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How to Calculate Percentage of a Number in Batch Script

Calculating percentages is a fundamental task, but it presents a challenge in batch scripting. The SET /A command, which is the only native tool for arithmetic, does not support floating-point numbers (decimals). This means a standard formula like 50 / 200 = 0.25 is impossible to calculate directly. SET /A would simply return 0.

To work around this, we must use integer math creatively by changing the order of operations to preserve our data's precision.

The Key Technique: Multiply Before You Divide​

The core principle for all percentage calculations in batch is to perform the multiplication before the division.

  • Wrong: (50 / 200) * 100 --> In integer math, this becomes 0 * 100 = 0. Incorrect.
  • Correct: (50 * 100) / 200 --> In integer math, this becomes 5000 / 200 = 25. Correct!

By scaling our number up first, we avoid the fractional part being truncated by the integer-only division.

Example 1: What is X% of a Number?​

This is the most common calculation: finding a percentage of a given value.

Goal: Calculate 25% of 200. Formula: (TotalValue * Percentage) / 100

@echo off
set "TotalValue=200"
set "Percentage=25"

set /a "Result = (TotalValue * Percentage) / 100"

echo %Percentage%%% of %TotalValue% is: %Result%

Output:

25% of 200 is: 50
note

The %% is used to display a literal percent sign in a batch script.

Example 2: What Percentage is A of B?​

Here we want to find the percentage that one number represents of another.

Goal: Find what percentage 50 is of 200. Formula: (Part * 100) / Whole

@echo off
set "Part=50"
set "Whole=200"

set /a "Result = (Part * 100) / Whole"

echo %Part% is %Result%%% of %Whole%.

Output:

50 is 25% of 200.

Creating a Reusable Percentage Function​

For any real script, creating a reusable function is far more practical. The function below can handle the most common percentage operations in one place.

How It Works:​

The :percent function takes a mode as its first argument to determine what to do:

  • OF: Calculates X% of Y. (e.g., 25 OF 200)
  • ISOF: Calculates what % X is of Y. (e.g., 50 ISOF 200)
  • INCREASE: Increases a number by a percentage. (e.g., 200 INCREASE 25)
  • DECREASE: Decreases a number by a percentage. (e.g., 200 DECREASE 25)
@echo off
SETLOCAL

ECHO --- Reusable Percentage Function Demo ---
ECHO.

ECHO 1. What is 35%% of 500?
CALL :percent OF 35 500 result
ECHO    Result: %result%
ECHO.

ECHO 2. What percentage is 60 of 240?
CALL :percent ISOF 60 240 result
ECHO    Result: %result%%%
ECHO.

ECHO 3. Increase 1500 by 10%%.
CALL :percent INCREASE 1500 10 result
ECHO    Result: %result%
ECHO.

ECHO 4. Decrease 80 by 25%%.
CALL :percent DECREASE 80 25 result
ECHO    Result: %result%
ECHO.

GOTO :EOF

REM ======================================================================
:percent mode val1 val2 [return_var]
:: Performs various percentage calculations using integer arithmetic.
::   mode [in] - Operation: OF, ISOF, INCREASE, DECREASE
::   val1 [in] - The first value or percentage
::   val2 [in] - The second value or percentage
::   return_var [out] - Variable to store the result

SETLOCAL
    set "mode=%~1"
    set /a "v1=%~2"
    set /a "v2=%~3"
    set "res=0"

    if /i "%mode%"=="OF" (
        set /a "res = (v2 * v1) / 100"
    ) else if /i "%mode%"=="ISOF" (
        set /a "res = (v1 * 100) / v2"
    ) else if /i "%mode%"=="INCREASE" (
        set /a "res = v1 + ((v1 * v2) / 100)"
    ) else if /i "%mode%"=="DECREASE" (
        set /a "res = v1 - ((v1 * v2) / 100)"
    ) else (
        echo ERROR: Invalid mode '%mode%' in :percent function.
    )
(
    ENDLOCAL
    IF "%~4" NEQ "" (SET %~4=%res%) ELSE (ECHO %res%)
)
EXIT /B

Output:

--- Reusable Percentage Function Demo ---

1. What is 35% of 500?
   Result: 175

2. What percentage is 60 of 240?
   Result: 25%

3. Increase 1500 by 10%.
   Result: 1650

4. Decrease 80 by 25%.
   Result: 60

Limitations and Considerations​

  1. Integer Overflow: All calculations are still bound by the 32-bit integer limit of 2,147,483,647. If an intermediate step (like Part * 100) exceeds this limit, the calculation will fail.
  2. Rounding: This method always rounds down (truncates). For example, (3 * 100) / 8 results in 37, not 37.5. There is no simple way to perform banker's rounding in pure batch.
  3. Advanced Math: For any calculations requiring higher precision, multiple decimal places, or more complex functions, using a more powerful scripting tool like PowerShell is highly recommended.

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