The first thing we notice about a diamond engagement ring is its size. But diamonds are sold by weight, not size. Diamond carats are a unit of gem weight that’s equivalent to one-fifth of a gram. In other words, a five-carat diamond weighs one gram.

Of course, weight does tell you quite a bit about diamond size (especially if you have this handy diamond carat size chart). But it doesn't really tell you everything: all one-carat diamonds aren’t the same exact size.

Generally one carat round diamonds are about 6.5mm across but they could be as little as 6.1mm or as much as 6.8mm. Why the difference in size if they are the same diamond carat weight? Because some diamonds are cut deeper and some diamonds are cut shallower. Some diamonds have a thicker girdle. And that’s just the variation in the carat weight of round brilliant diamonds.

Each diamond fancy shape has different proportions. A two-carat round brilliant is about 8.1mm across. A two-carat Asscher cut is about 7.0mm square. In contrast, a two-carat oval is about 10.5x7.0mm: the oval looks much larger than the other two shapes. In general, round brilliants, Asscher cuts and cushion cuts look smaller than oval shapes, pear shapes and marquise shapes. To compare the carat weight of your favorite shape, check out our guide to diamond shape and size.

For most things we buy that are priced by weight, like coffee or chocolate, two pounds costs twice as much as one pound. But buying diamonds isn’t like buying coffee. A diamond’s price per carat increases with the size of the diamond because bigger diamonds are more rare. Let’s say a nice one-carat diamond is about $6,000. A two-carat diamond of the exact same quality might be $20,000. It’s more than three times the price of the one carat.

And it’s not twice as big in size. A one carat diamond is about 6.4mm and a two carat diamond is about 8.1mm. The face-up surface area of the diamond that weighs twice as much is only about 60% more. That’s because a lot of the weight is in height, not diameter.

There’s one more reason to think dimensions not carat weight.

How big a diamond are you shopping for? I bet you gave your answer in a round number of diamond carats, like one carat or two carats. Am I right? The diamond business knows that if you are looking for a one-carat diamond, you have your mind set on that nice round number.

Diamond pricing works that way too: diamonds from 0.90 to 0.99 carats have one price per carat and diamonds from 1.00 to 1.49 carats have another. So cutting a diamond that weighs 0.99 carats means leaving money on the table. A tiny weight difference becomes a significant price difference.

So diamond cutters do all they can to make sure that hit those magic round numbers, even if it means cheating a little on the angles and proportions that make a diamond beautiful and hiding some weight. That 1.00 carat diamond or 1.01 carat diamond may be one of those compromises where beauty was sacrificed to add weight.

So to make sure you are buying a beautiful diamond, don’t obsess too much about exact diamond carats over diamond cut quality.

Especially when you remember that size matters too: that 0.95 carat diamond and a one-carat diamond might be the exact same diameter. And you will definitely not notice the difference in diamond carats when the diamond is set in a ring.

So, to save money without sacrificing quality, look for diamonds that are just below the round numbers. And always pay attention to diamond dimension as well as diamond carat weight.