Rebuild math from scratch in a year
A full year of daily work — 45 minutes most days — takes an adult who has forgotten everything since school back through arithmetic, algebra, geometry, trigonometry and into pre-calculus. About 280 hours. At the end you can open a calculus textbook without flinching.
12 months · ~280 hours · ready to start calculus
1.Khan Academy — full math missions
Start at Arithmetic, then move through Pre-algebra, Algebra 1, Geometry, Algebra 2, Trigonometry, Pre-calculus. Take the unit tests honestly. If you score below 80%, redo the unit. The exercises are the point — not the videos. Most adults try to skip ahead and stall; the missions exist precisely to catch the gaps you've been hiding from yourself.
Free
Khan Academy Math →2.Art of Problem Solving — Introduction series
Once you're past pre-algebra on Khan, start working through AoPS's Introduction to Algebra alongside it. The problems are harder than anything Khan asks of you, and that's the point — Khan teaches procedures, AoPS teaches mathematical thinking. Buy the textbook and the solutions manual together. Skip a problem set and you'll know.
~$57 textbook + $20 solutions per book
AoPS bookstore →3.Paul's Online Math Notes — drill problems
Paul Dawkins (Lamar University) has hosted clean, solved problem sets in algebra, trig and pre-calculus for two decades. Use them as your end-of-week reality check. If you can't do his algebra cheat-sheet problems cold by month six, you're not ready for what comes next. Bookmark the cheat sheets and revisit them every few weeks.
Free
Paul's Online Math Notes →If this doesn't fit you
If you only need math for a specific applied goal — programming, finance, a stats class — skip this entire path. Go directly to the math your goal requires and look up missing pieces as you hit them. The full rebuild is for people who want the structure itself, not a credential.
Why this path
Most adults relearning math buy three textbooks and finish none. Khan keeps you honest about prerequisites in a way no book can — its mastery system catches the cracks. AoPS forces real thinking on top so you don't end up able to follow steps but unable to solve anything new. Paul's notes are the bridge most curricula skip. Be honest about pace: a year is the floor, not the ceiling, and skipping the unit tests is how people fail this quietly.