The scale converts any measurement you enter into centimeters or inches, as these are the most common units used when designing LEGO MOCs.

For instance, if a car is 5m (500cm) in real life, converting it to a 1:50 scale means dividing 500 by 50, giving a 10 cm long LEGO model.
This makes it easy to create accurate, proportional builds.

You don't need to enter all measurements. The converter will only convert the values you enter. If you haven't entered anything if will display nothing.

A scale is a ratio that represents the size relationship between a model and the real object. A 1:1 scale means the model is identical in size to the original object.
As the second number in the ratio increases, the model becomes smaller.

For example, a 1:100 scale means the model is 100 times smaller than the original, where 1 unit on the model equals 100 units on the real object.

That would depend on the amount of LEGO bricks you have (or want to use) and on the final size of the model. If you want to make a miniaturized version of a modern ship, I suggest a scale between 1:1000 and 1:4000.

However something more detailed or smaller in size (like an airplane or automobile) would need a smaller scale like 1:50, 1:75, 1:400.

For context, the LEGO Titanic set (10294) has a scale of approximately 1:200 while LEGO Concorde set (10318) has a scale of approximately 1:90.

A LEGO stud is the width and length of a standard 1x1 LEGO Brick. Measuring at about 8mm, this unit can helps you visualize the length and width of the model when building with real LEGO bricks. It can also help you create border limits of your model when designing it.

For height measurements, LEGO studs are measured stacked upwards on their side (8mm) and aren't much relevant unless you are building a symmetrical design on all 3 axes.