I believe that you need to go up 2 sizes ...so if you are a 16.5 western , you would need a 18.5 English ?
Someone else can correct me if I am wrong , but that is what I have always heard.
I agree though ,to consult a saddle fitter and sit in as many different saddles you can to find the right fit /comfort.
I ride is a 16 " seat western but my english saddle is 17.5 and seems to fit me ok, I think it just depends on the saddle.
Western + 2 inch - sorry- it doesn't really work that way.
An 18.5 inch English saddle is very long and unless the rider has a really big butt then it will be too big and she'll be slopping about.
Fit the horse first - width and length - then sit on the saddle and ask yourself if it is comfortable.
There is also the question of cut - ie dressage, working hunter or GP
And of course whether it is deep seated ie high cantel and pommel
Then whether it is made of soft leather or hard leather,
Then does it have big knee rolls
And does it go round the horse's barrel
And is the horse's back wide and flat or narrow and high
But most importantly - does it fit the horse and does it suit you and do you feel comfortable on it?
Ladies, would you buy a wedding dress over the internet ?
You are only going to wear that dress once - or maybe twice, or perhaps three times or possibly four times - all depends on the number of husbands - but the saddle - well you are going to to sit on that maybe 5 or 6 times a week.
Choose that saddle as carefully as you are going to choose the husband.
So, fitting english saddle to the human is ALOT easier than fitting them to a horse. The main thing about fitting them to a human is to know that the seat length is less about the size of you butt and more about the length of your thigh. You should be able to sit in a saddle and have 4 fingers of room between the back of your butt and the top of the cantle and have 1-2 fingers between the front of your knee and the front of the flap. They measure the seat size of an english saddle by taking the measurement from the top of the cantle to one of the two pins (one on either side) that run through the side of the pommel. I reccommend that you go to a tack store and ask to try a few saddles in varying sizes to get an idea of what seat size you're looking for, as that's the most accurate way to find out, but if you wan't to buy via the internet, you can always ask the seller to measure from cantle to kneeflap to check that length. Hope this helps:) Best of luck finding a saddle!