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Agtron and measuring the degree of roast

The degree of roast is one of the most important factors affecting the flavour of coffee, but for a long time it was assessed subjectively, by eye, comparing the colour of the beans. This led to misunderstandings: one persons medium roast could be anothers dark. The solution became the Agtron scale - an objective, measurable system of assessing the degree of coffee roast, based on the measurement of light reflectance. Thanks to it roasteries can speak a common language of roast level, regardless of subjective judgement. In this post you will get to know what the Agtron scale is, how the measurement of light reflectance works, how it relates to SCA roast levels and why it is so important for quality control. It is a journey into the science of measuring coffee roast. Let us start with why an objective measurement was needed at all.

Why measure the degree of roast

The degree of coffee roast fundamentally affects its flavour: from light, acidic and fruity, through medium, balanced, to dark, bitter and roasty. For a long time roasters assessed the degree of roast subjectively, comparing the colour of the beans with memory or samples, by eye. The problem is that such judgement is inaccurate and inconsistent: what one roaster calls a medium roast, another may consider light or dark. There was no common, objective language. Without precise measurement it is hard to have repeatability, quality control and understanding between roasteries and customers. Therefore a need arose for an objective, measurable system of assessing the degree of roast, independent of human subjectivity. The answer became the Agtron scale. Measuring the degree of roast is the foundation of a modern, professional roastery. It lets you control the process, ensure repeatability and communicate unambiguously. Understanding why measurement is needed is the key to this post. So let us get to know what the Agtron scale is and how it works.

What the Agtron scale is

The Agtron scale is a standardised colour measurement system, designed specifically to assess the degree of roast of coffee beans, widely used in the coffee industry for quality control, product development and sensory evaluation. It uses devices working in near-infrared (NIR) or visible light reflectance technology. The principle is elegant in its simplicity: the Agtron scale directly correlates the surface reflectance of the bean with the degree of roast. The darker the beans, the more light they absorb, which results in less reflectance, and so a lower Agtron value. The Agtron value usually falls in the range from 0 to 150, although in practice a narrower range is used for coffee. It is an objective, repeatable tool: the device measures how much light reflects off the coffee sample and converts it into a number. The Agtron scale replaced subjective eyeballing with a precise, numerical measurement. It is the standard of roast level control in the professional world of coffee. The Agtron number says unambiguously how dark the coffee is roasted.

How light reflectance measurement works

The heart of the Agtron scale is the measurement of light reflectance. The physical principle is simple and intuitive. When coffee is roasted, its beans darken - from light brown at a light roast to almost black at a dark one. A darker surface absorbs more light and reflects less of it, while a lighter one reflects more. The Agtron device illuminates the coffee sample with light (in NIR or visible technology) and measures how much of this light reflects back. The more reflected light, the lighter the coffee and the higher the Agtron number. The less reflectance, the darker the coffee and the lower the number. It is a direct, physical translation of the colour of the bean into a measurable value. The measurement can be done on whole beans or on ground coffee, although the results for the two can differ slightly (the inside of the bean is sometimes lighter than the surface). The measurement of light reflectance is an objective, physical method, free of the subjectivity of the human eye. That is why Agtron is so reliable and repeatable. Light does not lie - it measures colour precisely.

Agtron numbers and roast levels

It is key to understand how Agtron numbers translate into roast levels. The lower the number, the darker the roast. The Specialty Coffee Association (SCA) defines lighter roasts with higher Agtron values (for example 80-100), and darker ones with lower (often 25-45), which facilitates clear and consistent language across the industry. In practice approximate ranges are used: a light roast is roughly 95-75, a medium roast 65-55, and a dark roast 45-25. The SCA developed a classification system made up of 8 coloured discs, establishing the degree of roast of coffee, with numbers from 25 to 95 (in order 25, 35, 45, 55, 65, 75, 85, 95), where 95 is very light and 25 very dark. These discs are physical samples for comparison, and the Agtron device gives a precise number. Thanks to this the degree of roast can be unambiguously determined and communicated. Agtron numbers are the common language of roasteries and tasters. They translate subjective concepts like light or dark into concrete, measurable values. We write more about roast levels themselves in our post on coffee roast levels.

History and origin

The Agtron scale has an interesting history, reaching back to the 1970s. It arose as a result of collaboration between Agtron Inc., an American instrumentation company, and the Specialty Coffee Association of America (SCAA). The reason for its creation was the growing need for an objective, repeatable assessment of the degree of roast - in the times when specialty coffee was developing, subjective eyeballing was ceasing to suffice. The industry needed a tool that would give consistent, comparable results regardless of the person assessing. Agtron, with its light reflectance measurement technology, fit this need perfectly. Over time it became the industry standard, and the SCA system of coloured discs linked the numbers with visual samples. The history of Agtron is the history of the professionalisation of coffee roasting: the transition from a craft based on intuition to a science based on measurement. It is proof of how the development of specialty coffee drove the need for precision. Agtron is the fruit of the pursuit of objectivity and repeatability. It is a tool that changed the way the industry talks about roast.

Limitations and context

Although the Agtron scale is extremely useful, it has its limitations, which are worth knowing. First, it measures only colour, and colour is not everything: two coffees of the same Agtron number can taste different if they were roasted with different profiles (roast curves). The degree of roast is not only the final colour, but also the path the bean took to reach it. Second, measurement on whole beans and on ground coffee can give different results, because the inside of the bean is sometimes lighter than the scorched surface - which is why how you measure matters. Third, Agtron tells about the degree of roast, but not about the quality of the coffee nor about the flavour profile - it is only one dimension of assessment. Therefore Agtron is a supporting tool, not a replacement for tasting and roast profile control. It is a precise measurement of colour, but colour is the beginning, not the end of the story. Understanding the limitations of Agtron lets you use it wisely, as one of the tools, and not an oracle. The Agtron number says a lot, but not everything. It is an important, but not the only element of assessing coffee.

Agtron in a table

Let us set Agtron numbers and roast levels side by side:

The table shows how Agtron numbers translate into roast levels. High values (75-95) are a light roast of an acidic, fruity profile. Medium (55-65) are a balanced roast. Low (25-45) are a dark roast of a bitter, roasty character. The lower the number, the darker the coffee - because a darker bean reflects less light. It is an objective, numerical translation of colour into degree of roast. Thanks to this roasteries and tasters speak one, precise language. Agtron turns a subjective impression into a measurable value.

Why it is worth knowing this

Understanding the Agtron scale enriches the knowledge of coffee and its roasting. First, it shows that behind the seemingly simple concept of the degree of roast stands precise science and measurement. Second, it explains how the industry achieves repeatability and consistency: thanks to an objective scale, and not subjective eyeballing. Third, it helps understand coffee descriptions and roastery communication: the Agtron number is concrete information about the degree of roast. Fourth, it makes you realise that colour is not everything - the same Agtron number can hide different roast profiles, so it is worth looking wider. A conscious coffee lover knows that behind the precision of modern roasting stand tools like Agtron. Next time, seeing an Agtron number in a coffee description, it is worth knowing what it means and what its limitations are. It is knowledge that deepens the understanding of roasting and lets you better match coffee to your own preferences. Agtron is the objective language of the degree of roast.

The key points in a nutshell

The Agtron scale is an objective, standardised system of measuring the degree of coffee roast, based on the measurement of light reflectance (NIR or visible technology). The principle is simple: darker beans absorb more light and reflect less of it, which gives a lower Agtron number - the darker the roast, the lower the number. The values correlate with roast levels: light about 95-75, medium 65-55, dark 45-25. The SCA created a system of 8 coloured discs (from 25 dark to 95 light). The scale arose in the 1970s from the collaboration of Agtron Inc. and the SCAA, replacing subjective eyeballing. Limitation: it measures only colour, not the profile or quality. Want to compare coffees of different roast and record your impressions? Keep tasting notes in the GustoNote app. See also our posts on coffee roast levels and on first and second crack.