"This perfume is 1.7 oz, so how many ml is that?"
When shopping for cosmetics abroad or ordering a drink in the US, there's one unit that always confuses us: the Ounce (oz).
It's tempting to think "It's roughly 30ml" and move on, but the world of ounces is far more complex. We use ounces for weight, and we use ounces for volume. Did you know that US ounces and UK ounces are actually different?
Today, we will clear up this confusion once and for all. By the end of this guide, you'll understand exactly why a Starbucks 'Venti' is called 'Venti'.
1. Two Types of Ounces: Weight (oz) vs. Volume (fl oz)
The most common mistake is mixing up "Weight Ounces" and "Fluid Ounces". They share the same name but are completely different beasts.
① Avoirdupois Ounce (Weight)
- Symbol: oz
- Use: Measuring Weight (Solids)
- Standard: 1 oz ≈ 28.35g
- Examples: A steak, a bag of chips, or gold (though gold uses Troy ounces, which are 31.1g, but let's stick to everyday items!).
② Fluid Ounce (Volume)
- Symbol: fl oz
- Use: Measuring Volume (Liquids)
- Standard: 1 fl oz ≈ 29.57ml (US Standard)
- Examples: Soda, perfume, lotions, shampoo.
Key Takeaway: When you see "1.7 oz" on a perfume bottle, it actually means "1.7 fl oz". It's a liquid. However, if you are calculating shipping weight for a package, that 'oz' refers to weight.
2. US Ounce vs. UK (Imperial) Ounce
If you memorized "1 ounce is about 30ml", you are half right. The standards for the US and UK differ.
🇺🇸 US Fluid Ounce (US fl oz)
- 1 fl oz = 29.5735 ml
- Most global products (US cosmetics, Starbucks, Coca-Cola cans) follow this standard.
- Fact: 1 US Gallon = 128 fl oz.
🇬🇧 Imperial Fluid Ounce (UK fl oz)
- 1 fl oz = 28.4131 ml
- It is slightly smaller than the US ounce.
- Think of a British pint of beer. A UK Pint is 20 Imperial fl oz (568ml), whereas a US Pint is 16 US fl oz (473ml).
- Fact: 1 Imperial Gallon = 160 fl oz.
If you order a pint in a London pub and it feels different from a pint in an American bar, this "Imperial System" is the reason. However, for most modern shopping (Amazon, cosmetics), assuming US fl oz is usually the safer bet.
3. Real Life Examples: Starbucks & Perfume
Let's get a feel for 1 ounce using everyday items.
☕ The Secret of Starbucks Cup Sizes
While the names sound Italian, the volumes are strictly US Fluid Ounces.
- Short: 8 fl oz (≈ 237ml)
- Tall: 12 fl oz (≈ 355ml)
- Grande: 16 fl oz (≈ 473ml)
- Venti: 20 fl oz (≈ 591ml) - 'Venti' literally means '20' in Italian. It's named Venti because it's 20 ounces!
- Trenta: 30 fl oz (≈ 916ml) - 'Trenta' means '30'.
💄 Cosmetics & Perfumes
Imported cosmetics usually follow standard sizes.
- 1.7 fl oz: The most common perfume size. It is roughly 50ml.
- 3.4 fl oz: A large perfume bottle. It is roughly 100ml.
- 6.7 fl oz: Large toner or lotion. It is roughly 200ml.
Now when you see "3.4 oz", you can instantly translate it to "Ah, that's the 100ml bottle!"
4. Why isn't 1 oz of water (weight) equal to 1 fl oz (volume)?
"Water has a density of 1, so 1g is 1ml. Shouldn't 1 oz weight equal 1 fl oz volume?" That's a very sharp question!
Unfortunately, the US Imperial system isn't that scientific.
- Weight of 1 US fl oz of water ≈ 29.57g
- 1 Weight oz = 28.35g
As you can see, there is a difference of about 1.2g. This means you need approx. 1.043 fl oz of water to equal 1 oz of weight. So for precise baking or chemistry, never assume "Liquid Oz = Weight Oz". (Though fo instant ramyun, it doesn't really matter.)
5. Frequently Asked Questions (FAQ)
Q1. A recipe calls for "8oz of water". What does it mean?
A. Since it's a liquid, it usually means 8 fl oz (1 Cup, approx. 240ml). (In the US, 1 Cup = 8 fl oz).
Q2. What about "8oz of flour"?
A. Flour is a solid, so this refers to Weight Oz (approx. 227g). In volume, that would be about 1.5 to 2 cups depending on how packed the flour is.
Q3. I'm buying from a UK site. Which ounce is it?
A. If it's a UK site, it's likely Imperial fl oz (28.41ml). However, global brands often list 'ml' on the back, which is the only truly universal standard. Always check the 'ml' if you can!
Ounces are confusing. Weight and volume share the same name but have different values, and the US and UK can't even agree on what that value is.
But just remember one thing: "1 US Fluid Ounce (fl oz) is about one 30ml shot." (29.57ml to be exact). knowing this will help you navigate Starbucks menus and Amazon listings with confidence.
For precise calculations, don't guess—use our converter below. It gives you the answer in a second.