In detail
Prime Video is one of the few services that tells you its data usage directly. In the app's streaming and downloading settings you can pick Good, Better or Best quality, and Amazon shows an estimate next to each option: roughly 0.4 GB per hour for Good, about 0.75 GB per hour for Better and up to around 6–7 GB per hour for Best.
'Best' adapts to your screen and connection and goes up to 4K Ultra HD with HDR on supported titles and devices. On a big TV with a fast connection, a Prime Video evening uses about as much data as Netflix or Disney+ at the same resolution; on a phone, the app usually streams far below the maximum.
Prime Video includes ads in many countries unless you pay for the ad-free option. Ads are streamed as video too, so a two-hour movie with ad breaks can use slightly more data than the same movie ad-free.
To keep mobile data under control, enable the data saver for cellular streaming and download episodes on Wi-Fi in Good or Better quality. Downloads in 'Best' can take several gigabytes per episode, so check your storage before downloading a whole season.
The calculator above starts with a typical Prime Video session in Full HD. Change the quality to match your setting, add other services and devices, and see the monthly total and the plan that fits.