Your OFP will give you fuel weight, so only need to divide by density to get volume. No need for a conversion.
Even then, you should get a warning when doing PERF INIT before departure. And even then be aware of your current fuel burn in flight and time to destination - less than an hour spare and you should be asking questions. Divert to nearest suitable is always an option, don't press on to destination with low fuel.
Maybe the most useful point for you to consider is, unlike a car, aircraft fuel should always be thought of in terms of time.