It’s usually an error to add quantities that have different units. If you have an equation that adds meters and kilograms, for example, something is almost certainly wrong. A formula that multiples meters and kilograms might be correct, but not one that adds them.
Why is this? It’s because the laws of nature don’t depend on our choice of units. For example, F = ma whether you use English, metric, or any other system of units.
Changing units introduces a multiplicative factor. You convert inches to centimeters, for example, by multiplying by 2.54.
If you were to add a length to a mass, and say you cut your unit of length in half but left your unit of mass the same, this wouldn’t be a multiplicative change (unless the mass were zero). But when you multiply or divide quantities with different units, the result does scale multiplicatively.