Order of Operations (PEMDAS) — Definition, Formula & Examples
Order of Operations (PEMDAS) is the set of rules that tells you which calculations to perform first when an expression has more than one operation. The acronym PEMDAS stands for Parentheses, Exponents, Multiplication, Division, Addition, Subtraction.
The order of operations is a convention that establishes a hierarchy for evaluating mathematical expressions: (1) evaluate expressions inside grouping symbols (parentheses, brackets, braces) first, working from the innermost group outward; (2) evaluate exponents and roots; (3) perform multiplication and division from left to right; (4) perform addition and subtraction from left to right. Multiplication and division share the same priority, as do addition and subtraction.
How It Works
When you see an expression like , you need a rule to decide whether to add first or multiply first—different choices give different answers. PEMDAS provides that rule. Start by simplifying anything inside parentheses, then handle exponents. Next, scan left to right and perform all multiplication and division in the order you encounter them. Finally, scan left to right again for addition and subtraction. The key detail many students miss is that multiplication and division are equal in rank (same for addition and subtraction), so you resolve ties by working left to right.
Worked Example
Problem: Evaluate:
Parentheses: Simplify inside the parentheses first.
Exponents: Evaluate the exponent next.
Multiplication and Division (left to right): Multiply first (it appears before the division when reading left to right), then divide.
Addition: Finally, add.
Answer:
Another Example
Problem: Evaluate:
Division and Multiplication (left to right): Division comes first when reading left to right, then multiplication.
Subtraction: Subtract last.
Answer:
Why It Matters
Every math course from 6th-grade pre-algebra onward assumes you follow the order of operations. Without it, formulas in science, finance, and engineering would be ambiguous—two people could read the same expression and get different results. Standardized tests like the SAT and ACT regularly include problems designed to test whether you apply PEMDAS correctly.
Common Mistakes
Mistake: Always doing multiplication before division (or addition before subtraction) because of the letter order in PEMDAS.
Correction: Multiplication and division share the same rank. So do addition and subtraction. Within each pair, work left to right. For example, , not .
Mistake: Forgetting to simplify everything inside parentheses before moving to the next step.
Correction: Resolve all operations inside grouping symbols first, including nested parentheses (innermost first), before applying exponents or anything outside.
