For most apps, the only difference is that the Free version has ads. For Calculator HD Pro and Calculator Pro Elite, the paid versions also have 2 exclusive features available as In App Purchases (Widget and Multiple History Tapes).
No, you only have to pay once and then you can download it again for free onto all of your devices.
The app is universal and works with both the iPhone and the iPad.
You can also share your purchase with others in your family if you have Family Sharing enabled on your account.
For more information about Downloading Previous Purchases and Family Sharing, please visit Apple at https://support.apple.com/billing
The Paid version of the app is a totally separate app, so look for a new icon on your device without the word Lite on it.
Start using the Paid version from now on, and delete the Free version from your device so that you do not open it by mistake.
From within the app, go to Settings, and scroll down to the bottom of the first page and tap Restore Purchases.
There are two reasons why you may get a unit conversion that you do not expect:
1. You may have mistakenly selected the wrong category of unit. Make sure to select the correct category. When you are converting a Mass or a Volume unit, you usually want to select units in the same category for both the From and To unit. Some units have similarly named Volume and Mass versions. For example:
It is possible to do a mixed category conversion between Mass and Volume or Volume and Mass in which case a material density is required as a third input. While you are searching for the second unit, the app will let you know if it is going to be a mixed category conversion by highlighting the unit category in Red instead of Green, and the unit conversion itself will have a material indicated (the default is water).
2. You may have mistakenly selected the wrong version of unit. Make sure to select the correct version. Many units have multiple versions of the same named unit. For example, the Volume unit of gallon (gal) has 3 different definitions (Imperial, US dry, and US liquid). If a unit has multiple version with the same name, the app will indicate the version of the unit by highlighting it in Red brackets at the end of the name of the unit:
If you are not sure which unit you want, you can click on the Wikipedia links next to each unit in the app to learn more.
The Widget can be accessed in your Today View or by 3D Touch on the app icon.
To see the in app purchase in action and how to use it, watch this video: https://www.youtube.com/watch?v=ECDGya7Zrco
For more information on how to access widgets, visit Apple at https://support.apple.com/en-us/HT207122
To see the in app purchase in action and how to use it, watch this video: https://www.youtube.com/watch?v=0I8NdWyPXP0