PSI (pounds per square inch) measures how much compressive load concrete can bear before failing. Higher isn't automatically better — it's about matching strength to the job, since higher-PSI mixes cost more and aren't always necessary.
2500–3000 PSI covers non-structural fill and light-duty slabs like sidewalks. 3500 PSI is the residential workhorse — driveways, garage floors, and footings. 4000+ PSI shows up in structural work, columns, and anywhere freeze-thaw cycling is a concern, since denser mixes resist that damage better.
A sidewalk only ever carries foot traffic, so 3000 PSI is plenty. A driveway carries vehicle weight repeatedly over years, which is why 3500 PSI (and sometimes 4000 in cold climates with de-icing salt exposure) is the standard recommendation.
When you order from a ready-mix supplier, you simply request a PSI rating and they batch-design the mix, additives included, to hit it. Hand-mixed ratios (like 1:2:3 cement:sand:gravel) are rule-of-thumb approximations of that same target strength for small DIY batches.
Use the PSI & Mix Calculator to see the typical ratio and use case for any target strength, or the Mix Ratio Calculator if you're hand-mixing a small batch.