Hydrogen production and storage equipment are often located inside confined enclosures such as containers or buildings. Deflagration vent panels are one of the most common and simplest methods to mitigate the consequences of gas deflagrations in confined systems. There are well-established methods for sizing vent panels, such as the National Fire Protection Association standard NFPA 68, but these are often challenging to apply to hydrogen (a highly reactive gas) if the conservative worst stoichiometric fuel/air concentration is used. This paper presents a new engineering model, Ventdef, based on NFPA 68 (2018) and applied section 7.2.3.2 in NFPA 68 (2018) for the calculations of S-u and P-max using actual gas concentrations. In addition, Ventdef also predicts overpressure external to the deflagration vent and has a new, less conservative method for predicting the size of the external fireball. The model is validated against a series of recent full-scale hydrogen confined volume deflagration tests performed under the EU-funded project "Improving hydrogen safety for energy applications through pre-normative research on vented deflagrations" (HySEA). Since the modifications to NFPA 68 are based on combustion fundamentals, the approach should be equally applicable to other flammable gases.