Two questions arrive at our factory almost every week, usually in the same message: “Can I roast a small batch in the bigger machine?” and “How many kilos per hour will it actually do?” Capacity is the single most common subject in our enquiries — we counted it in more than three hundred buyer conversations — and it is also the one where the brochure number misleads people most.
A roaster’s rated capacity is a maximum, not an operating setting. The number that decides your daily output, your profile consistency and whether your coffee tastes the same on Friday as it did on Monday is your batch size: how much green you actually charge, batch after batch. This guide sets out the usable batch window for a drum roaster, what breaks when you go under or over it, and the between-batch discipline that keeps a profile repeatable. We have been building drum roasters since 1990, so the numbers below come from machines we assemble and test, not from a spec sheet.
1. Rated Capacity Is a Maximum, Not a Setting
When we call a machine a 6 kg roaster, we mean the drum will accept 6 kg of green coffee and the burner and fan are sized to bring that load to first crack in a sensible time. It does not mean 6 kg is the batch you should run every day. The other figures on that page deserve the same scepticism — our guide to reading a coffee roaster spec sheet works through burner kW, airflow and electrical load line by line.
Three things move together inside the drum: the mass of beans, the heat the burner delivers, and the airflow that carries heat into the bean pile and chaff out of it. All three are matched at the design point. Charge far below it and the bean mass is too small to buffer the heat coming off the drum wall. Charge above it and the airflow no longer reaches the middle of the pile. Both directions cost you the same thing — control.
Two more consequences of the same fact are worth stating plainly, because they change purchase decisions:
- Roasted weight is not green weight. Coffee loses roughly 14–18% of its mass in the roast, more for darker profiles. A 6 kg green charge yields about 5 kg roasted. Every capacity calculation below is in green kilograms.
- Buying one size up is not a free option. A 12 kg machine will not politely roast 2 kg batches for you while you grow. If most of your production will be small batches for a while, the honest answer is often a smaller machine now, or two machines later — see our roaster capacity selection guide.
2. The Usable Batch Window
As a working rule for a conventional drum roaster: the usable window is about 40% to 100% of rated capacity, and the sweet spot is 60–80%. Inside that band the bean-mass probe is properly buried, the drum has enough thermal load to behave predictably, and you have burner headroom in both directions.
The table below is what we tell buyers to plan around. Batch cycle assumes a 12–15 minute roast; on machines with a separate cooling tray and a powered cooling fan you can charge the next batch while the previous one cools, which is what makes the batches-per-hour figures achievable.
| Rated capacity | Practical minimum | Sweet spot (60–80%) | Batches / hour | Green kg / hour at sweet spot |
|---|---|---|---|---|
| 500 g sample | 150 g | 300–400 g | 4–6 | 1.2–2.4 kg |
| 1 kg | 400 g | 600–800 g | 3–4 | 1.8–3.2 kg |
| 2 kg | 800 g | 1.2–1.6 kg | 3–4 | 3.6–6.4 kg |
| 3 kg | 1.2 kg | 1.8–2.4 kg | 3–4 | 5.4–9.6 kg |
| 5 kg | 2 kg | 3–4 kg | 3–4 | 9–16 kg |
| 6 kg | 2.5 kg | 3.6–4.8 kg | 3–4 | 11–19 kg |
| 12 kg | 5 kg | 7–10 kg | 3–4 | 21–40 kg |
| 20 kg | 8 kg | 12–16 kg | 3–4 | 36–64 kg |
| 30 kg | 12 kg | 18–24 kg | 3 | 54–72 kg |
| 60 kg | 25 kg | 36–48 kg | 3 | 108–144 kg |
Figures are planning guidance for our SD and YS drum machines under normal conditions. Dense high-grown arabica, very dark profiles and cold roasting rooms all push cycle times up. Ask us for the batch window of the specific model you are considering — it is on the commissioning sheet we ship with every machine.
Two practical readings of this table. First, if you need 60 kg of green through the machine in a working day, a 6 kg roaster at four batches an hour gets there in about four hours — comfortable. Second, if you need 300 kg a day, no amount of batch-size cleverness will get a 12 kg machine there; you need industrial capacity. Sizing against your weekly volume, not your best day, is covered in the roaster range overview.
3. What Actually Breaks When You Underload
Small batches in a big drum are the more common mistake, because it feels harmless. It is not, and the reasons are mechanical rather than mystical.
- The bean probe stops telling the truth. The bean-mass thermocouple is positioned to sit inside the bean pile at normal load. At 20–25% load the pile no longer covers it, so the probe reads a mix of bean surface and hot air. Your curve looks fine on screen and has no relationship to what the coffee is doing. This is the single biggest cause of “the machine is wrong” support tickets that turn out not to be.
- Rate of rise becomes twitchy. Thermal mass is what smooths a roast. A thin bean bed responds to every burner change instantly and overshoots, which makes a controlled approach to first crack much harder.
- Scorching and tipping. Fewer beans against the same hot drum surface means each bean spends more time in contact with metal. On a preheated drum you get scorch marks on the flat face of the bean and, at the extreme, tipped ends.
- Chaff behaviour changes. Airflow tuned for a full load can pull light chaff aggressively from a small charge, and in the other direction a nearly empty drum can let chaff settle where you do not want it. Chaff management matters for more than tidiness — see our roastery fire safety guide.
If you genuinely need small batches — cupping lots, single-farm micro lots, product development — that is what a sample roaster is for. A 500 g machine alongside a production roaster is a far better answer than running 1 kg batches in a 12 kg drum, and it is the configuration most serious roasteries end up with.
4. And When You Overload
Charging past rated capacity looks like free productivity for the first week. What follows:
- The drying phase stretches. More water to drive off with the same burner means a longer, flatter start, which is the classic route to baked, papery cups.
- Airflow cannot reach the core of the pile. You get a wider spread between the beans at the drum wall and the beans in the middle — visible as uneven colour in the cooling tray.
- The roast stalls late. Approaching first crack the burner is already near maximum, so there is no headroom left to manage the exothermic phase. Crashes and flicks show up on the rate-of-rise curve.
- Mechanical load rises. Drum motor, gearbox and bearings all work harder. Bearings are the component buyers ask us about most after the burner, and chronic overloading is the fastest way to shorten their life. Our maintenance schedule covers what to check and when.
5. Between-Batch Discipline: Where Repeatability Is Won
Most consistency problems are not batch-size problems at all. They are the problem of a machine that gets hotter as the morning goes on. A roaster that has run five batches back to back has a hot drum, hot ducting and a hot cyclone; identical settings will now roast faster than they did at 8 a.m.
The protocol below is what we teach during commissioning, and it is worth writing on the wall next to the machine:
- Warm up properly, then run a ballast batch. Bring the machine to charge temperature and hold it — 15 to 20 minutes on a small machine, 30 or more on a 30 kg. Then roast one cheap batch and discard or downgrade it. The first roast of the day on a cold machine is never the same as the fifth, and no software setting fixes that.
- Charge at the same temperature, every time. Fixed charge temperature is the foundation of a repeatable curve. Wait for it rather than charging early because the queue is building.
- Watch the turning point. The turning point — the moment the bean temperature stops falling and starts to climb, typically around 1:00 to 1:45 into the roast — is your best early diagnostic. Same coffee, same batch size, same charge temperature should give you the same turning point within a few seconds. If it drifts earlier across the morning, your machine is accumulating heat and you should lower charge temperature or extend the between-batch rest.
- Give the machine a fixed rest between batches. Pick a number — two minutes, three minutes — and keep it. Variable rest is variable heat.
- Do not change two things at once. If a profile needs correcting, move charge temperature or gas or batch size — one of them, then re-roast.
If you are still in the first fortnight with a new machine, the break-in behaviour is different again and worth reading first: commissioning and drum seasoning.
6. Locking the Profile: PLC and Artisan
Once you have a batch size and a between-batch routine that work, the job becomes recording them so a second operator can reproduce them. Two layers do this:
Machine-side automation. Our YS series runs a Siemens PLC that can store profiles and drive burner and airflow set points automatically, so the same recipe executes the same way regardless of who pressed start. What a PLC does and does not change is set out in Siemens PLC roaster automation, and the machines themselves are on the YS series page.
Logging software. Artisan is the standard open-source logger and works over USB or Bluetooth on our machines. Log every batch with its charge weight in the field — a curve without a batch weight attached is close to useless six months later, when you are trying to work out why a coffee changed. Setup and troubleshooting are in how to connect your roaster to Artisan.
7. Quick Troubleshooting Table
| What you see | Most likely cause | What to change first |
|---|---|---|
| Batches get faster through the morning | Heat accumulating in drum and ducting | Lower charge temperature slightly, or fix the between-batch rest |
| Curve looks erratic, coffee tastes fine | Batch too small — probe not buried | Raise batch to at least 40% of rated capacity |
| Uneven colour in the cooling tray | Overloaded drum, or airflow too low | Drop batch size 15%, then revisit airflow |
| Flat, baked, papery cup | Drying phase too long for the load | Raise charge temperature or reduce batch size |
| Scorched flat faces on the beans | Charge temperature too high for a small batch | Lower charge temperature and increase batch size |
| First batch of the day always different | No ballast roast; drum still cold | Add a warm-up hold plus one sacrificial batch |
Batch size also drives your running costs — gas per kilogram is meaningfully better at 70% load than at 25%, which we work through with real numbers in the roaster running costs guide.
Frequently Asked Questions
What is the minimum batch size for a commercial coffee roaster?
As a rule, about 40% of rated capacity — 2.5 kg in a 6 kg machine, 5 kg in a 12 kg machine. Below that the bean probe is no longer buried in the bean mass, the readings stop being trustworthy and scorching risk goes up. Some machines will tolerate 30% with a lower charge temperature; ask us for the tested window on the specific model.
Can I roast 1 kg in a 6 kg roaster?
You can put it in the drum, but you should not build production on it. One kilogram is under 20% of capacity, so the probe reads air rather than beans and the roast becomes very hard to repeat. If small batches are a regular need, pair a production roaster with a 500 g sample machine instead.
How many batches per hour can a commercial roaster do?
Three to four for a 12–15 minute profile, on machines that cool the previous batch while the next one roasts. Larger machines sit at the lower end because charging, cooling and discharging all take longer. Continuous back-to-back running for a full shift also needs the between-batch heat management described above.
Why is my first roast of the day always different?
Because the drum, ducting and cyclone are cold and are absorbing heat that later batches do not lose. Hold the machine at charge temperature for 15–30 minutes depending on size, then run one ballast batch you are willing to downgrade. Almost every consistent roastery does this.
Does batch size change how much gas I use per kilogram?
Yes, substantially. A large part of the burner’s work is heating the machine itself, and that cost is fixed whether you charge 2 kg or 5 kg. Roasting near the sweet spot spreads it across more coffee, which is why underloading is expensive as well as inconsistent.
Batch size sets the boundaries of the roast; the profile fills them in. See coffee roast profile development for the phase ratios, RoR crash and flick behaviour, and how to build your first three profiles inside that window.
Get a Quote
Tell us the batch size you actually want to run and the kilograms per week you need, and we will size the machine to your production rather than to a headline number. Send it through the enquiry form on our contact page and we will come back with a factory-direct configuration, the tested batch window for that model, cycle times for your profile length, and utility requirements for your electrician and gas installer. We do not publish price lists because every machine is built to a site, but you will get a real figure rather than a range.
If it is easier, message the factory directly on WhatsApp — include your weekly volume and we can usually narrow it to two models in a single exchange.
WhatsApp the Factory: +86 184 0771 4607
Last updated: August 27, 2026
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