What it is
The Weight Converter moves between milligrams, grams, kilograms and tonnes on one side, and ounces, pounds, stones and US tons on the other.
The three places this comes up
Luggage. Airlines outside the metric world quote allowances in pounds. A 50 lb limit is about 22.7 kg, and a 23 kg allowance is about 50.7 lb, which is close enough that a bag packed to one can fail the other.
Body weight. The United Kingdom and Ireland still use stones for people: one stone is 14 pounds, about 6.35 kg. Someone described as "eleven stone" weighs roughly 70 kg. The United States uses plain pounds, so 154 lb is that same 70 kg.
Cooking. Recipes from North America measure solids by weight in ounces and pounds, and one ounce is about 28.35 g. Baking is where the conversion has to be right: flour and leavening are sensitive enough that rounding to the nearest 10 g changes the result.
Mass and weight are not the same thing
Physics separates them: mass is how much matter something contains, weight is the force gravity applies to it. The kilogram measures mass and the newton measures force.
In practice everyone says weight and means mass, and every scale on earth is calibrated for Earth's gravity, so the distinction only matters in a classroom or in orbit. This converter, like every kitchen scale, deals in mass.
How to use it
Choose both units, type the amount in either box, and read the other. Nothing is uploaded and there is no limit on how many conversions you make.