Stress %: An Experimental Irregularity Index
Estimated from a single camera (PPG) reading — an educational estimate, not a medical measurement. State labels (Flow, Focus, Alarm…) are rough, experimental patterns, not emotion detection.
What this number is
Stress on this page is an experimental index. It is simply the Cardiac Stability Index (CSI), the variation of your beat-to-beat intervals relative to their average, rescaled to a percentage. It does not measure stress hormones, emotions or how stressed you feel, and it is not a validated stress score.
How this tool calculates it
Stress = 130 × CSI, limited to 0–100%. A typical camera CSI of about 0.3 gives roughly 40%. Heart rate and breathing rate are not part of this formula. The tool works on a rolling 15-second window of fingertip video (about 30 frames per second), finds each pulse peak, keeps only beat-to-beat (RR) intervals between 400 and 1,500 ms that are within 22% of the median, and smooths the displayed numbers so they change gradually.
How to read it
Higher
Your beat intervals varied more relative to their mean. That can accompany tension, but the most common cause on a phone camera is signal noise: moving your finger, pressing too hard, cold hands or weak light.
Lower
The intervals were more even and the signal was cleaner.
Important limits
This is an experimental heuristic built from a few camera-derived inputs. It is not a validated physiological measure, it is not emotion detection, and it is not a medical test. Treat it as a rough hint about the direction your signal is moving, not as a verdict about how you feel. In real physiology, acute stress usually lowers short-term heart rate variability rather than raising it, so a higher irregularity score is not a reliable stress sign. If stress is affecting your daily life, talking to a professional is more useful than any reading.
What actually helps
Slow breathing with a longer exhale
Several minutes of slow breathing is one of the simplest ways to calm the body.
Better readings
Rest your forearm on a table, cover the camera lightly and hold still for 30 seconds or more before looking at the number.
Daily habits
Sleep, regular exercise, time outdoors and social contact all help with long-term stress.
The ONDA app is a separate product: it focuses on pulse, breathing and, with an Apple Watch, coherence. It does not use the formulas on this page.
The ONDA Principle
Stress here is 130 × CSI: a rough rhythm-irregularity hint, strongly affected by signal quality.
Ready to measure?
The ONDA app works with your iPhone camera, Apple Watch and Apple Health.