Preoperative evaluation of liver functional reserve is important in hepatobiliary surgery. Although the indocyanine green retention rate at 15 minutes (ICG-R15) is the gold standard for this purpose, a new method without technical complexity would be preferable. We assessed the usefulness of the previously established index of convexity (IOC). In total, 159 consecutive patients who underwent both technetium-99m-galactosyl human serum albumin (^99mTc-GSA) scintigraphy and the ICG-R15 were included. Correlation coefficients between indices from ^99mTc-GSA scintigraphy and blood examinations including ICG-R15 were evaluated, and a conversion formula from the IOC to the ICG-R15 was established. The IOC was calculated as [L(15) × 2 − L(3) − L(27)] / [L(27) − L(3)], where L(t) indicates the radiation counts within the whole liver at t minutes after ^99mTc-GSA injection. The IOC showed a significantly stronger correlation with the ICG-R15 (r = −0.532, p < 0.001) than the index of blood clearance (HH15) and the receptor index (LHL15). A formula for estimating ICG-R15 from IOC was "ICG-R15 = −31.0 × IOC + 30.1". In conclusion, the IOC is a better index for evaluating preoperative liver functional reserve than the conventional indices. A formula for estimating ICG-R15 from the IOC will be useful.