday purposes, “kg” gets used to mean “how heavy something feels here on Earth.” Nobody’s using it wrong by doing that. The distinction only turns practically important off-planet, or in physics coursework.

Why this distinction almost never matters in daily life

Unless you’re doing physics calculations, or working somewhere gravity genuinely differs — spacecraft design, lunar mission planning — you can ignore the distinction. Earth’s gravity stays constant enough across its surface. Mass and weight track together predictably as a result.

A bathroom scale, a kitchen scale, and a shipping label all use “weight” units to communicate what is technically mass. That’s a perfectly fine practical shorthand everywhere gravity reads as 1g — which covers essentially everywhere a person will ever stand on a scale.

How weight and mass units show up in daily situations

• Cooking: US recipes typically measure dry ingredients by volume — cups, tablespoons. UK and most European recipes measure by mass (grams) instead. That’s actually more accurate, since a “cup of flour” can vary in weight depending on how tightly it’s packed.
• Body weight: the US reports body weight in pounds. The UK often uses stone and pounds together — a “12 stone 4” reading. Most other countries use kilograms exclusively.
• Shipping: US carriers price by pounds and ounces. International carriers and most manufacturers price by kilograms and grams, so cross-border shipments often list both.
• Produce and groceries: US grocery scales weigh in pounds and ounces. Most of the world weighs in kilograms and grams, which is why imported packaging sometimes carries both units printed side by side.

Frequently Asked Questions

What is the base unit for measuring mass in the metric system?
The kilogram. Unusually, it’s the one SI base unit that carries a prefix (“kilo-“). That’s a historical quirk — the metric system was originally defined against a physical reference object, not the gram.

What unit is used to measure weight in science?
In formal physics, weight is a force, measured in newtons, while mass gets measured in kilograms. Outside strict physics contexts — engineering, everyday conversation, most chemistry and biology — “weight” commonly gets expressed in kg or lbs interchangeably with mass. That’s fine in non-technical writing.

How many grams are in an ounce?
About 28.35 grams (28.3495 to be precise). A standard US letter-sized envelope weighs roughly 5 grams. A slice of bread runs roughly one ounce, or about 28 grams.

What are the units used to measure mass for very small or very large quantities?
For very small masses, metric uses milligrams and micrograms, common in medicine and chemistry. For very large masses, metric uses metric tons; US customary uses short tons.

Both scale from the same base unit — grams on the metric side, pounds on the customary side. Only the multiplier changes, whether that’s powers of ten or fixed ratios.

Is a kilogram the same everywhere in the world, or does it vary like weight does?
Mass, and therefore the kilogram, stays the same everywhere. It’s a fixed property of matter. What changes by location is weight — the force of gravity acting on that mass. That’s why an astronaut’s mass in kilograms stays constant in orbit, even though they experience apparent weightlessness.

What is the unit for measuring weight of a person versus weight of a package?
There’s no separate unit. A person’s weight and a package’s weight both use the same scale — pounds/ounces in the US, kilograms/grams elsewhere. The only practical difference is which subunit is convenient. People typically get weighed in whole pounds or kilograms, while packages and ingredients often get weighed down to the ounce or gram for precision.

Why does the US still use pounds instead of kilograms?
Historical inertia, mostly. The US customary system predates widespread metric adoption, and consumer habits, road signs, and product packaging never fully converted. Scientists and the military in the US already use metric units for precision work — it’s daily commerce and casual speech that stuck with pounds.