More information from the unit converter. How many mJ in 1 gigajoule? The answer is 1000000000000. We assume you are converting between millijoule and gigajoule.You can view more details on each measurement unit: mJ or gigajoule The SI derived unit for energy is the joule. 1 joule is equal to 1000 mJ, or 1.0E-9 gigajoule. Note that rounding errors …