Troy Ounce, Gram, Tola, Karat: A Gold Units and Purity Conversion Guide

Prices of gold are quoted in one unit, but gold is sold in many. You might compare prices from a jeweler in Mumbai who works in grams and tolas, a bullion dealer in Hong Kong who works in taels, and a coin buyer in London who works in fractions of an ounce. Mixing up units, or using the wrong ounce, is one of the most common and costly errors.

This guide covers the fundamental gold units, how to convert between them, how to express gold purity and how to combine weight with purity to calculate value for any piece of gold.

Use the OroTracker unit and karat calculator

Don't confuse the troy ounce with the kitchen ounce!

The troy ounce is a medieval unit still used for precious metals. One troy ounce = 31.1034768 grams. The common avoirdupois ounce used for food weighs approximately 28.3495 grams. A troy ounce is roughly 10% heavier. An error here creates a discrepancy of about 3 grams per ounce - a significant difference at current gold prices.

All gold prices quoted 'per ounce' are troy ounces. This is exact: the troy ounce is defined in terms of the metric system by the U.S. National Institute of Standards and Technology (NIST).

Core conversion factors

• 1 troy ounce = 31.1034768 grams
• 1 gram = 0.0321507 troy ounces
• 1 kilogram = 32.1507 troy ounces
• 1 troy pound = 12 troy ounces (about 373.24 grams)
• 1 grain = about 0.0648 grams (480 grains make a troy ounce)
• 1 tola = 11.6638 grams, or 0.375 troy ounces

To convert a price per troy ounce to a price per gram, divide by 31.1035. For example: if spot is $2,000/troy oz, the price per gram of pure gold is about $64.30.

The tola (and other local units)

The tola is used across the Indian subcontinent (India, Pakistan, Nepal and Bangladesh) and the Gulf countries. It derives from the weight of a British-Indian rupee coin - 11.66 grams. In South Asia, 10-tola bars (roughly 116.64 grams or 3.75 troy ounces) are the standard gold bar size sold.

Other regional units worth knowing:

• Tael - used in Hong Kong, China and Southeast Asia. Confirm the local definition before agreeing on a price, as tael weight varies by region.
• Baht - Thailand's unit, approximately 15.244 grams of gold.
• Mithqal - used in parts of the Middle East and North Africa, metered locally.

When comparing prices across regions, always convert to grams first. It is the universal unit for comparison.

Fineness and karats

Pure gold is soft, so most jewelry is alloyed with other metals. Purity can be expressed two ways:

Fineness is the proportion of gold in 1,000 parts of the total metal. A '999' stamp means 99.9% gold. Karats (K or kt) divide the metal into 24 parts - 24K is pure gold.

• 24K = 24/24 = 99.9%+ (fineness 999 or 9999 for some bullion)
• 22K = 22/24 = 91.67% (fineness 916)
• 18K = 18/24 = 75.0% (fineness 750)
• 14K = 14/24 = 58.33% (fineness 585)
• 10K = 10/24 = 41.67% (fineness 417)
• 9K = 9/24 = 37.5% (fineness 375)

Do not confuse 'karat' with 'carat'. Karat measures gold purity out of 24. Carat is a weight unit for gemstones - 1 carat = 0.2 grams.

How to calculate the value of gold

Value = weight (in grams) × purity × price per gram of pure gold

Or in troy ounces: Value = weight (troy oz) × purity × price per troy ounce

Worked Example 1: One-tola 22K bar

Assume spot = $2,000/troy oz. Weight: 1 tola = 0.375 troy oz. Purity: 22K = 0.9167. Pure gold content: 0.375 × 0.9167 = 0.3438 troy oz. Melt value: 0.3438 × $2,000 = about $687.50.

Worked Example 2: A 25 g 18K chain

Price per gram of pure gold: $2,000 / 31.1035 = about $64.30. Pure gold in the chain: 25 g × 0.75 = 18.75 g. Melt value: 18.75 × $64.30 = about $1,205.60. The retail price will be higher due to workmanship, design, brand and sales tax.

Worked Example 3: Convert a bar to ounces

A 100-gram bar of 999 gold contains 100 × 0.999 = 99.9 grams of pure gold. Divide by 31.1035 to get 3.212 troy ounces of fine gold.

Disclaimer: This article is for general educational purposes only and should not be construed as financial, investment, tax or legal advice. Prices shown in examples are illustrative only and do not reflect current market prices.