Go Calc makes it easy to create your own mini calculator apps right on your iPhone or iPod Touch. I am going to describe how to create a calculator to track your mortgage and quickly see what happens when interest rates rise or fall. Hopefully this tutorial will inspire you to create your own mini calculator apps using Go Calc.

Here is a newly installed Go Calc:

I am going to start by creating a new calculator. Just tap the + button at the top of the screen.

I have given the calculation a name and typed in some notes. Once this is done I can tap Save.

Go Calc works a little bit like a spreadsheet. All calculators need some variables which will be used to compute one or more results. For a simple mortgage calculator we need three variables:

- Interest Rate
- Years (that is how many years I have left to pay off the remainder of my mortgage)
- Amount (how much money is left to pay back to the bank)

In my calculator I want to know what my monthly payment is going to be. So **Payment** will be my result.

Let’s start by adding the variables. Add the first variable by tapping the **Add Variable** button.

I can now tell Go Calc all about my first variable **Interest Rate**.

You can see in the screenshot that I have given quite a few details about my variable:

**Variable name**– notice that you can include spaces, however Go Calc ignores spaces and case when working out results so “Interest Rate” and “interestrate” are treated as the same value**Min**– the lowest the interest rate can be is 0% (unfortunately!)**Max**– heres hoping interest rates don’t go above 20%**Prefix**– I don’t want a prefix for interest rate. Prefix is text that will appear before the value**Suffix**– this appears after the interest rate number**Decimal places to show**– interest rates are usually shown to two decimal places**Slider**– I want to be able to quickly play with the interest rate to see what will happen to my monthly payments so I have turned the slider on**Step**– when I play with the slider I want the interest rate to go up or down in increments of 0.1

Now my first variable is set up I can save it – just tap Save at the top of the screen.

The other two variables are Years and Amount. Here is what they look like when defining them in Go Calc:

Once the variables have been defined I can go ahead and create the result.

Tap **Add Result** and you will be taken to the **New Result** page.

Just like a variable a result needs a name. This is because results can be used in other results formulas. However in this case the result name is just for our benefit.

For this result I am defining both a prefix and a suffix. This will make the result really easy to understand on the page. For example it might show £200/month indicating that I need to pay £200 per month.

Since we are dealing with money I am going to show the result to two decimal places.

If you are building a complex calculator with lots of intermediate results you might choose to hide a result that the user doesn’t really need to see. In this case though I don’t want to hide my result.

Once these details have been entered I can start editing the formula.

One great thing about Go Calc is that if you have used Microsoft Excel before (or Numbers, OpenOffice Calc, …) then you will be able to write formulas. Tap **Edit Formula** to begin.

The formula builder in Go Calc gives you quick access to all of your variables and any other results you might have defined. To include a variable in your formula tap **Variables**, select the one you want to insert and then tap the + button. It will appear in the text field showing the formula. You might want to play with this a little to get the hang of it. Notice that you can easily move the cursor using the buttons on the toolbar. This really helps as positioning the cursor precisely can be a little tricky with your finger.

You could write some really long, complicated formula to work out the payment from first principles. Go Calc however has many built in functions to help with many common tasks. Most of the functions work in exactly the same way as they do in Excel.

The functions are split into four categories:

- Finance
- Logical
- Math
- Statistical

You can get help on any function by selecting it and tapping the ? button. If it still looks obscure then search the Internet for how Excel’s version works – it will probably work the same in Go Calc!

I am now going to enter the formula using the formula builder and the keyboard.

The formula I need is: abs(pmt(Interest Rate/100/12,Years*12,Amount))

Here is what it looks like in the Go Calc formula builder:

I am now ready to use my calculator. Save the result by tapping Save and then come out of edit mode by tapping Done.

To set the values of the variables you can use the sliders or double tap the variable and use the built in calculator keypad.

I can now at a glance see how much my mortgage will be per month.

The real power comes with exploring **what-if** scenarios. In the UK interest rates are fairly low at the moment, but expected to rise. Using Go Calc I can very quickly see what will happen to my monthly mortgage repayments just by dragging the slider under Interest Rate. As I drag the slider under Interest Rate the Payment result will automatically update. This is the same for any of the other variables. If I suddenly come into some money and want to pay off some of my mortgage I can see what will happen just with a slide or double tap of my finger.

Of course there are lots of mortgage calculator apps available on the App Store. Go Calc however lets you create your own calculator apps for problems which might be specific to you or the area you work in.

If you create any good calculators you can export them and share them with your friends or work colleagues. Your calculators can be organised into folders for easy access.

Have fun creating your own mini calculator apps using Go Calc. Let me know how you get on!

(Just in case you haven’t yet bought Go Calc you should now pop on over to the App Store…)