The link posted at the very start of this thread, which goes to a post here on the board, isn't an online calculator but the advice of a longtime cavalier breeder.
People often want to have some sort of estimate of size, and as she notes very clearly, you really can only make a very broad guess but she gives a rough way of making a prediction and I think most of us have found it is roughly accurate when it comes to puppies we've owned!
Cavaliers come in all sorts of sizes. The breed standard is 12 to 18 pounds in the UK and Ireland, and 13 to 18 pounds in the US–generally one or the other in other countries. That said, the standard is primarily the standard for show purposes, and often dogs will be over that size, and some will be undersized (though this is definitely less common). Also many and perhaps most of the males in the show ring seem to be at least a pound or 2 over breed standard and several breeders that I've talked to over time say they would never use males that fall into the lower to middle areas of the breed standard as they think this produces males that are too slight for showing. In other words, it would be very common to find male show dogs over the breed standard. Perhaps time to change the breed standard– it would be pretty unusual to see dogs being shown that are down at the smaller end of the breed standard so maybe it makes more sense for the size to start at 14 or 15 pounds and go up into the low 20s. There is also an ongoing debate about whether breeding for small size creates more health problems. It is known that miniaturizing breeds into toy size creates a range of health issues that are particular to that process.
I've come across plenty of cavaliers that weigh 25 to 30 pounds, and a handful that are even larger than that. A variety of breeds were used to reconstruct the breed way back at the start of the last century, and depending on the line, it is said these include cockers and even springers and that every now and then, some puppies will throw back to those larger sizes. We had a member here with a cavalier that was in the mid to high 30 pound range or maybe even low 40s, and the largest one that I had come in to rescue over the years was probably in the mid-30s–a big boy, but quite fit and a very beautiful blenheim! It has also said that some less reputable breeders and puppy farmers will use a springer to produce larger litter sizes as cavaliers typically have fairly small litters.
As some have noted, perhaps one of the biggest problems owners face is that whatever the healthy size is for a given cavalier, most cavaliers themselves would prefer to be much larger–as in, they would like to eat themselves into obesity! In common with a handful of other breeds -- and therefore perhaps it's a genetic issue–the breed as a whole does not seem to have any shut off point for eating food once they hit adulthood and with rare exceptions, they easily become overweight and should never be left to free feed or to eat as much as they want. :roll: