As surf forecasters we measure waves from the trough (the lowest point) to the peak (the highest point). Typically we do this in feet but of course it's equally valid in meters. Surfers however indulge in a range of different measuring systems depending on their context which all usually involve underestimating the face height of the waves.
The 'Hawaiian' system is a well established alternative to measuring the face height and tends to equate consistently to about half the face height. So a 4ft Hawaiian wave is 8ft on the face, or a couple or three feet over the head of a riding surfer. You'll hear this explained as 'measuring the back of the wave' but in reality this doesn't work, or if it does for a very small selection of breaks. There's a broad consensus of agreement around the Hawaiian scale, so regardless of the confusion it leads to with inexperienced surfers none the less a '6ft Hawaiian scale' wave will look the same for most surfers in most locations.
Surfers outside Hawaii tend, in our experience, to measure somewhere between face height and the Hawaiian scale, with more experienced surfers tending to under call the wave. It can turn into the most ridiculous of chest beating exercises and, of course, lacking a consistent and understood scale it is a long way less than useful.
Check out this photo:
There were surfers describing this day as '4ft' – they've every right to call it as they see it but you can start to see that this system isn't much use for us as surf forecasters. Most of us would be pretty surprised to head to the beach on a 4ft forecast and see that at the beach. It's for that reason that on MSW and everywhere we discuss waves we talk in feet, and we talk about the actual trough to peak swell height or the wave face height. Given this the scale looks something like:
1ft – Ankle to Knee High
2ft – Knee to Thigh
3ft – Waist Hight
4ft – Chest High
5ft – Head High (for average rider slightly crouched down)
Interestingly in more recent years with the advent particularly of the Billabong XXL which aims to objectively measure the largest waves ridden each year there has been a strong and (for us at least) encouraging trend to measure waves based on their actual size again!