Why the unit is where quotes go wrong
Gold is quoted per troy ounce, weighed in grams by most of the world, bought in pennyweight by a large part of the US scrap trade, and gifted by the tola across South Asia and the Gulf. Between the smallest and largest of those there is a factor of eleven. A price quoted "per unit" without naming the unit is not a price at all, and asking which one is the fastest way to make a quote comparable.
Every unit here is a fixed multiple of the troy ounce, so all these conversions are exact rather than approximate. A troy ounce is 31.1034768 grams by definition. A pennyweight is one twentieth of that, 1.5552 grams. A troy grain is one four-hundred-and-eightieth. A tola is exactly three eighths of a troy ounce, 11.6638 grams.
The two ounces, and the mistake they cause
The avoirdupois ounce — the one on a kitchen scale — is 28.349523125 grams. The troy ounce is 31.1034768. The troy ounce is about 9.7% heavier, which means weighing gold on a kitchen scale and pricing it per troy ounce overstates the answer by roughly a tenth.
This is the most common unit error in the subject and it runs in both directions: someone who weighs in troy and prices in avoirdupois underestimates by the same margin. If a scale does not state which ounce it uses, weigh in grams. Grams are unambiguous.
Tola, baht and the regional units
The tola was standardised at 180 troy grains, which makes it exactly three eighths of a troy ounce. It is the customary unit across India, Pakistan, Bangladesh — where it is called a vori or bhori — and much of the Gulf, where gold is bought as a store of value as much as an ornament.
Thailand prices everything in baht, and there are two of them. The bullion baht is 15.244 grams and the jewellery baht is 15.16, a difference of about half a percent. The Thai Gold Traders Association publishes the daily price against the bullion figure. Half a percent sounds trivial until it is applied to a kilogram.