This may vary but are mattress toppers generally the same size as the mattress they are on? I am looking at King toppers, should I expect it to be 76 x 80?
Not always, no. Depends on the manufacturer. I’ve owned all kinds of toppers over the years (memory foam and latex). Both materials can range from the exact size of the mattress to 2-3" smaller.
Hi colorfinger,
Ditto to what jkozlow3 said 
In most cases a better supplier will be tell you the dimensions of a topper that they sell if it’s not listed on their site but there can also be smaller variances in the same topper as well.
Phoenix
If a manufacturer doesn’t specify, odds are they are cut 2-4" short on each side.
I had a problem with this once and the manufacturer tried to turn a negative into a postive by saying “by cutting it short on each side we make sure the sheets still fit and there isn’t any over lap”, but personally i feel it should match up with the size of the bed nearly perfectly. And if a manufacturer/retailer tries to tell you nobody will cut to the exact size, just know that is an outright lie (believe it or not, there are companies saying that).
So just make sure the company specifies, or give them a call and ask. IT would be of particular annoyance to me if I were to lose a few inches of sleepable space on a king bed, especially if there were two people sleeping it.
Matt
I bought this horrible topper from Kohl’s that turned out to be almost 6" short on the sides. What did they figure, I don’t really use my whole bed anyway? lol
Hi colorfinger,
Topper dimension will also depend on were the manufacturer takes his foam to cut his toppers. We use the same foam blocks to cut layers of foam to build our mattresses and our toppers. Therefor the toppers are the same size as a standard size mattress.
Other manufacturers specifically pour smaller blocks of foam to make their toppers. With this method they use less foam and therefor can sell the toppers for less, since they are not covering the entire mattress.
We also suggest to ask what size you are getting before you make a purchase…
good luck 