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COBOL has dedicated verbs for each arithmetic operation. They read like English.

ADD

       ADD 5 TO TOTAL.
       ADD A TO B GIVING RESULT.
       ADD A B C TO TOTAL.                     *> add multiple
       ADD 1 TO X ON SIZE ERROR DISPLAY "overflow".

ADD ... TO X modifies X. ADD ... GIVING X puts the result in X (X needn't be initialized).

SUBTRACT

       SUBTRACT 5 FROM TOTAL.
       SUBTRACT A FROM B GIVING RESULT.

MULTIPLY / DIVIDE

       MULTIPLY 2 BY X.
       MULTIPLY A BY B GIVING RESULT.
       DIVIDE A BY B GIVING RESULT.
       DIVIDE A BY B GIVING QUOTIENT REMAINDER REM.

Note BY in MULTIPLY but INTO/BY in DIVIDE:

• DIVIDE A INTO B GIVING X. → X = B / A
• DIVIDE A BY B GIVING X. → X = A / B

Keep them straight or you'll spend an afternoon debugging.

COMPUTE

For anything more complex, use COMPUTE — write expressions in normal infix:

       COMPUTE RESULT = (A + B) * 2.
       COMPUTE INTEREST = PRINCIPAL * RATE / 100.
       COMPUTE AVG = SUM / N.
       COMPUTE X = A ** 2 + B ** 2.            *> exponentiation

Operators:

• + - * / — arithmetic
• ** — exponentiation
• Parentheses for grouping

ROUNDED and SIZE ERROR

       COMPUTE PRICE ROUNDED = COST * 1.07.
       ADD A TO TOTAL
           ON SIZE ERROR DISPLAY "overflow"
           NOT ON SIZE ERROR DISPLAY "ok".

• ROUNDED — round to PIC's precision (default truncates)
• ON SIZE ERROR — handle overflow (result wouldn't fit in target)

Type considerations

COBOL math respects PIC. COMPUTE X = 7 / 2. with X PIC 9(3) gives 003 (truncated). With PIC 9(3)V99 you get 003.50.

PIC S9(...) allows negatives. Without S, results are unsigned (negative results may underflow weirdly).

For heavy math, consider COMP (binary integer) or COMP-3 (packed decimal) — both faster than display PIC for arithmetic.

Example: Compound interest

       01 PRINCIPAL  PIC 9(7)V99 VALUE 1000.00.
       01 RATE       PIC V99   VALUE .05.        *> 5%
       01 YEARS      PIC 9(2)  VALUE 10.
       01 RESULT     PIC 9(9)V99.
           COMPUTE RESULT = PRINCIPAL * (1 + RATE) ** YEARS.
           DISPLAY "after " YEARS " years: " RESULT.