Khan Academy

Khan Academy is a free platform covering school mathematics from arithmetic through calculus and beyond.

Although it is not designed for competition math, but it is the most thorough freely available resource for building the prerequisite knowledge that competitions assume.

What's Available

Video lessons and practice exercises for every core math topic
Covers arithmetic, pre-algebra, algebra, geometry, trigonometry, statistics, and calculus
Immediate feedback and progress tracking
Completely free

Who It Is For

Khan Academy is useful if you have gaps in foundational knowledge that are holding you back in competition problems. For example:

Struggling with algebraic manipulation before moving to olympiad algebra
Needing to solidify geometry basics before tackling competition geometry problems
Reviewing number properties before approaching number theory

Khan Academy alone will not prepare you for competitions — the problem style is very different. Think of it as the prerequisite layer. Once your foundations are solid, move to AoPS or Brilliant for competition-specific practice.