How to let the user choose between typing price inclusive VAT or exclusive VAT in TextField?

I am implementing an back-office application where the user type in prices for products. Sometimes it is preferred to type the price inclusive value-added-tax, VAT and sometimes exclusive VAT.

How do I best let the user choose between the inclusive or exclusive VAT in a usability perspective?

I could have two TextFields above eachother one inclusive VAT 开发者_StackOverflow社区and one exclusive, and reflect the input. But I don't think that reflecting the input in another TextField at realtime is good when it comes to usability, it distracts the user.

I could also have two Radiobuttons above the TextField or below, or maybe besides the TextField that let the user make the choice.

Or should I have a single button for turning between inclusive/exclusive VAT? like On/Off-buttons. But what text should I have on the button and how should the button be designed? I think this is good because it takes less space and it is easy to have it close to the TextField, but it's very hard to design a good button in a usability perspective.

Please give me some recommendations. Maybe someone of you works with usability or have seen a similar problem.

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This question is duplicated on UXExchange, which is probably a more appropriate forum. My answer is there, along with potentially many other answers from UX specialists.

http://uxexchange.com/questions/2181/how-to-let-the-user-choose-between-typing-price-inclusive-vat-or-exclusive-vat-in

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I'd suggest using a single text field with a "includes VAT" checkbox next to it. Next to these, I would display the price as plain text (reflecting changes in real time) both with and without VAT. This way, there is only a single input area, and the results are visible and obvious.

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Your first idea is the best one. No, not that one, the one you rejected with two editable text boxes and no buttons where entry in one text box automatically updates what’s in the other. Any distraction caused by the updates is minor compared to the hassle of the alternatives where the user has to figure out and activate a button. If you are still concerned with the update being distracting, then have the update occur when focus leaves the text box or the Enter key is pressed.