VB Convert to Decimal: How-To & Examples

This article explains how to convert values to decimal in Visual Basic (VB.NET and VBA), covering the main conversion functions, their syntax, and when to use each one. You’ll learn the difference between CDec, Convert.ToDecimal, and Decimal.Parse, and see practical examples for each.

Formula

The core conversion in VB is done by the CDec function, which converts an expression to Decimal. The syntax is simply:

CDec(expression)

Where expression is any valid numeric or string expression. The result is a Decimal data type, which is a 128-bit number that can hold up to 28-29 significant digits and is ideal for financial calculations.

Worked Example

Using the calculator above with the default values:

1. Input value: 123.45 (as a string)
2. Input type: String (multiplier = 1)
3. Formula: 123.45 * 1
4. Result: 123.45

If you change the input type to Double, the multiplier becomes 2, so the result would be 246.90. The calculator simply demonstrates the multiplicative factor; in real code you would not multiply—you would call the appropriate conversion function.

Why Convert to Decimal?

In VB, the Decimal type is the go-to for precise numeric operations, especially when dealing with money, measurements, or any value where rounding errors from binary floating-point (like Double or Single) are unacceptable. For instance, 0.1 + 0.2 in a Double yields 0.30000000000000004, but in Decimal it gives exactly 0.3. Converting to decimal ensures accuracy in calculations that require a fixed number of decimal places.

Main Conversion Methods

VB offers several ways to convert to Decimal, each with its own nuances:

• CDec(expression) – The classic VB conversion function. It’s concise and works on strings, numbers, and objects. It uses the current culture settings for decimal separators.
• Decimal.Parse(string) – Converts a string to a Decimal. It throws a FormatException if the string is not a valid number.
• Convert.ToDecimal(value) – A .NET Framework method that converts various types (including String, Double, Integer) to Decimal. It’s more explicit and can handle Nothing (returns 0).
• Decimal.TryParse(string, result) – A safe version that returns a Boolean indicating success, avoiding exceptions.

How to Choose the Right Method

The choice depends on your context:

• If you’re writing classic VB (VBA) or want a short, familiar syntax, use CDec.
• If you’re in VB.NET and need to convert from a non-string type (e.g., Integer), Convert.ToDecimal is straightforward.
• If you’re parsing user input from a text box, use Decimal.TryParse to handle invalid entries gracefully.
• If you need to control formatting (like thousands separators), Decimal.Parse with a CultureInfo parameter is best.

Common Mistakes and Pitfalls

Here are frequent errors when converting:

• Ignoring culture: CDec("123.45") works in en-US, but in locales where the decimal separator is a comma, it will fail. Use Decimal.Parse with CultureInfo.InvariantCulture for consistent behavior.
• Overflow: Decimal has a max value of about 7.9×10^28. Converting a huge Double like 1e30 will throw an OverflowException.
• Trailing zeros: CDec(1.2300) yields 1.2300, but when displayed it may show 1.23. This is normal.
• Null values: CDec(Nothing) in VB.NET returns 0, but in VBA it may cause an error. Always check for Nothing first.

Performance and Precision

Decimal is slower than Double for arithmetic because it’s implemented in software, but for most business applications the difference is negligible. The precision is far superior: while Double has 15-16 significant digits, Decimal has 28-29. This makes it ideal for financial calculations, but overkill for scientific computations where performance is critical.

Alternatives to Decimal

If you don’t need the full precision of Decimal, consider:

• Integer/Long – For whole numbers, use these for speed and memory.
• Double – For scientific calculations where a tiny rounding error is acceptable.
• Currency (VBA only) – A fixed-point type with 4 decimal places, useful for money but with less range than Decimal.

Frequently Asked Questions

What is the difference between CDec and Convert.ToDecimal?

CDec is a VB-specific function that uses the current culture for parsing, while Convert.ToDecimal is a .NET method that also respects culture but offers more overloads and can handle Nothing gracefully. In practice, they are similar, but Convert.ToDecimal is more consistent across .NET languages.

How do I convert a string with commas to decimal?

If the string uses commas as decimal separators (e.g., “123,45”), you need to specify the appropriate culture. For example: Decimal.Parse("123,45", New CultureInfo("fr-FR")). Alternatively, replace commas with periods and use InvariantCulture.

Why do I get an OverflowException when converting?

This happens when the value is too large for the Decimal type (max ~7.9×10^28) or when converting from a Double that has a fractional part beyond the decimal’s precision. Check the value and consider using Double if it’s extremely large.

Can I convert a boolean to decimal?

Yes, Convert.ToDecimal(True) returns 1, and Convert.ToDecimal(False) returns 0. CDec also works, but it may not be as intuitive.

How do I handle invalid input without exceptions?

Use Decimal.TryParse. It returns True if successful and sets the result, otherwise False. This is the recommended way to parse user input.

Is Decimal the same as a floating-point type?

No. Decimal is a fixed-point type (though it can hold a large range), while Double and Single are binary floating-point. Decimal stores numbers as integers scaled by a power of 10, which is why it avoids binary rounding errors.

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in val | Value to convert | - | 123.45 | 0
in type | Input type | String=1, Double=2, Integer=3 | 1 | 0
out dec | Decimal result | - | val * type | 2