Sizing a turbo is an airflow problem, not a boost problem. Boost is just the pressure that results when a compressor pushes more air into an engine than it would breathe on its own — so the useful question is how many pounds per minute of air your power target needs, and whether a given compressor can move that at the pressure ratio you are asking for.
Roughly, one pound per minute of air supports about 10 hp at a sensible air-fuel ratio. Work out the airflow first, convert your target boost into a pressure ratio (absolute outlet over absolute inlet, so 14.7 psi of boost at sea level is a PR of 2.0), and only then go looking at compressor maps. A turbo picked on airflow will land inside its efficient island; a turbo picked on a forum recommendation often will not.
Boost also changes the engine underneath it. Effective compression ratio combines your static ratio with the pressure ratio, and it is what actually decides whether you detonate. An 9:1 engine at 14.7 psi behaves, thermodynamically, closer to an 18:1 engine — which is why boosted builds drop static compression and lean on intercooling and fuel quality.
Altitude matters more than people expect: a compressor has to work harder for the same boost in thin air, because pressure ratio is measured against ambient. The same turbo at 2,000 m is running a higher PR for the same gauge reading, spooling later and running hotter.
FAQ
How much boost is safe on a stock engine?
It depends on static compression, fuel and knock control far more than on a single psi figure. A low-compression factory turbo engine may take a few extra psi safely; a high-compression naturally aspirated engine can detonate on very little. Calculate effective compression first, then decide.
Why does my turbo make less power in summer?
Hot air is less dense, so each pound of boost carries less oxygen, and a hotter intake charge brings you closer to knock so timing gets pulled. Intercooler efficiency drops too. The same car on the same boost genuinely makes less power on a hot day.