Don’t Add Decimal Places to Currency Input

This is yet another example where keeping it simple is the best option. Many currency input situations (sending a bank payment or adding a tip, for example) require the user to enter a value, which could be a whole amount ($10) or an arbitrary amount (£5.99).

Products sometimes try to be too helpful by auto-adding the decimal place or adding “.00” to the end of the value, which tends to lead to errors. Not the fun kind of errors either, but the kind where you bid $1000.00 for some underpants on eBay when you meant to offer $10.00 for them as the absolute maximum. To avoid this, allow the user to type the decimal themselves, but assume a “.00” if they don’t.

You can improve some experiences by offering pre-set amounts. Tipping is a great example—many ride-hailing apps will have pre-set tip buttons for $1, $5, and so on.

Figure 59.1: Please don’t do this

This...