This tool allows you to find out how much a Single fare costs between two stations of your choice. At present, this only works for stations within the TfL network.
All you need to do is select two stations and the type of fare you want to find - and then click "Calculate Fare". You will then be shown the available fares, which often includes the Peak and Off Peak Oyster/Contactless fares and, if available, the cost of a paper Single ticket.
Some stations, for example Reading, are outside the Oyster zones, so you won't be able to use an Oyster card for that journey. If this happens to be the case, you will see a note at the top of the page.
For some journeys, you will be shown an "Alternative Fares" section. These journeys often avoid Zone 1 and are cheaper but may be slower or require more changes.