Percent change vs. percentage points: read health numbers clearly

A change from 30% to 27% is a drop of 3 percentage points. Relative to the starting value, it is a 10% decrease. Both statements describe the same arithmetic, but they answer different questions.

Percentage points: subtract the two percentages

For a value already expressed as a percentage, subtract the starting percentage from the ending percentage: 27% − 30% = −3 percentage points. This tells you the size of the change on the percentage scale.

Percent change: compare the change with the starting value

Subtract the starting value from the ending value, divide by the starting value, then multiply by 100: (27 − 30) ÷ 30 × 100 = −10%. This tells you the size of the change relative to where you began. The calculation is undefined when the starting value is zero.

A hypothetical tracking example

Suppose a habit dashboard reports that you completed 3 of 10 planned check-ins in one week and 6 of 10 the next. Completion rose from 30% to 60%. That is an increase of 30 percentage points, or a 100% relative increase. The number of completed check-ins doubled from 3 to 6.

Now change the denominator: 6 of 20 planned check-ins is still 30%. The count doubled, but the completion percentage did not. Always keep the count and its denominator beside the percentage. These numbers are invented examples, not Vea customer results or evidence of a health benefit.

Use this four-part note

1. Measure: What exactly was counted or measured?

2. Start and end: What are the two values, dates and units?

3. Denominator: Percentage of what, and did that base change?

4. Interpretation: What does the arithmetic establish, and what does it leave unanswered?

A precise note might read: “Completed check-ins increased from 3/10 to 6/10 across these two weeks. That is 30 percentage points. This calculation alone does not tell me whether my health changed.”

Calculation is not clinical interpretation

Arithmetic can describe a reported number without establishing whether a device measured it accurately, whether the difference is meaningful or what you should do about it. Keep those questions separate. When comparing an article's headline with its source, use our claim-check worksheet. Bring questions about personal results to your care team using the appointment checklist.

Method: Original worked examples using subtraction and relative-change arithmetic. No patient data, device validation or treatment outcomes are represented.

To keep planned and completed activity separate, use our workout-log template and example.