  The excess work performed in a heat-engine process with given finite
operation time \tau is bounded by the thermodynamic length, which measures the
distance during the relaxation along a path in the space of the thermodynamic
state. Unfortunately, the thermodynamic length, as a guidance for the heat
engine optimization, is beyond the experimental measurement. We propose to
measure the thermodynamic length \mathcal{L} through the extrapolation of
finite-time measurements
\mathcal{L}(\tau)=\int_{0}^{\tau}[P_{\mathrm{ex}}(t)]^{1/2}dt via the excess
power P_{\mathrm{ex}}(t). The current proposal allows to measure the
thermodynamic length for a single control parameter without requiring extra
effort to find the optimal control scheme. We illustrate the measurement
strategy via examples of the quantum harmonic oscillator with tuning frequency
and the classical ideal gas with changing volume.
