Concept 3 of 9
The Potion Mixer: Liquid Volume
Today's big idea
Match the units first, because volumes only add once the units agree.
Ten cups of 100 mL fill to 1,000 mL, which is exactly 1 L.
How it works
👀 What's going on?
How much juice actually fits in your bottle? Capacity is the amount a container holds, measured in millilitres for small amounts and litres for larger ones.
💡 Here's the trick
Look at the cups above. Ten cups of 100 mL each fill right up to 1,000 mL, and that is exactly 1 L. So 1 L = 1,000 mL, the same 1,000 link that ties kilometres to metres. Two amounts can only be added or subtracted once they share a unit.
🪄 How to do it
- 1.Put both amounts into the same unit, as the picture above puts everything into millilitres.
- 2.Add to combine them, or subtract to find what is left over.
- 3.Convert back into litres if that is the unit the question asked for.
✍️ Try it
A 2 L bottle is full and you drink 750 mL from it. How much is left, in millilitres and then in litres?
Match the units first, so 2 L becomes 2,000 mL. Then subtract: 2,000 minus 750 = 1,250 mL. Divided by 1,000 that is 1.25 L. Taking 750 straight off the 2 would have given nonsense.
⚠️ Watch out
You might be thinking you can subtract 750 mL from 2 L directly and land on 1.25. The numbers only line up once both are in the same unit. Mixing units inside one sum is the commonest way these go wrong.
🌟 Why this matters
Recipes, water bottles, petrol and medicine doses are all capacities. Getting both amounts into one unit before you calculate is what stops a cooking disaster.
Exam tip: Convert first and calculate second, because the question is usually checking whether you spotted that the units differ.
Watch me solve one
Quanta solves it
“Watch how I think through one of these — then you try.”
THE PROBLEM
“You have a 3 L jug. You pour out 1,200 mL. How much is left?”
- 1
Match the units first
3 L = 3,000 mL. Now both are in millilitres.
- 2
Pick the operation
Pouring out takes liquid away → subtract.
- 3
Subtract
3,000 − 1,200 = 1,800
- 4
Left in the jug ✓
1,800 mL (or 1.8 L)
Now you try one — same steps, different story 👇
See it in action
⚡ Time to play!
1 litre = 1,000 mL. Pour 250 mL scoops of potion to hit the target exactly — fill the flask to the top!
TARGET
1,500 mL
POURED
0 mL
Fill the flask to exactly 1,500 mL.
Quick check
✅ VERIFICATION
A jug holds 1.5 L. How many millilitres is that?
⚡ Finish the activity above first to answer.
Keep going!
Up next: The Kitchen Scale
Grams, kilograms and how to weigh things.