Gold Conversion Calculator: Weight & Purity

A gold conversion calculator converts between the common units used to weigh and price gold: troy ounces, grams, pennyweights, and kilograms, and it can also convert between purity levels (karats) to find the pure gold content in a given item. You enter a weight and optionally a purity, and the calculator returns the equivalent weights in other units and the amount of pure gold. This page explains the formulas, how to use them, and the common mistakes to avoid.

Formula

The core conversion is simple: multiply the input weight by the conversion factor of the selected unit to get grams, then multiply by the karat purity fraction to get the pure gold content. The formulas are:

• Weight in grams = input value × unit factor (where unit factor is 1 for g, 31.1035 for troy oz, 1.55517 for dwt, 1000 for kg)
• Pure gold (g) = weight in grams × (karat / 24)
• Troy ounces = weight in grams / 31.1035
• Pennyweight = weight in grams / 1.55517
• Kilograms = weight in grams / 1000

Variables: wt is the numeric weight you enter; unit is the conversion factor of the unit you selected; karat is the purity fraction (e.g., 0.75 for 18k). All weights are in grams for the pure gold output.

Worked Example

Using the default values: weight = 31.1035 g, unit = g (factor 1), karat = 24 (factor 1).

1. Weight in grams = 31.1035 × 1 = 31.1035 g
2. Pure gold = 31.1035 × 1 = 31.1035 g
3. Troy ounces = 31.1035 / 31.1035 = 1.0000 ozt
4. Pennyweight = 31.1035 / 1.55517 = 20.0000 dwt
5. Kilograms = 31.1035 / 1000 = 0.0311 kg

So 31.1035 grams of 24k gold is exactly 1 troy ounce, 20 pennyweight, and 0.0311 kg. This matches the calculator’s output.

Why Gold Uses Troy Ounces, Not Regular Ounces

Gold and other precious metals are weighed in troy ounces, not the avoirdupois ounces used for food and everyday items. One troy ounce equals 31.1035 grams, while a standard ounce is 28.3495 grams. That’s a difference of about 9.7%. If you use a regular kitchen scale that shows ounces, you might accidentally think you have more gold than you do. The troy system also includes the pennyweight (1.55517 g) and the troy pound (12 troy ounces), which are rarely used today but still appear in some jewelry and dental contexts.

Understanding Karats and Pure Gold Content

Karat (abbreviated k or kt) measures purity out of 24 parts. Pure gold is 24k. An 18k item is 18/24 = 75% gold by weight; the rest is usually copper, silver, or other metals. The calculator multiplies the total weight by the karat fraction to give the actual gold content. For example, a 10-gram 14k ring contains 10 × 0.5833 = 5.833 grams of pure gold. The remaining 4.167 grams are alloy metals. This is essential when estimating the melt value of scrap jewelry or when buying gold by weight.

How to Choose the Right Inputs

To get an accurate result, you need to know two things: the total weight of the item and its karat purity. Here’s how to determine them:

• Weigh the item on a digital scale that measures in grams or troy ounces. For small items, use a jeweler’s scale with 0.01 g precision.
• Find the karat stamp: usually 24k, 22k, 18k, 14k, 10k, or 9k. If there’s no stamp, you may need a professional test (acid test or XRF).
• If the item is plated or filled, the karat stamp refers to the outer layer, not the whole piece. The calculator assumes the entire weight is the stated karat.
• For coins, use the actual gold content (often listed as “AGWâ€� – actual gold weight) rather than the total weight, because coins may have small amounts of alloy.

Common Mistakes and How to Avoid Them

The most frequent error is mixing up troy ounces and avoirdupois ounces. Always check the scale’s unit setting. Another mistake is using the total weight of a gold-filled or plated item as if it were solid karat gold. Also, rounding too early can cause noticeable errors: if you round 31.1035 to 31.1, you lose 0.0035 g per ounce, which adds up over large quantities. Finally, remember that karat is a weight fraction, not a volume fraction – 18k gold is 75% gold by weight, not by volume.

Limits of the Calculator

This calculator provides weight conversions and pure gold content. It does not determine the current market price of gold, because that changes daily. To estimate value, you would multiply the pure gold weight (in grams or troy ounces) by the current spot price per gram or ounce. Also, the calculator assumes the entire item is homogeneous at the stated karat. In reality, soldered joints, hollow areas, or non-gold components (like gemstones) will reduce the actual gold content. For precise valuation, professional appraisal is recommended.

Frequently Asked Questions

How many grams are in a troy ounce of gold?
Exactly 31.1035 grams. This is the standard used for all precious metals, including gold, silver, and platinum.

What is the difference between 18k and 24k gold?
24k is pure gold (99.9% or higher). 18k is 75% gold and 25% other metals. The alloy makes 18k harder and more durable for everyday jewelry, while 24k is softer and more prone to scratching.

How do I convert pennyweight to grams?
Multiply the pennyweight value by 1.55517. For example, 10 dwt × 1.55517 = 15.5517 grams.

Can I use this calculator for gold scrap?
Yes, but only if you know the exact karat and the weight of the gold-bearing parts. Remove any non-gold attachments like clasps or stones, and be aware that some scrap buyers use a different purity assumption.

Why does my kitchen scale show a different ounce weight?
Because kitchen scales typically measure avoirdupois ounces (28.35 g), not troy ounces (31.10 g). If you weigh gold on a kitchen scale, convert the result to troy ounces by multiplying by 0.9114.

What does “dwt� stand for?
Dwt stands for pennyweight, a unit used in the troy system. One pennyweight equals 1.55517 grams, and there are 20 pennyweight in a troy ounce.

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in wt | Weight | g | 31.1035 | 0
sel unit | Weight unit | g=1, ozt=31.1035, dwt=1.55517, kg=1000
sel karat | Purity (karat) | 24=1, 22=0.9167, 18=0.75, 14=0.5833, 10=0.4167, 9=0.375
out pure | Pure gold weight | g | wt * unit * karat | 4
out ozt | Weight in troy ounces | ozt | wt * unit / 31.1035 | 4
out dwt | Weight in pennyweight | dwt | wt * unit / 1.55517 | 4
out kg | Weight in kilograms | kg | wt * unit / 1000 | 4