The short answer: no — ounces are not a metric unit. Ounces belong to the Imperial / U.S. customary measurement system and coexist with the metric (SI) system through fixed conversions.
In this article you’ll learn what ounces measure, why they are not metric, how they relate to grams and liters, their various types (avoirdupois, troy, fluid), and when you will still see them used in the U.S. and elsewhere.
What Does “Metric System” Mean?
The metric system (or International System of Units, SI) is built around a base of units like the meter, liter, and gram — and uses standard prefixes (kilo-, centi-, milli-) to scale up or down. It aims for simplicity, consistency, and ease of conversion (factors of 10).
Because ounces derive from an older British / Roman tradition, they do not fit into that base-10 structure. They are part of a different measurement tradition with roots in historical trade, land, and craft practices.
What Is an Ounce? (Weight, Volume, and More)
When people say “ounce,” they may refer to:
- Avoirdupois ounce (mass / weight) — The common ounce used in everyday U.S. life.
- Troy ounce (mass) — Used for precious metals.
- Fluid ounce (volume) — Used for liquids.
Each version has a precise definition and a fixed relation to metric units.
Avoirdupois Ounce: Everyday Weight
The avoirdupois ounce, used widely in the U.S. and formerly in the U.K., is defined as exactly 28.349523125 grams.
Sixteen of these ounces make a pound. That means one pound = 16 oz = 453.59237 grams.
You will see this ounce used on food packaging, postal scales, body weight in recipes, and so on.
Troy Ounce: Precious Metals
The troy ounce is a slightly heavier ounce (31.1034768 grams). It is exclusively used in the trade of precious metals like gold, silver, and platinum.
This usage persists because the precious-metal markets worldwide adopted and standardized on the troy ounce long ago — today it is internationally recognized in that domain.
Fluid Ounce: Volume of Liquids
A fluid ounce measures volume, not weight.
- In U.S. customary units, one fluid ounce is exactly 29.5735295625 milliliters.
- In the British (imperial) system, one fluid ounce is 28.4130625 milliliters.
So when you pour “8 fl oz” of milk, you are measuring space, not mass. (Though for water, 1 fl oz ≈ 1 avoirdupois oz in weight, but only approximately.)
Why Ounces Are Not Metric
- Ounces do not belong to the base units of SI (meter, kilogram, second, ampere, kelvin, mole, candela).
- Their conversions to metric units are not simple base-10 shifts — you multiply by 28.3495 (or 31.1035 for troy) or by ≈29.57 (for fluid).
- They carry irregular subdivisions (e.g. 16 oz = 1 lb; not 10 or 100).
- The design of the metric system purposely excluded legacy units so that everything scales consistently.
Thus ounces are legacy, parallel units rather than integral to metric.
How to Convert Ounces to Metric
Here are key conversion factors you’ll often use:
- 1 avoirdupois ounce = 28.349523125 grams
- 1 troy ounce = 31.1034768 grams
- 1 U.S. fluid ounce = 29.5735295625 milliliters
- 1 Imperial fluid ounce = 28.4130625 milliliters
In practice, many folks approximate: 1 oz ≈ 28.35 g, or 1 fl oz ≈ 29.57 mL.
Where Ounces Still Appear (Especially in the U.S.)
Even though most of the world uses metric, ounces remain deeply entrenched in U.S. life:
- Food labels state ounces.
- Many recipes use ounces.
- Postal and shipping weights in oz and lbs.
- Retail packaging (e.g. 12 oz cans).
- Cooking measurements (fluid ounces).
In the U.K. and other formerly British-influenced places, ounces have mostly faded in official use but may survive colloquially.
Are there “Metric Ounces”?
Some confusion arises because in certain countries, local “ounce” terms were redefined in a metric context. For example, Dutch “ons” once meant 100 grams (a “metric ounce”). But that is a local vernacular use — it is not part of the international metric system.
Common Misunderstandings
- Weight vs. volume confusion — People often confuse fluid ounces (volume) with ounces (mass). They are distinct.
- Mixing units in recipes — A recipe may call for “4 oz milk” meaning fluid ounces, not weight ounces.
- Thinking ounces are metric — Some believe all measures are metric today, so they assume ounces are part of it — this is not true.
- Assuming simple conversion — Because ounces convert to grams via multiplication, some think it’s metric; but the relationship is purely defined by agreement, not structural alignment with SI.
Metric System Advantages vs Ounces’ Legacy
The metric system gives you:
- Seamless scaling by powers of 10
- Uniform prefixes (milli-, centi-, kilo-)
- Simpler learning and fewer conversion errors
Whereas ounces offer:
- Historical continuity
- Familiarity for U.S. users
- Legacy support in markets and industry
But for scientific, technical, and international usage, metric is preferred.
Quick Reference Table
| Unit | Measures | Metric Equivalent |
| 1 avoirdupois oz | mass / weight | 28.35 g |
| 1 troy oz | mass / precious metals | 31.10 g |
| 1 U.S. fl oz | liquid volume | 29.57 mL |
| 1 Imperial fl oz | liquid volume | 28.41 mL |
When You Must Convert
If you work internationally, do science, share recipes globally, or handle trade, you’ll need to convert ounces to metric. Use a scale or precise conversion tool. Always check whether the “ounce” in question is weight (avoirdupois or troy) or fluid.
Closing Thoughts
Ounces are not metric. They exist in a parallel measurement tradition. But thanks to agreed conversions, they can translate into the metric world. In the U.S., ounces remain essential and ubiquitous.
Internationally and scientifically, metric dominates by design. In daily use, understanding which ounce you mean (weight or fluid) and applying the correct conversion ensures clarity and accuracy.