Why CMD set /a Gives the Wrong Disk Offset

CMD set /a uses signed 32-bit integer arithmetic, so an LBA-to-byte calculation can overflow even when the LBA itself fits. A negative result, or a suspicious zero, may be a calculation problem rather than evidence about the disk.

For disk addresses, recalculate from the original number using exact integer arithmetic in PowerShell. Preserve the original numeric base and use the logical block size that belongs to that address.

For a decimal LBA, start with the short PowerShell calculation below. For a hexadecimal LBA, use the input-checking function. Then check the result against the original units, address origin, and device bounds before using it.

Recalculate a decimal address in PowerShell

Open Windows PowerShell. No administrator rights are needed: this example only calculates numbers. Replace both quoted values with the original decimal LBA and the confirmed logical bytes per block. Do not reuse CMD’s overflowed result.

$lba = [bigint]'8388608'
$logicalBytes = [bigint]'512'
if ($lba -lt 0 -or $logicalBytes -le 0) {
    throw 'Use a nonnegative LBA and a positive logical block size.'
}
$byteOffset = $lba * $logicalBytes
$byteOffset.ToString()

With the example values, the result is 4294967296 bytes, or 4 GiB from the LBA’s origin. Your own inputs will produce a different result. Keep the exact byte count for the next check.

This short example assumes you have already checked the input format. For validation that rejects signs, commas, fractions, and expressions, use the function below. Use quoted decimal strings for large input values. In Windows PowerShell 5.1, a sufficiently large unquoted literal can lose precision before a later [bigint] cast gets to it. Converting an already rounded number cannot restore its missing digits.

Also avoid [int] for disk offsets: it is the 32-bit integer type. A 64-bit type is useful within its range, but arbitrary-precision [bigint] provides a straightforward way to keep these calculations exact.

Why CMD set /a overflows an LBA conversion

A signed 32-bit integer ranges from -2,147,483,648 to 2,147,483,647. Disk byte addresses can exceed that positive limit very quickly.

These are safe arithmetic demonstrations to enter in Command Prompt, without administrator rights. They do not access storage:

set /a 4194304*512
set /a 8388608*512

Enter the lines separately in Command Prompt. These arithmetic examples produce the CMD results below; the final column shows the exact mathematical result.

Both LBAs fit in the signed 32-bit range. Multiplying by a 512-byte logical block size is what crosses the boundary.

The second example is especially easy to miss: zero looks like a plausible disk offset, but the correct result is 4 GiB from the origin. Successful command completion does not certify an overflow-free result.

Changing the display to hexadecimal or taking the absolute value of a negative result does not recover a generally valid calculation. Start again from the original input.

Check leading zeros and signed hexadecimal

CMD set /a treats ordinary numeric input as decimal. However, a 0x prefix means hexadecimal, and a leading 0 selects octal.

This matters when a tool pads an address with zeros. The text 010 might mean decimal ten in the tool’s output, yet become eight when handed to CMD arithmetic.

Non-numeric text can also be interpreted as an environment-variable name. An undefined name may contribute zero instead of producing the input-validation error you expected.

Keep the original output and establish its documented base. Do not remove a prefix or leading zeros merely to make an error message disappear.

Parse unsigned hexadecimal explicitly

A storage address is often intended to be nonnegative. Direct hexadecimal conversion can introduce a sign interpretation that is inappropriate for that address.

The following function accepts decimal digits or a 0x hexadecimal prefix, validates the input, and returns a nonnegative arbitrary-precision integer. Paste it into Windows PowerShell first:

function ConvertTo-UnsignedInteger {
    param([string]$Text)
    $valueText = $Text.Trim()
    $culture = [Globalization.CultureInfo]::InvariantCulture
    if ($valueText -match '^0x[0-9a-f]+$') {
        return [bigint]::Parse(
            ('0' + $valueText.Substring(2)),
            [Globalization.NumberStyles]::AllowHexSpecifier,
            $culture
        )
    }
    if ($valueText -match '^[0-9]+$') {
        return [bigint]::Parse($valueText, $culture)
    }
    throw 'Use unsigned decimal digits or hex with a 0x prefix.'
}

In the same PowerShell window, replace 0xFFFFFFFF with your original address and 512 with its confirmed logical block size, then run:

$lba = ConvertTo-UnsignedInteger '0xFFFFFFFF'
$logicalBytes = ConvertTo-UnsignedInteger '512'
if ($logicalBytes -le 0) {
    throw 'Logical block size must be positive.'
}
$byteOffset = $lba * $logicalBytes
$byteOffset.ToString()

The LBA is 4294967295 in decimal, and its 512-byte offset is 2199023255040.

The extra zero added before hexadecimal parsing preserves a positive sign when the highest hexadecimal digit would otherwise imply a negative two’s-complement value. It does not change the intended unsigned magnitude.

This function deliberately treats 010 as decimal ten and 0x10 as sixteen. It rejects minus signs, commas, fractions, and arithmetic expressions. Use the original source’s base when choosing the input; the parser cannot infer it for you.

Do not repair the wrong number

Once you have an exact result, check what it describes:

1. Units: an LBA is a block number. Multiply by logical bytes per block; do not substitute physical sector size or cluster size.
2. Origin: the result has the same origin as the LBA. A disk-relative address has not yet become volume-relative.
3. Bounds: compare it with the verified device or partition’s raw byte range.
4. Identity and time: confirm the source refers to this device and layout, especially if it came from an older event.

If you later need a cluster index, use integer quotient and remainder rather than rounding a fractional answer to the nearest whole number. A filename lookup based on an overflowed or wrongly rounded address can point at an unrelated location.

For the complete block-to-byte conversion and unit checks, see LBA to Byte Offset: Decimal, Hex, and Sector Size.

For the full path from a verified disk address to its current NTFS allocation, see From LBA to LCN: Which File Uses the Cluster Now?. A current filename does not prove which file used the location when an older error occurred.

None of these arithmetic commands fixes file system damage or confirms a failing drive. Their job is narrower: give you a trustworthy number before you interpret the storage evidence.