This article contains information about two of the calculation settings
for Microsoft Excel: iteration and maximum change. This article also
includes information about how the settings effect calculations when you
use circular references in formulas.
Iteration is the process of repeatedly calculating values in a worksheet
until a specific numeric condition is met. In Excel, the condition is the
limit you set in the Maximum Iterations box on the Calculation tab in the
Options dialog box (click Options on the Tools menu).
To specify the maximum amount of change you will accept between
calculation results, type the amount in the Maximum Change box. The smaller
the number, the more accurate the result and the more time Microsoft Excel
requires to calculate a worksheet.
Maximum change is the maximum amount of change you will accept between
calculation results. To set this change value, type the number in the
Maximum Change box on the Calculation tab of the Options dialog box.
The two calculation settings, Maximum Iterations and Maximum Change, are only
enabled for that session of Excel. When you quit and restart Excel, the
options return to their default settings. However, you can save a workbook
that has custom calculation settings, so that when you open that workbook,
the settings exist for the entire session, even if you close that workbook.
When Microsoft Excel calculates values in the worksheet, it repeats
calculations until it reaches the number of iterations you set in the
Maximum Iterations box or until it changes all cells by less than the
amount you set in the Maximum Change box, whichever is reached first.
Unless you change the default iteration settings, Excel stops calculating
after 100 iterations or when all calculated values change by less than
0.001 (the default maximum change value) between iterations. During
iteration, values move closer to the correct solution with each iteration.
This is called "convergence."
If the change in the result, or the delta, is greater than or equal to the
value in the Maximum Change box, Excel continues to calculate as long as
the limit in the Maximum Iterations box has not been reached. If the
change in result is less than the maximum change value, Excel stops
calculating.
Example 1: Large Iterations Value
To see an example of this calculation methodology, follow these steps:
- In a new workbook, click Options on the Tools menu. Click the
Calculation tab and click Manual. Click Iteration. In the Maximum
Iterations box, type 100. In the Maximum Change box, type 1. Click OK.
- Click cell A1 and type the following circular formula:
=A1+1
- The value 1 is returned in cell A1. Press the F9 key to recalculate the
workbook.
Note that the value in cell A1 is 101. Each successive calculation
increments the result by 100.
This is because each incremental change is never less than the maximum
change value of 1. The circular formula adds 1 to the result. Because
the maximum iteration value is 100, each iteration adds 1 to the value,
which results in 1*100.
- Change the value in the Maximum Change box from 1 to 1.0001 and press
F9 to recalculate the workbook. The result is incremented by 1 instead
of by 100.
This behavior occurs because the value returned by the first calculation
reaches the maximum change limit you set (that is, the change in result
is less than the value you specified in the Maximum Change box).
Example 2a: Small Iterations Value
To see this example, follow these steps:
- Create a new workbook. On the Tools menu, click Options. Click the
Calculation tab and click Manual. Click Iteration. In the Maximum
Iterations box, type 1. In the Maximum Change box, type 0.001. Click
OK.
- Enter the following in Sheet1:
A1: 1000
A2: =(A1+A2)/10
The initial result of the formula in cell A2 when you press ENTER is
100, or (1000+0)/10=100.
Press F9. The resulting value is 110, or (1000+100)/10=110. The
change in result is 10.
Press F9 again, the resulting value is 111, or (1000+110)/10=111. The
change in result is 1.
Press F9 again, the resulting value is 111.1, or (1000+111)/10=111.1.
The change in result is 0.1.
Press F9, the resulting value is 111.11, or (1000+111.1)/10=111.11.
The change in result is 0.01.
Press F9, the resulting value is 111.111, or (1000+111.11)/10=111.111
The change in result is 0.001, which is equal to but not less than the
value you specified in the Maximum Change box (0.001).
Press F9, the resulting value is 111.1111, or (1000+111.111)/10=111.1111
In this case the change in result is 0.0001, which is less than the
value you specified in the Maximum Change box. We would expect
Excel to stop calculating if you set maximum iterations above 6.
If you set maximum iterations to 100, and you reenter the formula in cell
A2, the initial resulting value in A2 is 100. When you press F9, the
resulting value is 111.1111 (as expected). When the workbook is
recalculated again, Excel calculates once, and then halts because the
change in result after first calculation is less than the maximum change
value (The limit of 15 significant digits in Excel applies).
Example 2b
NOTE: This example is a continuation of Example 2a; you must follow the
steps for Example 2a before you follow the steps for the following
example.
To see this example, follow these steps:
- On the Tools menu, click Options. Click the Calculation tab and click
Manual. Click Iteration. In the Maximum Iterations box, type 1. In the
Maximum Change box, type 0.001. Click OK.
- Type the following in cell D1:
=D1+1
- Retype the formula in cell A2 as follows:
=(A1+A2)/10
- Press F9 to recalculate the formulas in the workbook. Excel repeats the
calculation 100 times.
This behavior occurs because the amount by which the value in cell D1
changes never exceeds the limit you set in the Maximum Change box (the
change in the result remains below 0.001).
In this case, Excel recognizes at least one formula in which the limits
for halting calculation are not met and Excel continues to calculate the
formula. Since, by definition, a circular reference is never completed,
Excel calculates all circular reference formulas until they meet either
the maximum iteration or the maximum change limits.
Article ID: 177856 - Last Review: November 20, 2001 - Revision: 1.0
APPLIES TO
- Microsoft Excel 97 Standard Edition
Retired KB Content DisclaimerThis article was written about products for which Microsoft no longer offers support. Therefore, this article is offered "as is" and will no longer be updated.