Portfolio beta calculator

How much your portfolio moves when the benchmark moves — computed from paired return series, with r² attached so you know how much of the movement the beta actually explains. Runs in your browser; nothing is sent or stored.

Portfolio returns — periodic, in % (commas, spaces, or one per line) Benchmark returns — same periods, same order (e.g. S&P 500)

The formula

β = Cov(R_p, R_m) / Var(R_m) — the covariance of portfolio and benchmark returns over the variance of the benchmark. β = 1 moves with the market; above 1 amplifies it; below 1 damps it; negative moves against it.

Why the r² matters as much as the beta

A beta of 1.2 with r² of 0.9 describes a portfolio that really is a levered market proxy. The same beta with r² of 0.2 describes a portfolio whose moves are mostly not market moves — the beta is statistically real but practically minor, and idiosyncratic drivers (sector events, single-name news, geopolitical exposure) dominate. Reading a beta without its r² is reading half the sentence.

Limits: beta is sample- and frequency-dependent (daily vs monthly returns give different answers), assumes the relationship is linear and stable, and — like correlation — tends to jump in crises.