How Many Grams in an Ounce? Troy vs Avoirdupois Explained

"How many grams in an ounce?" has two correct answers depending on which ounce you mean. The difference is not academic: buying precious metals with the wrong conversion in mind means paying 10% more — or less — than you intended. This article explains both ounce types, when each applies, and gives you every conversion factor you need.

Quick Answer: 28.3495 Grams (Avoirdupois)

The ounce used in everyday life — for food, body weight, shipping, and most commerce — is the avoirdupois ounce:

1 avoirdupois ounce = 28.3495 grams (exactly 28.349523125 g)

Sixteen avoirdupois ounces make one avoirdupois pound (453.592 g). This is the ounce on US nutrition labels, postal scales, and kitchen scales.

For quick mental math: multiply ounces by 28.35 to get grams, or divide grams by 28.35 to get ounces.

Troy Ounces: 31.1035 Grams (Gold, Silver, Gems)

The troy ounce is the standard unit for precious metals and gemstones in global markets:

1 troy ounce = 31.1034768 grams

A troy ounce is approximately 10% heavier than an avoirdupois ounce (31.10 g vs 28.35 g). The troy ounce is subdivided differently: 12 troy ounces = 1 troy pound (373.24 g), which is lighter than an avoirdupois pound (453.59 g).

When you see a gold price quoted as "$2,100 per ounce," that ounce is always troy. Quoting 28.35 g instead of 31.10 g would undervalue gold by about 9.7%.

Avoirdupois vs Troy: When to Use Which

†The apothecary ounce is equal to the troy ounce (31.1035 g) but uses different subdivision names (scruples, drams). It is largely obsolete outside historical contexts.

The rule is simple: if it's food, your body, or a package — avoirdupois. If it's a precious metal or gem — troy.

Converting Ounces to Grams and Back

Avoirdupois formulas

Troy formulas

The abbreviation for troy ounce is ozt (sometimes written oz t) to distinguish it from the avoirdupois oz. Bullion dealers and market data always use this context implicitly — you rarely see "ozt" written out, but the meaning is clear from context.

Common Weight Conversions Table

Avoirdupois reference

Troy ounce reference (precious metals)

The last two rows highlight a useful reference: 1 kilogram of gold equals 32.151 troy ounces.

Visit the weight converter for live calculations, or use the direct ounce to gram converter for fast lookups.

FAQ

Why does the US use avoirdupois ounces for food but troy ounces for metals? The two systems developed independently for different trades. Avoirdupois (from Old French "goods of weight") evolved for general commerce in medieval Europe. Troy weight (named for Troyes, France, a major medieval trade fair city) was established for precious metals. Both systems were codified in the British Weights and Measures Acts and inherited by the US. There was never a move to unify them because the communities using each system — commodity traders vs metallurgists — had no overlap.

Is the fluid ounce related to either of these? No. The fluid ounce measures volume (29.5735 mL), not mass. It is a coincidence that water at room temperature has a density such that 1 US fluid ounce of water weighs approximately 1.04 avoirdupois ounces. The two units have separate definitions.

What is a "pennyweight" and does it still matter? A pennyweight (dwt) is 1/20 of a troy ounce, equal to 1.5552 g. Jewelers and dentists still use it, particularly for gold. If you see gold quoted "per dwt," multiply by 20 to get the troy ounce price.

How many grams are in an ounce of weed (US cannabis market)? US cannabis is sold using avoirdupois ounces. One ounce = 28.3495 g. An eighth (1/8 oz) = 3.5437 g, typically rounded to 3.5 g in practice.

My kitchen scale shows both oz and g — which should I use? For precision cooking and baking, use grams. They produce whole numbers for most recipe quantities and eliminate the fluid-ounce/weight-ounce confusion entirely.