Magicseaweed delivers some very precise information about expected swell heights at your local beach for the next ten days. However anyone who's ever been surfing will know that even on the perfect day not every wave that comes through the line up will be the same size. We go into some detail here about this but to summarise the heights we (and every other surf forecaster around) give are 'significant height' which is the average of the largest third of waves. It's close to saying the same as 'the average height of the set waves'. Fortunately there's a fairly good relationship between this number and the height of the largest waves you're likely to see. A good general guide is to multiple the height by 1.5x to find the largest waves and hence the range of likely surfable waves:
2ft = 2-3ft range
4ft = 4-6ft
6ft = 6-9ft
12ft = 12-18ft
20ft = 20-30ft
There's good mathematical justification for this and it also works well when compared to actual observed data. You'll see that it also means the range of wave sizes gets bigger the bigger the swell gets. If you can imagine that you're surfing a deep water spot that needs 8ft of swell to start to break you'll almost certainly see infrequent breaking waves on a forecast 6ft swell as those maximum height waves hit the reef. However head out on an 8ft swell and not only will every set wave be breaking on the reef but you'll also be dodging occasional 12ft bombs – a potentially big change in conditions for just a couple of feet on the swell.
In addition to this 1.5x multiple being a great guide to the general wave range bear in mind that the theoretical maximum height is actually around 2x the significant height – this is the largest wave of the largest set you just might have to deal with (although observations suggest that rouge waves larger than this can occur see comments below).
The main reason I'm writing this now is we've been keeping a very close eye on the wave buoys to check the forecast for the swell currently building below Tasmania and the current data is a near perfect illustration of how this theory all pans out in practice:
You can see that the red line (the largest wave in the period, which normally is around 20 minutes) trends at very close to 1.5 x the blue line (the significant height). But interestingly you'll notice on Tuesday 5th at 12:00 a single reading hits fractionally over the 2x limit. Now we are aware that occasionally a buoy can just get itself in a knot and report spurious data, but it looks entirely consistent with the theory and it makes a great real world example.