The Stack Force;
A Handy Tool for Mentalism & Mind Reading
Developed and written by Bob Goodwin
Edited by Karl Rabe
Spreadsheet calculation utility by Karl Rabe
Copyright 2022 by Robert Goodwin. All rights reserved.
The Stack Force enables you to force a predetermined number on a spectator by having them select several numbers from a list. No matter what numbers they select, the total will be the force number.
Examples of numbers you may want to force:
 Year in which a significant event occurred (Houdini's birth  1874)
 Anniversary (S.A.M. formed in 1902)
 Specific word in a book ("book test")
 Military unit designation (101st)
 Addresses
 Phone numbers
References and Acknowledgements
In “Secrets Of Mental Math", (A. Benjamin & M. Shermer, 2006 p.212) teaches an effect called AN “AMAZING” SUM. Per the authors, it was first shown to them by James “the Amaz ing” Randi, who has used it effectively in his magic. The forcing of a single number is described.
I have “reverse engineered” this, so to speak, to provide a stepbystep guide to calculating the necessary numbers to force ANY four or five digit number you choose.
Worksheet Example of a Stack Force Routine
Houdini's Spirit Is Asking From The 'Other Side': "When was I born?"
 Choose one number from each category
 Add Your Chosen Numbers Here. The numbers are “stacked” vertically, not written horizontally. So for example, 719 chosen from column A is written vertically in column A below.
A  B  C  
+  
+  
=  Houdini Birth Year 
Notes:
 This routine is based on the birth year of Houdini (1874).
 Try it yourself. No matter what combination of numbers you choose from A, B and C they will always add to 1874.
 The numbers shown above were developed using the target calculated using the method explained below – The digits of the numbers in Category A all add to 17, the digits of the numbers in Category B all add to 16, and likewise, the digits of the numbers in Category C all add to 14.
How To Set Up A Stack Force To Force A Certain Number
Step  Number  Example  
  a  b  c  
1  Identify the Target Force Number: 34 digits work best.  1  8  7  4 
This 4digit target is based on the year Harry Houdini was born. With a 4digit target, 3 columns are needed (labeled A, B, & C) to stack vertically and add the numbers.

2  Calculate the number totals needed in each column to force the number: Work from right to left, and top to bottom. Select numbers so the sum of the digits in each column add to the target number for that column.  1  4  This # is same as last # in force no., with 1 preceding (col. C)  
1  6  Next # is 6, with 1 preceding (col. B).  
7  Next # is 7, with 1 preceding (col. A).  
8  7  4  Target # totals (14, 16, & 17) add to 1874  
3  Confirm the column targets by adding some sample numbers and checking the total.  7  8  4  #s in col. C add to target (14)  
1  8  8  #s in col. B add to target (16)  
9  0  2  #s in col. A add to target (17)  
8  7  4  
After confirming the numbers needed for your stack force (step 3), you now need to prepare the worksheet to be reviewed with the volunteer during your routine  see Worksheet Example of a Stack Force Routine above for an example.
You may find that some target force numbers don't require different numbers in the columns. If your calculation in Step 2 above results in the same value for all three numbers. (Example: A target of 1776 will require all three numbers to have digits adding to 16. In this case, you do not need to have three categories of numbers. All numbers the spectator choose can be lumped together in one group.
Stack Force Calculator
This section by Karl Rabe.
After editing Bob's Stack Force above, I began playing around with Excel to see if I could automate the calculations. The result was the Stack Force Calculator spreadsheet. Click the icon below to open it. You have to enable macros and of course you need to have Microsoft Excel installed.
The Stack Force Calculator will automatically generate a set of "choice" numbers to force a desired target
number as per Bob Goodwin's method for creating a stack force. On the Stack Calc tab, enter the desired force number in the yellow cell. Any number up to 10 digits can be used. The calculcate will generate all possible choice number to use. Select your numbers from the list for each digit. You can view the Tester tab which will show the manually calculated result for each set of choices.
Here is an example of the output of the calculator, using the force number in Bob's example above, Houdini's Year 1874. You can see it produces the same target totals... 14, 16 and 17. It then provides 20 (only 10 shown here) randomly generated 3 digit numbers where the digits add to those target totals.
If you are even just a little comfortable with excel, give it a try.
Column  1  2  3 
Target  14  16  17 
Choice 1  590  565  359 
Choice 2  752  556  584 
Choice 3  707  448  944 
Choice 4  860  196  566 
Choice 5  428  565  575 
Choice 6  905  529  386 
Choice 7  824  367  629 
Choice 8  914  880  719 
Choice 9  653  880  476 
Choice 10  365  178  368 