PPI = √(Width² + Height²) ÷ Diagonal. Higher PPI means sharper text and images.
Retina threshold: ~220 PPI at arm's length. Most phones exceed 400 PPI.
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A common mistake is misidentifying which formula applies to the specific version of the problem. Many everyday calculations have superficially similar variants that use different formulas — always check which scenario matches your situation.
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109 PPI (24" 1080p) is standard. 163 PPI (27" 4K) is excellent. Above 200 PPI is retina-quality.