To replace diesel with hydrogen, the calculation is based on Energy Density. Hydrogen has an energy density of approximately 120 megajoules per kilogram (MJ/kg), which is nearly three times higher than diesel (about 42.5 MJ/kg). This means that 1 kg of hydrogen contains roughly the same amount of energy as 2.8 to 3 kg (or about 3.3 liters) of diesel. However, in 2026, the real-world "replacement ratio" also depends on the efficiency of the engine. A Hydrogen Fuel Cell is significantly more efficient (up to 60%) at converting fuel into motion than a standard internal combustion diesel engine (about 30-35%). Consequently, for many heavy-duty applications, you may only need about 1 kg of hydrogen to replace roughly 1 gallon (3.8 liters) of diesel in terms of actual range or work performed. The biggest challenge in 2026 remains the "volumetric" density; while 1 kg of hydrogen is energetically powerful, it occupies a much larger space than a gallon of diesel, requiring massive high-pressure tanks or cryogenic cooling to be practically used in trucks and ships.