It's weighed at check in and can be weighed at the gate for passengers connecting from a different airline to LH. And I have seen people sent away to consolidate their carry on weight down. Here's what I've done in some cases (chortle chortle). Note you are also allowed a smaller personal item (e.g. laptop bag).

Yes, Lufthansa does enforce weight limits for hand luggage, and they may weigh your bags at the check-in counter or the boarding gate, especially on fully booked flights or when using smaller regional aircraft. The standard allowance for Economy and Premium Economy passengers is one item of hand luggage weighing no more than 8 kg (17.6 lbs), plus a small personal item. Business and First Class passengers are permitted two items, with each also capped at 8 kg. While ground staff may not weigh every single passenger's bag on every flight, they are particularly observant of large "trolley" cases. If your bag is found to be overweight or over the dimensions (55 x 40 x 23 cm), you may be required to check it into the hold, which can incur an extra fee if you are traveling on an "Economy Light" fare. For 2026 travel, it is highly recommended to use a digital luggage scale before heading to the airport to ensure your carry-on meets the 8 kg threshold, as Lufthansa’s strict adherence to safety and overhead bin capacity means they are less likely to overlook excess weight.