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Sunday, July 8, 2012

Speed up your Excel Formulas [Speedy Spreadsheet Week]

Speed up your Excel Formulas [Speedy Spreadsheet Week]:
Optimizing & Speeding up Excel Formulas - 10 tips & best practices
Excel formulas acting slow? As part of our Speedy Spreadsheet Week, today lets talk about optimizing & speeding up Excel formulas. Use these tips & ideas to super-charge your sluggish workbook. Use the best practices & formula guidelines described in this post to optimize your complex worksheet models & make them faster.

10 Tips to Optimize & Speed up Excel Formulas

1. Use tables to hold the data

Starting Excel 2007 you can keep all the related data in a table. For example call center data in our recent dashboard is kept in a table. Tables can be used in formulas with structural references, can be used as a source for pivot tables etc. And since tables grow & shrink as you add / remove data, none of your formulas need to be dynamic. As an example, if you have table called cs, then the formula sum(cs[column_name]) refers to sum of all values in the column_name of table cs. Even if you add more data to CS, the formula still works.
Note: You can use Excel lists in 2003 version, which are similar to tables.
Resources to learn about Excel Tables:

2. Use named ranges, named formulas

By using names and named formulas, you can simplify your spreadsheet. Not only that, since named ranges & named formulas can hold arrays (ie lists of values), you can hold intermediate results or values that you need to refer many times in these named formulas. This will reduce the formula overhead and makes your workbooks faster.
Resources to learn about named ranges & named formulas:

3. Use Pivot Tables

Many times, even when we do not need formulas we use them, because we can. Pivot tables are an excellent way to calculate a lot of summary values with few clicks. Once the pivot is built, you can refer to the pivot values with GETPIVOTDATA or simple cell references. This will reduce a lot of unnecessary calculations. If you are changing the data, you can just go to DATA ribbon and refresh all pivots in one go. This process works smoothly when you use tables to hold the data.
One of the reasons for slow workbooks is lot of data. Since, pivot tables are designed to work with lots of data, by using them, you can speed up your workbooks.
Resources to learn Pivot Tables:

4. Sort your data

One of the reasons for sluggish performance is that you are searching for something in a lot of un-sorted data. You are making Excel look for a needle in a hay-stack. Many times we inherit un-sorted data thru data imports. By sorting the data & using correct operators in lookup formulas, we can instantly speedup a sluggish workbook. If you feel that sorting the data is a pain, you can even automate it so that as soon as data is refreshed, you can initiate sort procedure (thru a simple VBA macro).
Examples on Sorting:

5. Use Manual Calculation Mode

Speed is the hefty price you pay for complexity. But many times, we want our Excel workbooks to be complex, because only then they would reflect real world. In such cases, you can set formula calculations to manual mode.
Set Formulas to Manual Calculation Mode to Speedup / Optimize them
Just press F9 whenever you want to run the formulas. Please note that Excel runs formulas whenever you save the file too.

6. Use Non-volatile formulas

There are a class of formulas in excel called as volatile formulas. These formulas are re-calculated whenever there is a change in the workbook. Examples of volatile formulas are RAND, NOW, TODAY, OFFSET etc. So when your worksheet has a lot of volatile formulas, any time you make a change all these formulas must be re-calculated. Thus, your worksheet becomes slow.
Solution? Simple, do not use volatile formulas. For example, instead of using OFFSET to construct a dynamic range, you can use INDEX. Since INDEX is non-volatile, it tends to be faster. Or better still, use a table.
Resources to learn more:

7. Keep formulas in a separate sheet

Formulas are the driving force behind any Excel workbook or model. By keeping all them in a separate worksheet(s), you minimize the chance of mistakes, omissions or repetitions.  Debugging or investigating slow performance becomes an easy task when all formulas are in same place. I usually keep all the formulas in one sheet whenever I am designing a dashboard or complex workbook. This structure also helps me in thinking thru various calculations and planning the formulas in a structured way.

8. Write better formulas

Here are some guidelines that I follow when writing formulas.
  • Built-in formulas tend to better than your own version – for example SUMIFS is easier to write and just as fast as SUMPRODUCT.
  • Do not refer to entire column when you need just a few values. Do not write SUM(A:A), when you know values are only in A1:A10.
  • IFERROR instead of lengthy IF(ISERROR formulas. Use IFERROR to simplify your error checking.
  • Remove or Fix formula errors [how to, findout why formulas are not working]
  • Remove or Reduce array formulas
  • Remove or Reduce references to other workbooks.
  • Remove any named ranges that result in error or missing links.
  • Try to come up with alternative formulas: this not only sharpens your mind, but lets you discover better solutions.
  • Do not calculate something if you do not need it.
  • Do not calculate same thing twice. Use the first result second time too.
Resources to write better formulas:

9. Desperate times need desperate measures

Sometimes, no matter what you do, the workbook remains slow. Here are a few whacky ideas that I try in such cases:
  • Replace formulas with values. I take a backup of the formulas. Then I select everything, CTRL+C, ALT+ESV. Done!
  • Develop the workbook from scratch: Sometimes it helps to design the workbook afresh.
  • Replace external data links with actual data: And import data by copy-pasting if needed.
  • Reduce the functionality: See if the end user can live with fewer features in the workbook.
  • Find an alternative solution: Trying to do everything in Excel is foolish. See if there is any external tool that can do this better & faster.

10. Learn new formulas & play with them

Optimization is not a one-shot exercise. It is an ongoing-business. So you need to constantly learn new formulas, new uses & play with them. This way, you see new ways to improve a sluggish workbook. To begin with, explore our Formula homework & formula forensics pages and see how you solve these problems.

How do you speed-up your Excel formulas?

So how do you optimize & speed-up your Excel formulas? What techniques do you use? Please share using comments.
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