Understanding Roast Loss

During roasting, coffee beans undergo dramatic physical and chemical changes. Water evaporates, carbon dioxide forms, and the structure of the bean expands. These transformations leave roasted beans lighter than their raw form. The difference in weight, commonly expressed as a percentage, reveals how much mass was lost. Knowing your typical roast loss helps maintain consistency from batch to batch. If one roast loses notably more or less weight, it may taste different or store poorly. Many roasters track this figure in a log along with roast temperature and time, building profiles that yield their preferred flavor.

Roast loss typically ranges from 10 % to 20 % depending on how dark you roast. Lighter roasts retain more moisture, so the beans lose less weight. Dark roasts spend more time in the heat, driving out moisture and initiating pyrolysis—a process that breaks down complex carbohydrates into gases and oils. The result is significant mass reduction and a shiny surface from liberated oils. Our calculator captures this value using the weights you supply.

The Formula in Detail

The calculation is straightforward. Let represent the weight of the green beans and $R$ the weight after roasting. The percentage loss $L$ is:

$$L=\frac{G-R}{G}\times 100$$

In plain language, you subtract the roasted weight from the starting weight, divide by the starting weight, and multiply by one hundred. If you loaded 500 g of green beans and finished with 425 g roasted, the loss is $\frac{75}{500}$ × 100 = 15 %. By tracking this number across multiple batches, you can gauge whether your roasting process is stable.

Why Monitoring Loss Matters

Consistency is a hallmark of great coffee. If you sell beans or simply want your morning cup to taste the same each time, measuring roast loss alongside color and crack timing provides valuable feedback. A sudden drop in weight loss might indicate a lower charge temperature or insufficient development. Conversely, an unexpected increase could mean your roaster ran hotter than intended or your batch size was smaller, causing more heat exposure. Logging these metrics helps you diagnose issues before they influence flavor in the cup.

Typical Loss by Roast Degree

Roast Level	Approx. Loss (%)
Light	11 – 13
Medium	13 – 15
Medium-Dark	15 – 17
Dark	17 – 20

The numbers above vary by bean density and roaster design, but they offer a ballpark for comparison. If your measurements routinely fall outside these ranges, consider adjusting your process or verifying your scale calibration.

Practical Advice

Accurately weighing beans can be tricky due to chaff and residual dust. Use a container that fits easily on your scale and tare it before adding beans. Weigh both the green and roasted beans when they are at room temperature to avoid air currents from hot beans affecting the scale. If possible, weigh immediately after roasting but before degassing; beans shed some weight as carbon dioxide escapes in the hours after roasting, so capturing the loss early ensures consistency.

Interpreting Results

A high roast loss generally correlates with darker color, lower density, and oils appearing on the bean surface. Some roasters target a specific loss to achieve flavor profiles with more developed caramelization. Others intentionally keep loss below 13 % to preserve acidity and origin characteristics. By comparing your weight loss with cupping notes, you can refine roast curves that accentuate the flavors you enjoy.

Beyond Weight: Flavor Development

While weight loss provides a simple numeric target, it does not capture all aspects of roasting. Bean temperature, time in each roast phase, and airflow all interact to create complex reactions. A roast with 15 % loss could taste drastically different depending on how quickly it reached first crack or how aggressively the heat ramped afterward. Use roast loss as one indicator among many. Keeping notes on aroma, color, and sound will give you a more complete picture of each batch.

Conclusion

Whether you roast coffee at home or run a small operation, measuring roast loss is an easy step toward consistent flavor. Enter your starting and ending weights above to see the percentage of mass lost. Over time, you’ll notice patterns that align with roast level and taste. Small changes in loss percentage can highlight equipment quirks or reveal a need to adjust temperature. Combine this metric with careful tasting, and you’ll have a solid foundation for perfecting your personal roasting style.