Example:
15 mi³ = 1.651676e+13 gal (US)
1 mi³ = 1.101117e+12 gal (US) | 1 gal (US) = 9.081686e-13 mi³
Conversion Steps:
15 × (4.168182e+9 / 0.003785412) = 1.651676e+13