Spreadsheet to calculate how to cut pipes
You didn't declare y or x.

Always turn on Option Explicit. You can have it automatically by going to Tools -> Options in the VBE and checking the Require Variable Declaration option. This way if you have any variables not defined, the compiler will let you know.

When you don't define your variable, VBA will declare it as a Variant type that can hold any type of data. While this may be more flexible, it adds processing time to your macro as VBA decides or tests for the type. 

Additionally, since a Variant can be any type of data, you may miss out on valuable troubleshooting information on Type Mismatch

By not declaring variables, you could possibly be paying a penalty.



It's good practice to indent all of your code that way Labels will stick out as obvious. You also have a ton of unneeded white space. To begin, it might look like this:

Option Explicit

Sub CalcPipe()
    Dim StartPipeLng As Long, LastLng As Long, TotalLng As Long, PipeName As Long
    StartPipeLng = Cells(2, 4).Value
    LastLng = ActiveSheet.UsedRange.Rows(ActiveSheet.UsedRange.Rows.Count).Row
    TotalLng = Application.WorksheetFunction.SumProduct(Range("A2:A" &amp; LastLng), Range("B2:B" &amp; LastLng))
    TotalLng = Application.WorksheetFunction.RoundUp(TotalLng / StartPipeLng, 0)
    Cells(5, 4).Value = TotalLng
    PipeName = 1
    Dim y As Long
    Dim x As Long
        For y = 6 To TotalLng + 5
            Cells(1, y).Value = "Pipe " &amp; PipeName
            PipeName = PipeName + 1
            StartPipeLng = Cells(2, 4).Value
            For x = 2 To LastLng
                If StartPipeLng - Cells(x, 1).Value &gt;= 0 And Cells(x, 2).Value &lt;&gt; Cells(x, 3).Value Then
                    StartPipeLng = StartPipeLng - Cells(x, 1).Value
                    Cells(x, 3).Value = Cells(x, 3).Value + 1
                    Cells(x, y).Value = Cells(x, y).Value + 1
                    x = x - 1
                End If
            Next x
        Next y
    Columns(3).Clear
End Sub




Variable names - give your variables meaningful names. 
Standard VBA naming conventions have camelCase for local variables and PascalCase for other variables and names.

Dim initialPipeLength As Long
Dim lastPipeRow As Long
Dim totalPipes As Long
Dim pipeNumber As Long
Dim rowCounter As Long
Dim y As Long




A loop like this -

For x = 2 To lastPipeRow
    If firstPipeRow -  Then
        x = x - 1
    End If
 Next x


Could probably just be

For x = lastPipeRow to 2 Step -1
Next


But, since it's not a standard loop and that won't work, you should use variable names to make it more obvious what is happening there.



There is a standard way to find lastRow and lastColumn. That post explains why.

lastPipeRow = ActiveSheet.Cells(Rows.Count, 1).End(xlUp).Row




Also, since you reuse your range, I'd add something to catch if you're accidentally overwriting data

Dim result As VbMsgBoxResult
If Not IsEmpty(ActiveSheet.Cells(1, 6)) Then
    result = MsgBox("There is still data on this sheet, do you want to overwrite it?", VbMsgBoxStyle:=vbYesNo)
    If result = vbNo Then Exit Sub
End If


I'd also clear out old data

If Not IsEmpty(ActiveSheet.Cells(1, 6)) Then
    result = MsgBox("There is still data on this sheet, do you want to overwrite it?", vbYesNo)
    If result = vbNo Then Exit Sub
    Dim lastColumn As Long
    lastColumn = ActiveSheet.Cells(1, Columns.Count).End(xlToLeft).Column
    ActiveSheet.Range(Columns(6), Columns(lastColumn)).Clear
End If




To avoid using column 3, use an array. But, I'd say since you want to use an array, you could optimize your solution to the cutting stock problem with the Greedy algorithm.