A range, and the reason it is a range
Caffeine comes mostly from the grams you used and the beans they came from, so a single number would be a guess wearing a lab coat.
Estimated range per serving
—
This is arithmetic on a published range, not a measurement of your cup. It is not health advice.
The species fractions are literature ranges for caffeine as a share of dry mass. The share that reaches the cup is a CupTune-authored band of 70 to 95 per cent, shown in the working rather than hidden inside the multiply. Reference per-serving figures for common products are published in USDA FoodData Central caffeine values.
The FDA on how much caffeine is too much cites 400 milligrams a day — about two to three 12-fluid-ounce cups of coffee — as an amount not generally associated with negative effects for most healthy adults, while noting wide variation in sensitivity between people. It is a population reference, not your limit. The caffeine guide covers the roast argument and why it is unsettled.
It depends mostly on the dose. A 12 g filter mug estimates at roughly 67 to 160 mg on CupTune's model and an 18 g double espresso at roughly 101 to 239 mg.
Because caffeine per gram varies by species and the share that reaches the cup varies by brew. A single number would be a guess presented as a measurement.
Per millilitre yes, per serving usually not. A large filter mug often uses more grams of coffee than a double espresso does.
It is the FDA's cited amount for most healthy adults, not a personal limit. Sensitivity, body weight, medication and medical conditions all change it, and CupTune is not a source of medical advice.