I'm designing an inflation calculator. It's for a country that had three different currencies in the past 70 years. That means if you set a specific date, the entered value would be accepted in the currency that was valid back then.

So if I enter a value on a specific date, it will be taken as if I entered a value in the currency that was current on that date. For example if I enter value 1000 it's going to be 1000 XXX in the year 1952, and 1000 € in the year 2012.

That means that the prefix on a textbox would change, based on the date entered.

However the problem is, what if a user changes the date, and doesn't notice or forgets the currency has changed (even thought he can see the different prefix). Should I include a pop-up reminding him that the currency changed?


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Some calculators work in a way that you enter a value in €, no matter if the € existed back then or used, but the client doesn't have recalculations of that. So the user has to enter it as a different currency. The app will mostly be used for old contracts with the old currency on it.

thank you!

4 Answers 4


You can just make the currency a select filed like in this example but before or inside the value input.

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To ensure the user see the currency change, you could make a simple animation of 2 or 3 seconds that change the currency colour smoothly and back for the normal colour. Nothing definitive and nothing looks like an error, only a warning.


Interesting question, you could use a popup as you've suggested or an alternative might be to clear the field they have entered.

A popup is more intrusive, clearing the field may work better but also be confusing as to why the currency has changed.

By clearing the field you'd be trading off the annoyance of having to enter the value again vs the impact of getting it wrong.

You say:

doesn't notice or forgets the currency has changed (even thought he can see the different prefix)

This is very likely occurrence, especially since the change is not in the field the user has changed, their focus will not be on the field that changed. Changing a single character on a screen is not a noticable change for a user.


You can use tools like color or animation to bring the user's focus to the change. This would allow your system to say "Hey, I am updating the currency and the value based on the year you selected."

If this changes what the user wants for an amount, they will see it and can correct it.

One one thing this interaction does not do is explain why currency changed. That is something you likely want to address as part of the application itself if the inflation calculator is only for this country.

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This would be a good use for a wizard style approach. Step 1, prompt for the date (and don't show currency yet). Step 2, show currency once date is selected. Being a wizard, the user can't just edit the date, but must hit the "back" button if they wish to go back.

The one place this might be problematic is if the user has to enter many dates and many currency values. Then the wizard gets a bit more cumbersome to work with.

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