Ratio vs Proportion

Ratio

2:3 is a ratio. On its own it is a statement about relative size: two of one thing for every three of another. There is nothing to solve.

Proportion

2:3 = 8:12 is a proportion. It claims the two ratios describe the same relationship — and it is true here, because both reduce to 2:3. A proportion becomes useful when one of the four values is unknown, because cross-multiplication then recovers it.

Which tool do I need?

If you have quantities and want to describe or reduce them, you need a ratio calculator. If you have three values and one gap, you need the proportion calculator.