How do I balance RPG combat encounters without causing a TPK?

The rating on the box isn't the whole answer

Every system with an encounter-building formula — challenge rating, XP budgets, threat levels — is answering a narrower question than 'will this fight go well': roughly, how much raw statistical firepower is on each side. That's a real and useful number. It's also blind to several things that decide whether a fight actually feels balanced at the table.

Use the system-specific formula where one exists; don't throw it out. What follows is the judgement layer that sits on top of it, for the things no formula captures on its own.

What the formula usually misses

• Action economy Four enemies acting once each often hits harder than one enemy with four times the stats — more actions means more chances to land the bad roll, and less chance for the party to shut down a single threat and be done.
• Party resources and current condition The same encounter is a different encounter depending on whether it's the party's first fight of the day or their fourth, at full resources or running on fumes.
• Signalling danger An unusually dangerous threat should read as unusual before it acts, not only after someone drops. A visibly different monster, an NPC's warning, an obviously wrong scene — give the players something to notice.
• Objectives beyond zero HP A fight with a goal — hold the door for three rounds, protect the prisoner, reach the altar — gives the party ways to succeed or fail that don't run through grinding every enemy down, and gives you room to end it before it becomes a slog either direction.
• Terrain and positioning Cover, elevation, hazards and chokepoints that create choices are doing their job. Terrain whose only function is extra unavoidable damage is just a harder fight wearing scenery.
• Retreat, surrender or alternate success If running away, negotiating or achieving the goal some other way is fictionally plausible, make sure it's actually available — a fight the party can't leave is a fight you'd better be certain they can win.
• Adjustable pressure Reinforcements that are delayed or conditional, morale that can break, an environmental threat that escalates — these let a fight run harder or softer depending on how it's actually going, instead of being fixed the moment initiative is rolled.

Adjust the situation, not the numbers behind the screen

When a fight is clearly going worse than intended, the honest lever is something already established in the fiction: reinforcements that were conditional don't show up, the enemy's morale breaks and they flee, the environmental hazard the party spotted earlier becomes relevant. These were true possibilities before the fight went sideways — using them isn't rescuing the party, it's the encounter behaving the way it was built to.

Secretly changing a monster's HP or fudging a roll mid-fight is a different thing, and it has a cost even when nobody notices: it teaches you nothing about whether the encounter was actually balanced, and if it's discovered, it teaches the table that the dice don't matter. Build the pressure valves in ahead of time so you never need the secret version.