Which bit is supposed to be measured?
For details of routes to these waterfalls, see the waterfall walks page.
This is not an official list (though it is substantially better than any other lists I have encountered). I do not expect this list to be complete, since I may have missed important waterfalls in Mid and North Wales. Add to that the fact that most waterfalls have no information available at all, and it becomes quite hard to produce this list. In many cases, careful reading of high scale maps is the only option. If you can offer any details, please get in touch.
This lists waterfalls with a single drop of 20 metres or more, and gives the heights of the drops that are tall enough.
To qualify for the list of single drops, a waterfall must fall continuously from top to bottom. While it is allowed to make contact with the bedrock, in cases such as a horsetail fall, cascade, or a nearly-vertical plunge, it must not have any sections that are that are gentle enough to be classed as a stream, river or plunge pool separating parts of that fall into separate distinct falls. It is not always easy obtaining this information for waterfalls, as people sometimes only give total height (or in some cases, nothing at all), in an attempt to make the waterfall seem as tall as possible. This is definitely the case for Pistyll Rhaeadr, for example, for which the only reliable information is the total height, when the waterfall is in three very distinct sections.
This lists waterfalls with a total height of 40 metres or more (chosen because there are too many waterfalls requiring etimates below that height), and gives their heights.
To qualify for the list of total heights, the sections of the waterfall must fall without significant sections of stream or river between drops. Plunge pools and intermediate cascades are allowed. It can be difficult to decide where to draw the line. Small streams that tumble down mountain scarps in a series of repeated, broken cascades are intentionally discounted, not because they do not deserve recognition, but because there are simply too many of them to discover, and it is too difficult to obtain accurate information for them. If you are looking for these, the tallest I know are the streams in Cwm Idwal, with a few being about 200 metres tall, and one to the southeast of the Devil's Kitchen having 300 metres of height dropped in a series of cascades. There are also some small streams that drop very steeply for over 600 metres down the southwest slopes of Glyder Fawr. Examples in Mid Wales would include Rhaeadr Du and Rhaeadr Wen at Tarren Bwlch Gwyn near Aberhosan, both about 250 metres high, but never breaking into proper waterfalls.
It may be interesting to note that Berw Nant yr Ychen is probably the tallest waterfall in South Wales in terms of total height, at nearly 30 metres. Sgwd Henrhyd, on the other hand, has only a single drop, and is only a few metres shorter.