To scale up a porter recipe to 5 gallons, start by determining the percentage of each ingredient in the original recipe. For instance, if you know how much of each malt and hops was used in a smaller batch, you can calculate the proportions by weight. Next, use a brewing calculator or software like BeerSmith to determine the total amount of each ingredient required for a 5-gallon batch. Remember to adjust water quantities, too, based on factors like boil time and evaporation rates.