What is roast loss in coffee roasting?

Roast loss is the difference between green weight and roasted weight, expressed as a percentage of the original green weight. It represents the mass lost during roasting - primarily water (which evaporates during the drying phase) and CO₂ and other gases produced and released by the chemical reactions during roasting.

A typical roast loss for specialty coffee ranges from around 12% for a very light roast to 20% or more for a dark roast. The darker the roast, the more material is driven off and the higher the loss percentage. Higher starting moisture content in the green coffee also contributes to higher roast loss.

Roast loss is a useful quality and consistency metric. A batch of the same green coffee roasted consistently should produce a similar roast loss percentage each time. If roast loss varies significantly between batches at the same intended profile, it may indicate variation in green coffee moisture content, batch size inconsistency, or a profile that isn't being executed the same way each time. For commercial roasters, roast loss also has direct financial implications - a higher loss means less sellable product from the same green coffee input.