Mass vs. Weight: Is There Actually a Difference for Everyday Conversion
Mass is a measure of the amount of matter in an object and stays constant regardless of location. Weight is technically the force of gravity acting on that mass, which varies with gravitational strength (different on the Moon than on Earth, for example) — but for virtually all everyday purposes on Earth, gravity is effectively constant, so kilograms and pounds are used interchangeably as if they measured the same thing without practical consequence.
This is a genuine scientific distinction, but it's worth knowing specifically why it almost never matters for everyday conversion despite being technically real.
The technical distinction
In physics, mass (measured in kilograms) is an intrinsic property of matter, while weight (technically measured in units of force, like newtons) is the gravitational force acting on that mass — the same object has the same mass everywhere, but its weight changes if the local gravitational strength changes, such as on the Moon, where gravity is much weaker.
Why kilograms and pounds are still used as if they're the same
In colloquial and everyday use, "pounds" and "kilograms" are both used to describe an object's heaviness on Earth, and since Earth's gravity is effectively constant for everyday purposes, this colloquial use causes no practical problem — the technical mass/weight distinction only becomes practically relevant in contexts like spaceflight or physics calculations specifically involving different gravitational environments.
When the distinction actually matters
For anything involving different gravitational environments (space travel, comparing what an object would weigh on different planets) or precise scientific and engineering calculations, correctly distinguishing mass from weight (and using the correct units — kilograms for mass, newtons for weight) genuinely matters — for everyday conversion between kg, lb, oz, and stone on Earth, it doesn't.