What this chart shows
Stock-to-flow (S2F) is a scarcity ratio borrowed from precious-metals analysis: existing stockpile divided by how much new supply is produced each year. Gold has a high S2F because so little new gold is mined relative to the enormous stockpile already above ground; Bitcoin's S2F rises in steps at every halving as issuance is cut in half.
PlanB's 2019 model regresses log price against log S2F and claims the relationship is predictive. Rather than quote his original published coefficients, this page fits its own regression to the full Coin Metrics history on every load — the same transparency principle as the Power Law page — and plots the model's implied price directly against what Bitcoin actually did.
How to read it
The gauge shows actual price as a multiple of the model's prediction for today. A reading near 1× means price is tracking the fitted line; well below 1× means the model is over-predicting — which is exactly what has happened since the 2021 cycle, when S2F implied prices far above where Bitcoin has actually traded.
Limitations — why this model's own creator called it obsolete
S2F regresses two series that both trend upward across Bitcoin's history (supply's growth rate and price). Regressing any two long-term uptrends against each other tends to produce a high R² regardless of whether one causes the other — the same spurious-correlation critique leveled at the Power Law page, and the core argument critics used to dismiss S2F even before it started missing.
Scarcity (the supply side) says nothing about demand. A model with no demand term cannot explain multi-year stretches where price moves far from what the ratio predicts — which is what has occurred since the 2021 cycle.
PlanB himself acknowledged the original model failed to predict prices after 2021 and proposed a replacement chart (S2FX) with different inputs. This page still shows the original, simpler S2F specification because it's the one that made the model famous — and the one whose failure is most instructive.