The Hawaiian way of calling wave size: the number describes the back of the wave, roughly half the real face. Multiply the Hawaiian number by two to get an approximate face height.
When a local in Hawaii talks about a six-foot wave, he doesn't mean a 1.80-metre wall. The Hawaiian scale describes the back of the wave and therefore lands at roughly half of what you see from the front — a six- to eight-foot wave on that scale has a face of about 3.5 to 5 metres. The expensive mistake is confusing it with face height, which many contest and media reports use: treat the two as the same and you overestimate your own experience by a factor of two. Hence the rule of thumb, Hawaiian number times two — and when in doubt, ask which scale someone means. What often gets missed: the scale is a grown convention, not a calibrated measurement. Swell period usually tells you more about the quality of a day than any height figure does.