Updated 29 September 2026 · Science Brewing, Halmstad
What size coffee roaster do you need?
Short answer: You don’t choose a size by the batch. You choose it by three numbers: how many kilos of roasted coffee you sell and use a month, how many hours you can spend at the machine, and what your room can handle. For most cafés and restaurants that lands somewhere between a few dozen and a couple of hundred kilos a month. That fits a machine of 1–2 kilos per batch, if you roast three or four days a week. The 15-kilo drum is almost always too big, and it is the more expensive mistake.
The essentials in 30 seconds
- Work backwards: kilos roasted, plus about 20 percent in roast loss, divided by the batch size.
- The batch size the manufacturer gives is a ceiling. Most machines make their best coffee at 50–70 percent of it. Plan on what the machine roasts well, not on what it holds.
- The batch size tells you little. The hours someone has to stand at the machine tell you more.
- Above roughly four hours of roasting per roast day, roasting starts to take over the day.
- A bigger machine usually means a duct, a chimney, three-phase power or gas. That is where projects stall.
- The calculator below gives you your numbers in a minute.
Run the numbers for your business
Fill in your consumption to see the result.
Would you like us to look at your room too? Book a 15-minute review. We go through your volume and your premises, and tell you straight if another machine or buying roasted coffee is right for you.
Work backwards from the bag
Most people start at the wrong end. They look at the machine’s batch size and try to guess whether it is enough. Start with what you know instead.
1. What do you use today? Look at your coffee supplier’s invoices for the last three months. Divide by three. That is your consumption in kilos of roasted coffee a month.
2. What do you want to sell in bags? This is the number most people forget, and it is often what makes the numbers work. Write down a cautious figure, not a hopeful one.
3. Add the roast loss. Coffee loses about 16 percent of its weight when roasted, mostly water. So you need about 20 percent more green coffee than you get out roasted. 100 kilos in the bag takes about 120 kilos in the hopper.
4. Divide by the batch. That gives you batches a month. Multiply by the batch time and you have the machine hours. Use the batch you will actually run, not the one in the brochure. More on that below.
That is four lines of arithmetic. The calculator above does them for you. But the important step is the next one, which no calculator can do: of those hours, how many does someone have to stand there?
Three size classes, and what each one asks of your premises
Coffee roasters are usually grouped by batch size. That is a poor grouping for a business, because the batch is the least interesting number. Here are the same classes, described by what actually decides whether they happen.
| Tabletop roaster | Mid-size | Drum roaster | |
|---|---|---|---|
| Nominal batch | About 1 kg | 1–3 kg | 6–15 kg |
| Power | Usually an ordinary socket | From an ordinary 10 A circuit to three-phase or a dedicated 30 A circuit, depending on the machine | Gas, or an electrical supply of tens of kilowatts |
| Smoke | Often your problem | From built-in cleaning to a duct, depending on the machine | Duct and chimney |
| Changes to the building | Rarely | None to substantial | Almost always |
| Someone at the machine | The whole batch: charge and drop by hand | From the whole batch to just loading, depending on the machine | The whole batch |
| Suits | Someone who wants to learn to roast, sample roast or roasts one or a few dozen kilos a month | Cafés, restaurants, hotels or a small roastery, a few dozen to a few hundred kilos a month | A roastery with wholesale customers |
| Most common mistake | Bought for production and turns into a part-time job | Bought before power and smoke are sorted out | Bought for future volume and turns into a building project |
The mid-size class is where the differences are greatest. Two machines with the same batch can ask completely different things of the room: one an ordinary socket, the other three-phase and a duct through the roof. Compare, for example, a 3-kilo machine with three-phase power and a duct with an American machine that needs a dedicated 30 A circuit. That is why the question of size should always be followed by the question of power and smoke. We have written a separate guide on that: what your premises must handle without a chimney. And gas or electric has a page of its own: electric or gas roaster.
If you are looking for something smaller than the mid-size class, see the comparison tabletop roaster or production machine.
Read the batch size as a ceiling
The batch size the manufacturer gives is a nominal figure: the most the machine is built to hold. It is not the batch that makes the best coffee.
Scott Rao, who wrote one of the standard books on coffee roasting, holds that most machines make their best coffee at 50–70 percent of nominal capacity. The rule of thumb in the trade is not to go above about 80 percent, and several manufacturers recommend stopping there themselves. With a full drum, heat and airflow fall short, and the roast becomes harder to control.
Worked out that way, drum roasters look like this in practice:
| Nominal batch | Where most people roast, 50–80 % |
|---|---|
| 6 kg | 3–4.8 kg |
| 12 kg | 6–9.6 kg |
| 15 kg | 7.5–12 kg |
So a 15-kilo drum is in practice a 7.5–12-kilo machine. Plan on the batch you will actually run, not the one in the brochure.
How little a machine can roast is more disputed. Advisers give anything from 20 to 40 percent of the nominal batch as the floor, and the manufacturers’ own figures vary. Below the floor the roast is hard to control. If you want to try a new coffee in a small batch, that matters. So ask every manufacturer two things: what batch is the machine validated for, and what is the smallest batch that gives an even result?
Ask us the same. Science Brewing Roaster is validated at 1.2 kilos per batch in 15 minutes on 10 A. We still sell it as a 1-kilo roaster and use 1 kilo in every example. The extra 200 grams are margin. The smallest batch run is 500 grams.
Too big is the more expensive mistake
If you buy too small, you notice quickly. You roast more often, or buy another machine. It costs money, but you can put a number on it and you can put it right.
If you buy too big, you notice before the machine is even in place. A 15-kilo drum brings a duct through the building, often gas or three-phase power, a discussion with the landlord and floor space that has to come from something else. The project grows bigger than the business it was meant to serve, and that is how most roastery dreams stall: not with a no, but with a “next year”.
So don’t ask yourself “what will I need in five years?” Ask: what is the smallest machine that handles my volume today, in the room I already have? If you grow, there are two ways forward: more roast days, or another machine. Both are faster than a building project.
How often you roast decides the size
The same monthly volume can call for completely different machines, depending on how often you roast.
If you roast one day a month, you need a lot of capacity on that day. If you roast two days a week, a smaller machine is enough, and the coffee is fresher when it is sold. Most roasters reckon beans are at their best from a few days to a few weeks after roasting. Roast more often, in smaller batches, and you sell more of your coffee inside that window.
The rule of thumb the calculator uses: above roughly four hours of roasting per roast day, roasting starts to take over the day. At that point adding a roast day is cheaper than buying a bigger machine.
For a food truck or a seasonal café the same applies the other way round: roast more often in high season, less often the rest of the year, in the same machine.
100 kilos a month, worked out from a design
The installation page describes what the machine can do and what the room must have. This page works out what that means for your week.
This is what 100 kilos of roasted coffee a month looks like in the machine we build, Science Brewing Roaster. The numbers come from how the machine works, not from how any customer spends their time. Your day decides the rest.
| Kilos roasted a month | 100 |
|---|---|
| Kilos green, with about 16 % roast loss | about 120 |
| Batches of 1 kg, 15 minutes | about 120 |
| Machine time | about 30 hours |
| Loading and starting, 10 + 10 seconds a batch | about 40 minutes |
| Emptying the chaff, every 5–10 kg green | 12–24 times |
| Changing the water, roughly every tenth roast | about 12 times |
| Steps that need precision | None. The machine finishes, cools and empties the batch itself |
The machine holds three batches at different stages at once: one in the hopper, one in the drum, one in the cooler. Someone comes by roughly every fifteen minutes, but only to tip a one-kilo bag into the hopper. Starting is done in the app, remotely too, for example when electricity is cheap. Unattended operation requires your insurer’s approval, whatever the machine.
Two numbers, and what they mean. We use 1 kilo per batch and 4 kg of green coffee an hour for planning, on 10 A. Those are cautious numbers. The machine does 1.2 kg in 15 minutes, so 4.8 kg an hour, which comes to about 115 kg a day as a theoretical ceiling. That ceiling is the number to compare with other manufacturers’ nominal figures, because they give their ceilings. Plan on the lower one.
What the room needs: a dedicated 10 A circuit on ordinary 230 V, 70 × 30 cm of bench space with about 30 cm clear on the left (80 cm for servicing), and wifi. No chimney, because the smoke is cleaned inside the machine without an afterburner. So a socket on its own circuit is enough. All dimensions and requirements are in installation and technical requirements.
The smallest batch run is 500 grams of green coffee. That means the same machine works when volume is low and when you want to try a new coffee. There are ready-made roast profiles for coffee from our partner roasteries.
When you have outgrown a machine
If you roast more than about four hours per roast day and can’t add more roast days, you have outgrown a machine in this class. Then there are two ways forward:
- Another machine. Same room, one more circuit in the fuse box, double the capacity. If one breaks down, the other keeps roasting. What each machine asks of the power supply is in the technical requirements.
- A drum roaster with a duct. The right way when you sell wholesale volumes and have a room that can be rebuilt. Bear in mind that the choice between gas and electric comes with it.
Which one is right depends on the room, not on the machine.
Compare with a named machine
- Science Brewing Roaster or Bellwether Shop Roaster
- Science Brewing Roaster or Roest P3000
- Science Brewing Roaster or Aillio Bullet
- Sample roasters for roasteries
- Electric or gas roaster
Read more for your business
Two ways forward
1. Book a review, 15 minutes
Bring the number from the calculator. We go through your volume, how often you want to roast and what your room can handle. You get a straight answer, even if it is a different machine or to keep buying roasted coffee.
2. Check your room first
Take the premises test and reply to the email with a photo of your fuse box. We will tell you what your circuit can handle. It costs nothing and applies whichever machine you end up with.
This is not for you if
- You roast for yourself or for your home
- Your brewing equipment is on loan from your coffee supplier against volume
- You pay less than about €14 per kilo today
- You need wholesale volumes from a single machine
- You have no stable connection where the machine will stand
Frequently asked questions
What size coffee roaster does a café need?
Work backwards from consumption and what you want to sell in bags. For most cafés that comes to between a few dozen and a couple of hundred kilos of roasted coffee a month, which fits a machine of 1–2 kilos per batch if you roast three or four days a week. What decides it is how many hours someone has to stand at the machine and what the premises can handle.
What does nominal batch size mean?
It is the most the machine is built to hold. Many machines make their best coffee at 50–70 percent of it, and the rule of thumb is not to go above about 80 percent. So a 15-kilo drum is in practice a 7.5–12-kilo machine. Science Brewing Roaster is sold as a 1-kilo roaster, is validated at 1.2 and has 500 grams as its smallest batch run.
How much green coffee does a kilo of roasted coffee take?
About 1.2 kilos. Coffee loses about 16 percent of its weight in roasting, mostly water. Darker roasts lose a little more.
How many kilos can you roast an hour?
It depends on batch size and batch time. Ask every manufacturer two things: what the machine can do as a ceiling, and what you can plan for. Science Brewing Roaster does 4.8 kg of green coffee an hour as a ceiling on 10 A. We recommend planning for 4 kg.
Is it better to buy bigger so you can grow?
Rarely. A bigger machine usually needs a duct, gas or three-phase power, and that is where projects stall. Adding a roast day or another machine is faster than a building project.
Can you roast small batches in a big machine?
Up to a point. Advisers give between 20 and 40 percent of the nominal batch as the minimum, and below that the roast becomes hard to control. Ask the manufacturer for the smallest batch that gives an even result, not just the smallest batch the machine accepts. Science Brewing Roaster’s smallest batch run is 500 grams of green coffee.
How often do you need to roast?
Often enough that the coffee is sold while it is fresh. Most roasters reckon beans are at their best from a few days to a few weeks after roasting. One or two roast days a week is common for a café.
How long does it take to roast 100 kilos a month?
Machine time is about 30 hours in a machine that does 4 kg of green coffee an hour. How much of that time is yours depends on the machine. If it needs charge and drop by hand, it is all 30. If it only needs loading and starting, it is around 40 minutes.
Do you need an environmental permit to roast coffee?
It depends on the country and the volume. In Sweden, for example, notification is only required above 100 tonnes a year, whatever the machine. Ask your local authority.
Who we are
Science Brewing exists to do away with bad coffee. We sell green coffee, build the Science Brewing Roaster coffee roaster and a grinder for whole beans. We roast our own production on the machine described on this page, in Halmstad, Sweden.