A page of algebra can look convincing while describing something impossible. One of the quickest ways to question it is to check the units. The habit is modest, almost unglamorous, but it keeps an answer attached to the physical quantity it claims to represent.
Name the quantity
Distance, time, current, and energy are different kinds of quantities. A number by itself does not tell us which one we have found. Writing the unit beside a result forces that question into the open and makes the calculation more legible to someone else.
The same care belongs at the beginning. Write down the input units before substitution. A value in milliamperes and a value in amperes may describe the same kind of quantity, but using them without conversion can change a numerical answer by a thousand.
A number becomes an answer only when we know what it measures.
Make both sides agree
If an equation claims to calculate speed, its dimensions should reduce to length divided by time. If the units instead describe length multiplied by time, something needs attention. A missing division or an incorrect rearrangement may be easier to spot through units than through another pass at the algebra.
Consistency cannot prove that a formula is correct. A wrong dimensionless factor can pass a unit check. So can a relationship that uses the right dimensions but the wrong physics. The check is a filter, not a substitute for understanding.
Try an extreme case
Unit checking becomes stronger when paired with a simple limiting case. What should happen when an input approaches zero? Does a result become unreasonably large when a resistance increases? Does a predicted time remain positive? These questions ask whether the mathematics still tells a sensible physical story.
A rough estimate is useful too. An order-of-magnitude expectation makes it harder for a misplaced decimal point to hide behind many digits. Precision is valuable after the result is plausible; it cannot rescue an implausible model.
Use the habit outside physics
The idea applies to planning and building as well. Revenue is not profit, a monthly number is not an annual number, and users are not the same thing as visits. Naming what a number measures prevents unlike things from being compared as if they were interchangeable.
The practical routine is simple: label the inputs, track the units, check a limiting case, and explain the result in a sentence. It takes little space, but it can save a great deal of confusion.
