PackCal runs across AMOLED and 64-color MIP (memory-in-pixel) displays. The list below is grouped by product family, and it matches what the Connect IQ Store will actually offer you.
Why some older watches aren't here. PackCal is a paid app, and Garmin only sells paid Connect IQ apps on a specific list of products. That list does not include most older watches, so devices such as the Instinct 2, the first-generation Enduro, the Venu Sq, the vívoactive 3 and 4, and the Forerunner 55/245/645/745/945 cannot install PackCal even though the app itself is built for them. That restriction is Garmin's and applies to every paid Connect IQ app, not just this one. Garmin publishes the current list on its App Sales page.
The Connect IQ Store listing has the live compatibility check, install only completes on a supported device.
A data field's cell can be anything from a full single-field screen on a Forerunner 965 down to a quarter of the screen on a four-field Instinct. PackCal draws by hand for the cell it's given: on a spacious single-field screen it shows the calorie hero plus a live rail, the pack-bonus pill, and a footer; in a denser multi-field layout it degrades to a labelled number, then to a bare number when the cell is tiny. The layout is also display-shape-aware (round and rectangular watches each get a layout tuned to their bezel), with a condensed, sun-legible numeral face.
Watches with less memory show the simpler view. On the most memory-limited watches in the list — the fēnix 6 family and the Instinct E and Instinct 3 Solar — a full single-field screen shows the labelled number rather than the calorie hero with the live rail and pack-bonus pill around it. The calorie math is identical on every watch. Only what is displayed alongside it differs.
PackCal is built against Connect IQ API 3.2, so the app itself runs on hardware going back to the fēnix 6 generation. Which watches can actually install it is set by Garmin's paid-app product list rather than by the app, which is why the list above is shorter than the hardware PackCal supports. Newer watches use newer Connect IQ features where they exist, and older ones fall back automatically.