# Change formula recalculation, iteration, or precision

To use formulas (formula: A sequence of values, cell references, names, functions, or operators in a cell that together produce a new value. A formula always begins with an equal sign (=).) efficiently, there are three important considerations that you need to understand:

**Calculation** is the process of computing formulas and then displaying the results as values in the cells that contain the formulas. To avoid unnecessary calculations, Microsoft Office Excel automatically recalculates formulas only when the cells that the formula depends on have changed. This is the default behavior when you first open a workbook and when you are editing a workbook. However, you can control when and how Excel recalculates formulas.

**Iteration** is the repeated recalculation of a worksheet until a specific numeric condition is met. Excel cannot automatically calculate a formula that refers to the cell — either directly or indirectly — that contains the formula. This is called a circular reference. If a formula refers back to one of its own cells, you must determine how many times the formula should recalculate. Circular references can iterate indefinitely. However, you can control the the maximum number of iterations and the amount of acceptable change.

**Precision** is a measure of the degree of accuracy for a calculation. Excel stores and calculates with 15 significant digits of precision. However, you can change the precision of calculations so that Excel uses the displayed value instead of the stored value when it recalculates formulas.

- Change when a worksheet or workbook recalculates
- Recalculate a worksheet or workbook manually by using keyboard shortcuts
- Change the number of times Excel iterates a formula
- Change the precision of calculations in a workbook
- Change the number of processors used to calculate formulas
- Learn about calculating workbooks that were created in an earlier version of Excel

## Change when a worksheet or workbook recalculates

As calculation proceeds, you can choose commands or perform actions such as entering numbers or formulas. Excel temporarily interrupts calculation to carry out the other commands or actions and then resumes calculation. The calculation process may take more time if the workbook contains a large number of formulas, or if the worksheets contain data tables (data table: A range of cells that shows the results of substituting different values in one or more formulas. There are two types of data tables: one-input tables and two-input tables.) or functions that automatically recalculate every time the workbook is recalculated. Also, the calculation process may take more time if the worksheets contain links to other worksheets or workbooks. You can control when calculation occurs by changing the calculation process to manual calculation.

- Click the
**Microsoft Office Button**, click**Excel Options**, and then click the**Formulas**category.

- Do one of the following:
- To recalculate all dependent formulas every time you make a change to a value, formula, or name, in the
**Calculation options**section, under**Workbook Calculation**, click**Automatic**. This is the default calculation setting.

** Tip ** Alternatively, on the **Formulas** tab, in the **Calculation** group, click **Calculation Options**, and then click **Automatic**.

- To recalculate all dependent formulas—except data tables—every time you make a change to a value, formula, or name, in the
**Calculation options**section, under**Workbook Calculation**, click**Automatic except for data tables**.

** Tip ** Alternatively, on the **Formulas** tab, in the **Calculation** group, click **Calculation Options**, and then click **Automatic Except for Data Tables**.

- To turn off automatic recalculation and recalculate open workbooks only when you explicitly do so (by clicking
**Calculate Now**under**Calculation Options**in the**Calculation**group on the**Formulas**tab), in the**Calculation options**section, under**Workbook Calculation**, click**Manual**.

** Note ** When you click **Manual**, Excel automatically selects the **Recalculate workbook before saving** check box. If saving a workbook takes a long time, clearing **Recalculate workbook before saving** may improve the save time.

** Tip ** Alternatively, on the **Formulas** tab, in the **Calculation** group, click **Calculation Options**, and then click **Manual**.

- To manually recalculate all open worksheets, including data tables, and update all open chart sheets, on the
**Formulas**tab, in the**Calculation**group, click the**Calculate Now**button.

- To manually recalculate the active worksheet and any charts and chart sheets linked to this worksheet, on the
**Formulas**tab, in the**Calculation**group, click the**Calculate Sheet**button.

Changing any of the options affects all open workbooks.

** Note ** If a worksheet contains a formula that is linked to a worksheet that has not been recalculated and you update that link, Excel displays a message stating that the source worksheet is not completely recalculated. To update the link with the current value stored on the source worksheet, even though the value might not be correct, click **OK**. To cancel updating the link and use the previous value obtained from the source worksheet, click **Cancel**.

## Recalculate a worksheet or workbook manually by using keyboard shortcuts

To |
Press |
---|---|

Recalculate formulas that have changed since the last calculation, and formulas dependent on them, in all open workbooks. If a workbook is set for automatic recalculation, you do not need to press F9 for recalculation. | F9 |

Recalculate formulas that have changed since the last calculation, and formulas dependent on them, in the active worksheet. | SHIFT+F9 |

Recalculate all formulas in all open workbooks, regardless of whether they have changed since last time or not. | CTRL+ALT+F9 |

Recheck dependent formulas, and then recalculate all formulas in all open workbooks, regardless of whether they have changed since last time or not. | CTRL+SHIFT+ALT+F9 |

## Change the number of times Excel iterates a formula

- Click the
**Microsoft Office Button**, click**Excel Options**, and then click the**Formulas**category.

- In the
**Calculation options**section, select the**Enable iterative calculation**check box. - To set the maximum number of times Microsoft Excel will recalculate, type the number of iterations in the
**Maximum Iterations**box. The higher the number of iterations, the more time Excel will need to recalculate a worksheet. - To set the maximum amount of change you will accept between recalculation results, type the amount in the
**Maximum Change**box. The smaller the number, the more accurate the result and the more time Excel needs to recalculate a worksheet.

** Note ** Solver and Goal Seek are part of a suite of commands sometimes called what-if analysis (what-if analysis: A process of changing the values in cells to see how those changes affect the outcome of formulas on the worksheet. For example, varying the interest rate that is used in an amortization table to determine the amount of the payments.) tools. Both commands use iteration in a controlled way to obtain desired results. You can use Solver when you need to find the optimum value for a particular cell by adjusting the values of several cells or when you want to apply specific limitations to one or more of the values in the calculation. You can use Goal Seek when you know the desired result of a single formula but not the input value the formula needs to determine the result.

## Change the precision of calculations in a workbook

Before you change the precision of calculations, keep in mind the following important points:

**By default, Excel calculates stored, not displayed, values** The displayed and printed value depends on how you choose to format and display the stored value. For example, a cell that displays a date as "6/22/2008" also contains a serial number that is the stored value for the date in the cell. You can change the display of the date to another format (for example, to "22-Jun-2008"), but changing the display of a value on a worksheet does not change the stored value.

**Use caution when changing the precision of calculations** When a formula performs calculations, Excel usually uses the values stored in cells referenced by the formula. For example, if two cells each contain the value 10.005 and the cells are formatted to display values in currency format, the value $10.01 is displayed in each cell. If you add the two cells together, the result is $20.01 because Excel adds the stored values 10.005 and 10.005, not the displayed values.

When you change the precision of the calculations in a workbook by using the displayed (formatted) values, Excel permanently changes stored values in cells from full precision (15 digits) to whatever format, including decimal places, is displayed. If you later choose to calculate with full precision, the original underlying values cannot be restored.

- Click the
**Microsoft Office Button**, click**Excel Options**, and then click the**Advanced**category.

- In the
**When calculating this workbook**section, select the workbook you want, and then select the**Set precision as displayed**check box.

## Change the number of processors used to calculate formulas

A computer can have more than one processor (it contains multiple physical processors) or can be hyperthreaded (it contains multiple logical processors). On these computers, you can improve or control the time it takes to recalculate workbooks that contain many formulas by setting the number of processors to use for recalculation. In many cases, portions of a recalculation workload can be performed simultaneously. Splitting this workload across multiple processors can reduce the overall time it takes complete the recalculation.

- Click the
**Microsoft Office Button**, click**Excel Options**, and then click the**Advanced**category.

- To enable or disable the use of multiple processors during calculation, in the
**Formulas**section, select or clear the**Enable multi-threaded calculation**check box.

** Note ** This check box is enabled by default and all processors are used during calculation. The number of processors on your computer is automatically detected and displayed next to the **Use all processors on this computer **option.

- Optionally, if you select
**Enable multi-threaded calculation**, you can control the number of processors to use on your computer. For example, you might want to limit the number of processors used during recalculation if you have other programs running on your computer that require dedicated processing time.

- Under
**Number of calculation threads**, click**Manual**. Enter the number of processes to use.

** Note ** The maximum number is 1024.

## Learn about calculating workbooks that were created in an earlier version of Excel

To ensure that older workbooks are calculated correctly, Excel behaves differently when you first open an old workbook than when you open a workbook created in the current version.

When you open a workbook created in the current version, Excel recalculates only the formulas that depend on cells that have changed.

When you use open a workbook that was created in an earlier version of Excel, all of the formulas in the workbook — those that depend on cells that have changed and those that do not — are recalculated. This ensures that the workbook is fully optimized for the current Excel version.

Because complete recalculation can take longer than partial recalculation, opening a workbook that was not previously saved in the current Excel version can take longer than usual. Once you save the workbook in the current version, it will open faster.