Development Time Ratio (DTR) in Coffee Roasting: What It Is and How to Use It

Development time ratio is one of the most widely referenced metrics in coffee roasting - and one of the most misunderstood. It is a useful tool for tracking how your roasts progress and for comparing batches, but it is not a number that guarantees a good cup on its own.

This guide explains what DTR is, how to calculate it, what typical ranges look like at different roast levels, what it actually tells you about your coffee, and where its limitations are. (If you are newer to roasting, our guide on roasting coffee beans at home covers the fundamentals.

What is development time ratio?

Development time ratio (DTR) is the percentage of your total roast time that takes place after first crack begins.

The formula is:

DTR = (Development Time ÷ Total Roast Time) × 100

Development time is the time between the start of first crack and the end of the roast. Total roast time is the time from charge (when the beans go in) to the end of the roast.

So if your total roast takes 10 minutes and first crack started at 8 minutes, your development time is 2 minutes and your DTR is 20%.

That is all it is - a ratio that tells you what proportion of the roast happened after crack. It is one piece of information about how the roast progressed.

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Why DTR matters

The development phase - the time after first crack - is where the final flavour balance of roasted coffee is determined. During this phase, caramelisation continues and the relationship between acidity, sweetness, and body settles. The Maillard reaction does most of its work before first crack, but the development phase is where the roast character is finalised.

How long you spend in this phase relative to the total roast affects the balance between origin character and roast character in the cup. A shorter development preserves more acidity and origin-specific flavour. A longer development reduces brightness and brings forward sweetness, body, and roast-driven notes like chocolate and caramel.

DTR gives you a way to quantify and track this. If you roast the same coffee twice and one tastes brighter and the other more developed, checking the DTR of each batch helps you understand what changed and whether to repeat it.

Typical DTR ranges by roast level

These ranges come from Scott Rao's original work and are widely used across the industry as starting points. They are not rules - they are reference points.

Rao's recommended range of 20-25% was based on his experience tasting over 20,000 roasts for which he had bean temperature data. Of the best 20 roasts he had ever tasted from that set, 18 had a DTR of 20-25%. That is not proof that 20-25% is always correct - but it is a strong correlation from a very large dataset.

His recommendation was designed for medium and light-medium roasts on classic drum roasters, dropped between the end of first crack and the beginning of second crack. If you are roasting lighter than that, lower DTRs (15-20%) are common. If you are roasting darker, higher DTRs are expected.

What DTR does not tell you

A good DTR does not guarantee good development. A 22% DTR with a smoothly declining ROR through crack will taste very different from a 22% DTR where the ROR crashed and the roast stalled for most of the development phase. DTR tells you how long you spent in the development phase - it does not tell you whether the roast had enough energy to develop properly during that time.

DTR does not account for what happened before first crack. Two roasts with identical DTRs but very different first phases will taste different. If one had a sluggish drying phase and entered crack without enough internal energy, it may be underdeveloped even at 20% DTR. The development phase cannot fix problems that were created earlier in the roast.

DTR does not account for batch size and machine capacity. Rao notes an important exception: if the ratio of your burner capacity to your batch size is high - meaning you have a lot of heat available relative to the amount of coffee - you can achieve good development at a lower DTR. This partly explains why sample roasters and small-batch machines can produce well-developed coffee at DTRs that would leave a fully loaded production drum underdeveloped.

How to use DTR in practice

DTR is most useful as a tracking and comparison tool, not as a target to aim for in isolation.

Log it for every roast. Record the time first crack started, the time the roast ended, the total roast time, and the DTR. Do this alongside your other data - charge temperature, ROR shape, weight loss, colour. Over time, this builds a reference library that helps you understand your own roasting patterns.

Compare batches of the same coffee. If you roast the same green coffee twice and one cup tastes better, check the DTR alongside your other data. Did the better batch have a different DTR? A different ROR shape? A different charge temperature? DTR is one variable among several, and isolating its effect requires holding other things constant.

Cup at different DTRs. Roast the same coffee with deliberately different development times - everything else the same - and cup them blind. This teaches you what DTR actually does to that specific coffee on your specific machine. The ranges in the table above are starting points, but your palate and your equipment will determine what works best.

Do not chase a number. If your coffee tastes good at 18% DTR, do not extend development just to hit 20% because a guide told you to. Taste is the final judge. DTR is a tool for understanding and replicating what works, not a prescription for what to do.

How to calculate DTR: a quick reference

Example: Charge at 0:00. First crack at 8:00. Roast ends at 10:00. Development time = 2:00. Total roast time = 10:00. DTR = (2 ÷ 10) × 100 = 20%.

If you are using roast logging software - Cropster, Artisan, Kaffelogic Studio, or RoastTime - DTR is usually calculated automatically.

Common DTR mistakes

Treating DTR as the only metric that matters. DTR is one number among many. A smooth, declining ROR, appropriate charge temperature, and consistent batch size all matter as much or more. A well-calculated DTR with a crashed ROR will still produce [baked] coffee.

Applying the same DTR to every coffee. Different origins, densities, and processing methods respond to development differently. A dense Kenyan may need a different DTR from a soft Brazilian, even at the same target roast level. Let cupping guide your adjustments.

Extending development to hit a target number. If you are stretching development just to reach 20% and the roast is losing momentum, you are more likely to produce baked coffee than well-developed coffee. A shorter, well-managed development phase is better than a longer one that stalls.

Ignoring what happened before first crack. DTR only measures post-crack time. If the first phase was too fast (surface developing ahead of the core) or too slow (beans entering crack without enough energy), the development phase cannot compensate. Fix the whole roast, not just the end.

Dropping immediately when first crack ends. First crack ending does not mean development is complete. Some coffees need time after the last pop to finish developing. Equally, other coffees are done before crack fully finishes. Listen, watch, and taste - do not rely on the sound of crack alone.

Wrapping up

DTR is a useful metric. It gives you a number you can track, compare, and use to understand what your development phase is doing to the coffee. But it is one number in a complex process, and it works best when combined with everything else you know about the roast - the ROR shape, the charge temperature, the green coffee, and what the cup tastes like.

Log your DTR. Cup your roasts. Compare the two. Over time, you will develop a sense for what DTR ranges work on your machine with your coffees - and that personal reference is worth more than any guideline.