Example:
15 Btu (th) = 1.581525e+13 nJ
1 Btu (th) = 1.05435e+12 nJ | 1 nJ = 9.484517e-13 Btu (th)
Conversion Steps:
15 × (1054.349999974 / 1e-9) = 1.581525e+13