They are not comparable things
This comparison gets made constantly and it does not quite make sense, because the two answer different questions. A size chart is a marketing and grading artefact. A measurement app is an instrument pointed at a body.
Comparing them on accuracy misses that a chart is not trying to measure anything.
What the research says about charts
Tellar Research measured over 1,000 garments across 50 UK brands against their own published charts. Only 64% matched within 1 cm. Average within-brand and within-category variance was 4.2 cm.
68% of garments labelled UK 12 fell outside industry-standard parameters. Accuracy degraded with size, from 71% at UK 8-10 to 54% at UK 20-22.
A chart that disagrees with its own garment more than a third of the time is not a measurement instrument. It is a label.
What the research says about apps
Published smartphone validation reports root-mean-square errors of 2.5 cm at the arm, 4.6 cm at the hip, 5.1 cm at the thigh and 6.1 cm at the waist.
For context, a professional 20-camera scanner measured in the same study gave 2.9 cm at the arm and 9.2 cm at the waist. Professional hardware did not dominate.
The comparison that does matter
If your alternative to an app is guessing your size from a chart, an app wins comfortably. Both have error, but the chart's error is unknown to you and the app's is roughly 6 cm at worst.
That is the real argument: not that measurement is perfect, but that a bounded error beats an unbounded guess.
Where charts genuinely help
Charts are useful for one thing: they encode how a specific brand grades. If a brand runs small, its chart tells you that consistently even if the numbers are absolute nonsense. Over time you calibrate to it.
That calibration is genuinely useful information. A measurement app cannot give it to you, because it knows nothing about the brand.
The correct workflow
- Use measurements to know your body once.
- Use charts to understand how a particular brand grades.
- Use fit reviews to learn how the garment actually behaves.
- Never treat the label alone as a decision.
The honest summary
Apps describe you. Charts describe an intention. Confusing them is the root of most fit frustration, because people use a label as though it were a measurement.
Deeper reading in app accuracy and the chart problem.