How I Made Claude Actually Reliable at Math (5-Minute Setup)

I spent a week watching Claude confidently give me wrong answers. Not wrong opinions — wrong numbers. TDEE calculations off by 200 calories. Mortgage amortization that didn't add up. Compound interest that was close-ish but not quite right. The thing is, Claude sounds confident when it hallucinates math. It walks you through the reasoning, uses the right formula names, and arrives at a number that feels plausible. The problem only shows up when you check the work. This is a known issue with LLMs. They don't actually "do math" — they pattern-match from training data. Arithmetic is surprisingly unreliable, especially for multi-step calculations. Here's how I fixed it. The Problem: LLMs Are Not Calculators When you ask Claude to calculate your TDEE (Total Daily Energy Expenditure), it might use the Harris-Benedict formula and arrive at approximately the right answer. But "approximately" isn't good enough when you're tracking calories or modeling a 30-year mortgage. LLMs work by predicting