What is batch size in coffee roasting?

Batch size is the weight of green coffee loaded into the roaster for a single roast, typically measured in grams or kilograms. It's one of the most fundamental variables in roasting because it directly affects how the machine behaves: how quickly the bean mass absorbs heat, how milestones like yellowing and first crack arrive, and how much thermal capacity the roaster needs to maintain momentum through the roast.

Every drum roaster has an optimal batch size range - typically expressed as a percentage of the drum's stated capacity. Roasting at 50-70% of stated capacity is a common guideline for home drum roasters, providing enough bean mass for even agitation without overloading the machine's heating ability. Batches significantly below the minimum may not agitate properly in the drum, leading to uneven development or facing. Batches above the maximum may stall mid-roast as the machine can't deliver enough heat to keep pace.

Keeping batch size consistent between roasts is critical for profile repeatability. A batch 100g lighter than usual will absorb heat faster and hit milestones earlier - so the same profile settings will produce a different result. When logging roast data, recording green weight for every batch is a basic but essential practice that makes it possible to diagnose what changed between a roast that worked and one that didn't.