A grinder that is not calibrated costs a cafe money every single shot. Too coarse and the espresso runs fast, thin and sour. Too fine and it chokes, runs bitter and backs up the queue. This is the process King Carlos Coffee uses to set up a Mazzer Robur S Electronic for a 22 g double in a 25 to 31 second window, and then lock that dose into the grinder's timer so every barista on the bar pulls the same shot. It takes about 25 minutes and roughly 150 g of coffee, machine programming included.
If you run a cafe, pub, club or office and want a roaster that sets this up with you, start at our wholesale coffee page.
What you need
- Digital scales that read to 0.1 g. A bench scale that reads to whole grams is not accurate enough for this.
- Your double portafilter, clean and dry.
- A teaspoon, for removing excess grind.
- A timer or a stopwatch, if your machine does not have a shot timer.
- The coffee you will actually serve. Calibrate on the coffee in the hopper, not on whatever is left in the grinder from yesterday.
Know the grinder first
These are the parts of the Robur S you will touch during this job, straight from the maker's diagram.
The electronic model is the one on the right. From the top: hopper cover, hopper, the coffee grinding adjustment disk under the hopper, the locking slide on the hopper, the push button panel, and the funnel where the portafilter sits. The on-off switch is on the side.
- The menu and selection button. Three horizontal lines, top left of the panel. It opens the dose timer settings. Below it is the back button (the manual calls it cancel), a curved arrow, which exits without saving.
- The dose buttons 1, 2 and 3. The cup icons with one, two and three dots. The hand icon next to them is manual grind. Each dose button stores its own grind time. For this process, 2 is your double, 3 is your 2 second test dose, and 1 is your single, which you set last.
- The coffee grinding adjustment disk. The numbered ring under the hopper, with a handle. It is stepless, so the numbers are a reference, not a setting. There is a notch on the disk and a fixed index arrow on the body: that is how you read where you are.
- The locking slide. The gate on the hopper that stops beans dropping into the burrs. You close it before every purge.
Part 1: find the grind by hand
Do not touch the timer yet. The timer only repeats whatever grind you give it, so get the grind right first.
Step 1. Dose 22 g by hand
Check the portafilter sits level on the fork with the grind falling into the centre of the basket. The stop pin sets how deep it sits; the side tongues set the height. If coffee is piling to one side, fix that before you calibrate anything.
Put the empty portafilter on the scales and tare to 0.0. Grind into it using the manual button until it looks like about 22 g. Weigh it. If it is over, take grind out with the teaspoon until the scale reads 22.0 g. If it is under, add a touch more. Do not guess. The whole process depends on the dose being exactly 22 g every time you test.
Step 2. Tamp, lock in and time the shot
Tamp as you normally would, lock the portafilter into the group and start the shot and the timer together. The clock starts when the pump starts, not at the first drop. Stop when you would normally stop, which at this stage is the usual visual mark of about 60 ml in the glass. Write down the seconds.
You are aiming for a range: 25 to 31 seconds. Do not chase a perfect 28. A range is a target you can hit twenty times in a row on a busy morning. A single number is not.
Step 3. Adjust the grind, not the dose
Read your position off the adjustment disk notch against the fixed adjustment index arrow before you move anything, and write it down. Move the handle with the grinder running.
- Under 25 seconds: the grind is too coarse. Move the adjustment disk towards finer by a small amount, 1 to 2 mm of travel on the ring. Not a full number.
- Over 31 seconds: the grind is too fine. Move it coarser by the same small amount.
Small moves. A conical burr grinder like this one is sensitive, and the most common mistake is swinging the disk too far and overshooting in the other direction.
Step 4. Purge the old grind
Changing the disk changes what the burrs produce, but the chute and the burr chamber are still full of coffee ground at the old setting. Push the locking slide on the hopper closed so no new beans drop in, run the grinder on manual until it runs empty, and throw that away. Expect to waste about 30 g. Open the slide again, then re-dose 22 g by hand exactly as in Step 1.
Step 5. Repeat until it lands
Time the next shot. Adjust, purge, re-dose, test. Keep going until 22 g gives you a shot inside 25 to 31 seconds. Two or three rounds is normal. When you are there, stop touching the disk.
Part 2: lock the dose into the timer
Now the grinder knows the grind. Teach it the dose. The Robur S doses by time: the longer the burrs run, the more coffee comes out. The timer is what gives you consistency across every barista and every shift. It is not a scale, so treat it as very good, not perfect.
Step 6. Set button 3 as your 2 second test dose
This is the screen you are looking for. The dose icon tells you which button you are editing and the number is its time in seconds.
Press and hold the menu button (three lines), then press dose button 3, and release both. The display shows the current time for dose 3. Use the up and down arrows to set it to 2.00 seconds. Press menu to save. (The back button exits without saving.)
Step 7. Weigh the test dose
Tare the empty portafilter on the scales to 0.0. Put it on the fork and press button 3. Weigh what came out. Write it down. In our example the 2 second dose gave 8.85 g.
Step 8. Do the maths
Divide the test time by the test weight to get seconds per gram. Multiply by 22 to get your double dose time.
2 ÷ 8.85 = 0.226 seconds per gram. 0.226 × 22 = 4.97 seconds.
The Robur S adjusts in hundredths of a second, and the maker's own rule of thumb is that 0.10 seconds is about 0.5 g of coffee, so the second decimal place matters.
Step 9. Set button 2 as your double
Hold menu, press dose button 2, release both. Use the arrows to set the time you calculated, 4.97 in our example. Press menu to save.
Step 10. Test it and trim
Tare the empty portafilter, press button 2, weigh it. You want 22.0 g. If it is out, do not guess at the correction, calculate it:
new time = current time × 22 ÷ actual weight
Example: 4.97 seconds gave 21.4 g. 4.97 × 22 ÷ 21.4 = 5.11 seconds. Set 5.11, test again. One correction is usually enough.
Then pull a shot on the timed dose and confirm it is still inside 25 to 31 seconds. Now set button 1 for your single the same way.
Part 3: program the machine for a 44 to 46 g yield
Up to now you have judged the shot by time and by eye, roughly 60 ml in the glass. That is fine for finding the grind. It is not how you run the bar. Volume lies: crema, bubbles and the viscosity of the coffee itself change how much liquid 60 ml actually is. Weight does not lie. So the last step is to make the machine stop the shot at a weight, not a volume.
The target is 22 g in, 44 to 46 g out. The rule of thumb is 1 g of dry coffee gives 2 g of espresso in the cup. Here is how you set it.
- Put your cup on the scales under the group, tare to 0.0, and lock in a timed 22 g dose.
- Put the machine's double button into programming mode. On most commercial machines that means holding the button until it flashes, but the exact method is in your machine's manual, and knowing it is part of running a bar.
- Start the shot. Watch the scales, not the glass. When the scale reads 44 g, press the button to stop and save. The machine now stores that shot as a volume of water or a flow count and will stop there every time.
- Pull two more shots on the saved button and weigh them. You want 44 to 46 g each time, inside 25 to 31 seconds from pump start. If they land, you are set.
Why the machine, not the barista? Because a programmed button takes the stop decision away from whoever is on the machine at 8am. The grinder gives you the same dose, the machine gives you the same yield, and the only variable left is the coffee.
During service: the 0.3 g rule
Timed dosing drifts a little through the day as the beans settle, the burrs warm up and the hopper level changes. Spot check the dose by weight a few times a shift. If it reads between 21.7 g and 22.3 g, that is plus or minus 0.3 g, leave it alone. That is why you calibrated to a range in the first place: if your original shot landed around 28 seconds at 22 g, a 0.3 g swing either way still keeps you inside 25 to 31. Only step in if it drifts further than that, and then correct the time with the Step 10 formula, not the grind.
A word on grind-by-weight grinders
You will hear that a grinder with a built-in scale makes all of this unnecessary. We have tried many of them. They need calibrating too, in our experience more often than a timed grinder, and they have their own calibration standard to follow. A bench that vibrates, a knock from a portafilter or a machine pump running next door is enough to throw the load cell off and give you a wrong reading that looks right. For a busy bar our view is that a well-calibrated timer grinder is the way to go: simple, robust and checkable with a $30 set of scales.
When you must recalibrate
Any change to the grind disk means the dose time is wrong, because a finer grind comes out of the burrs slower than a coarse one. Same seconds, fewer grams. So if you move the disk for any reason, go back to Step 7 and redo the timer. A deep clean of the burr chamber or a change of coffee are the other times to start from Step 1.
A new grinder, or new burrs, is a special case. The burrs take roughly 10 to 20 kg of coffee to settle in, and over that period expect to make small grind and timer adjustments as the cut changes. After that, if the room temperature and humidity are stable, the grinder generally holds its setting with only minor touches. Either way, pull a few test shots every morning before you open. It takes two minutes and it means the first paying customer gets the same coffee as the last one.
Common questions
Why 22 g?
It is the dose King Carlos Coffee recommends for our blends in a standard double basket, and the one all our recipes are built around: 22 g in, 44 g out. It fills the basket properly for a full-size double and splits cleanly for two 8 oz cups.
Why a range of 25 to 31 seconds instead of one number?
Because coffee is a natural product and a cafe is a moving environment. A six second window gives your team something they can hit consistently, and the difference in the cup across that range is small. Chasing one exact second wastes coffee and time.
Why set the grind before the timer?
The timer only controls how long the burrs run. If you set the time first and then change the grind, the dose changes with it and you have to start again.
How much coffee does this use?
Allow around 150 g: two or three 22 g test shots, about 30 g purged each time you move the disk, and three more shots to program and verify the machine.
Why weigh the espresso instead of measuring volume?
Because crema and the thickness of the coffee change how much space the same amount of espresso takes up. Two shots can look identical at 60 ml and weigh 40 g and 46 g. Weight is the only measure that repeats.
How often should I recalibrate?
Redo the timer whenever the grind disk moves. Start from scratch after a deep clean or a change of coffee. A new grinder needs small adjustments for its first 10 to 20 kg while the burrs settle. And pull a few test shots every morning regardless.
What scales do I need?
Anything that reads to 0.1 g and fits a portafilter on the platform. Accuracy matters more than brand.
Want this done on your bar with you? King Carlos Coffee sets up and calibrates grinders for every venue we supply, and we train the staff who will use them. Start with the wholesale coffee page and the $40 venue sample pack.
Grinder diagrams: Mazzer Robur S Electronic user manual.
