This post is part of spreadcheats series.
Today we will learn a fascinating little feature in excel called “goal seek”.
But what good is a feature if we cant find a use for it? So we will build a simple retirement calculator using excel.
Before plunging in to the complex retirement calculations, let us spend a bunch of words understanding what this goal seek is all about.
What is goal seek in excel?
We can think of goal seek as opposite of formulas. Formulas tell you what is the output of a bunch of variables used in an equation (for eg. sumproduct is an equation involving + and *). Goal seek tells you what inputs you need to give in order to get certain output.
For example, you can use goal seek to solve a linear equation or find the internal return rate (IRR) of an investment.
Now that you understand goal seek, let us plan your retirement. 🙂
Make a financial model to estimate your monthly savings to meet retirement goals.
(Note: the image shows commas according to Indian currency formatting.)
In order to proceed, we would need some data, like,
(1) What is your current age?
(2) What is your expected retirement age?
(3) How much do you think you will spend every month when you retire (of course in today’s prices)
(4) Your expectation of inflation (%)?
(5) Your expected return (%) on investments?
Once the data is available, we will need to calculate the following,
I have shown the worksheet on the right with some dummy data.
(6) The yearly expenses at the time of retirement: (3) * (1+(4))^((2)-(1))*12
(7) Corpus required to generate the above amount every year (and leave the principle behind): (6)/(5)
(If these calculations are overwhelming, download the excel retirement calculator workbook here.)
We know how much corpus is needed.
We can use FV() formula to determine the future value of a series of payments made periodically and compounded at a given interest rate.
We know how much the FV() out come should be, but we don’t know how much the input (monthly investment) should be.
This is where goal seek is going to help us.
Let us assume the monthly investment amount will be in cell A5. Let us also assume, the interest rate is in cell A4, retirement age is in A3, current age is in A2.
We will write the FV formula in cell A6 like this = -FV(A4/12,(A3-A2)*12,A5)
(we have to negate FV since it uses weird accounting notations)
Since the cell A5 is blank, the FV will show the value as 0.
Now, we will use goal seek to find out how much cell A5 should have so that A6 will be calculated to the corpus amount required.
Go to Data tab and click on What if analysis and select goal seek. (In excel 2003, it should be in tools menu)
See this screen cast to understand how the goal seek works:
The goal seek window has 3 inputs. The cell you need to change. The cell you want to set and the value to set.
Once you use the goal seek it will find the correct (or closest) value to meet the goal and displays it. If you press OK, the value will be placed in the cell (in our case, in A5)
That is all.
Download the Retirement Calculator Excel Worksheet and play with it
Click here to download the retirement calculator worksheet. Follow the instructions in the workbook to see this example for yourself. Change values to find the amount that you need to save.
Do you find goal seek feature useful?
What do you do with excel goal seek? Do you use it in your modeling, planning worksheets? Tell me your experiences and ideas using comments.
Additional resources:
- Read remaining posts in Spreadcheats series: Become a spreadsheet guru by learning these nifty hacks.
- Excel financial formulas – Help on NPV, FV, PV and more
- Understand why you should start early when it comes to retirement savings
- Buy or rent calculator in Excel – calculate returns on property investments
PS: the retirement calculation steps are derived from this excellent article on smart investor
15 Responses to “A Gantt Chart Alternative – Gantt Box Chart”
That's a great idea.
Maybe the planned End Date should be highlight more.
I don't know how it would look like (nor how to do it yet), but what if instead of finishing the bold line to the best case End Date, it finishes to the realistic End Date?
The idea is ok, I think other project management tools have this, already? Maybe not.
Gantt charts in my view are about the signal most unless thing in the world, theres no way you can look at one thats more that a little complex and understand what it's telling you. I'm going to write a diatribe on project management at some point, its one of my pet areas I think!! 😉
The issue I have with this chart Chandoo, is that Tasks need to be linked to each other, so they should inherit the uncertainty, which would mean the as you moved down chart the lines would be miles apart for later tasks, and you might have to add lots of lines for subsequent tasks to cover the various outcome of it's parents.
Having said that, for the high level board summary, it's a nice way to go, it it appeals to the management 😉
thanks Chandoo, great post.
Ross
Whoooa !!! That's a very clever idea Chandoo. I really love it.
I think i'll update my gantt project sheet with that idea soon (remember my template ?)
@ross : you can link start date to the end date of the previous task in your data. The only problem I still se is to which end date (real ? planned ? best ?) in order to have average amount of information.
If best end date, you'll tend to increase uncertainty at the end of chain, although if you link to real end date, uncertainty will be decreased too much, leading in both cases to wrong management direction.
Maybe planned till the task is finished then real will do the job ?
Hey chandoo, this looks good and this would definite add value in production planning / scheduling. Uncertainity in finishing a task is very high in production scheduling and this could give an insight or a bird eye view of possible shipments we can have....
I've always been frustrated by the limitations of gantt charts. Will definitely use this, I've always struggled with how to succinctly communicate the uncertainty of certain tasks without confusing stakeholders.
I like this, I think it's a very effective way of showing how a timeline can change and which parts of a project need close attention.
@Cyril / @Ross: I would intially link the the start date to the planned end date of the previous task, with the chart updating when a task has been completed to reflect the true end date.
Or what about giving a drop-down selection box to allow the user to see the chart based on planned/best-case/worst-case end dates?
Like the idea. Have found that Excel is more flexible than MS Project for graphical solutions. The "Best Case"\"Worst Case" metrics are theoretically appealing but once the project and\or phase commences their reliability diminishes. A chart like the above that showed Planned Start, Planned End, Replan End Start, Replan End Date, Number of Replans the Start and End Dates, and Actual would provide an active, actionable view of each task\phase. It would also highlight the areas which are riskiest.
It is always amazing how flexible excel can be.
My question is how would the chart show a scenario where the date moved up? If a task is dropped or the duration of the task is significantly reduced by applying more people or machinery to the task, the dates will move up.
The gantt chart has been around for a long time, but it is still quite useful to show progress.
Cheers,
B
I like the idea but seems bit complicated in case of long projects involving numerous activity.
Also, reading and explaining is required hence not feasible where plans are just send to audience for approval.
Cheers
SY
Great idea Chandoo,
When I was reading this idea regarding delivery dates, another thought popped into my mind, how can you show the uncertainty with MONEY!!
In this case, applies to cost management or even a normal budget, you think?
Would Box Chart and Gannt Chart help to understand the best case, middle case and worst case when money is spend or planned with these three risks are involved?
I imagine that this chart could help people who write their budgets get a better understanding of risks affecting their spending.
Peter
Chandoo,
I like it. How would you display an entry once it has been completed (actual)?
Thank you,
Matt
From what you have shown so far I think that this box Gantt chart is awesome! I think that this could be an extremely useful tool.
I can't wait to learn how to make my own charts in Excel.
Will the methods that you are going to teach us work in 2003 as well?
[...] Firday, we proposed a new chart for showing project plans. I chose an ugly name for it and called it Gantt Box [...]
You need to read Eli Goldratt's Critical Chain. The uncertainty you are looking for should be accounted for in a project buffer. Not at each task level.
Further you should spend time understanding Agile Development. This would have you plan only in 1-3week iterations. This allows you to embrace changes to work not yet started, and for your customer to re-direct your course at regular intervals (after each iteration) throughout your project. keyword search: Agile Scrum
These items will show you that you are solving a tracking problem for something that you can entirely avoid!
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