The Pandolf load-carriage equation, in your browser. The same math PackCal runs on your watch, whether the weight is a weighted vest, a loaded pack, or a ruck.
Heart rate can't see weight, only strain. Most watches estimate calories from your pulse, which rises under a load but under-counts it, so a loaded walk reads lower than the work actually cost. This calculator uses the U.S. Army's Pandolf equation instead: body weight, pack weight, speed, grade, and terrain in; metabolic cost out. The loaded and unloaded columns update together so you can see exactly what your vest or pack is worth. The equation doesn't care what you call the weight: a weighted vest, a loaded hiking pack, and ruck plates all count the same.
This table is generated live by the same function as the calculator, using your body weight, speed, and grade from above. The pattern worth knowing: the pack bonus scales with the load-to-body-weight ratio. Grade and terrain raise the total burn a lot, but they raise the loaded and unloaded numbers together, so the percentage moves slowly. A heavy load on a lighter body is what pushes the bonus past 25 percent.
The calculator runs the original 1977 Pandolf form, exactly as PackCal does on the watch:
M is metabolic rate in watts, W body weight in kg, L load weight in kg, V speed in m/s, G grade in percent, and η the terrain coefficient. Calories per hour are M × 3600 / 4184. Two guardrails carry over from the watch implementation: speed is capped at 2.5 m/s (5.6 mph) because the equation was validated for walking, and on descents the result is floored at the standing cost (the Santee correction) so a downhill never reads as free.
The full write-up, including the terrain research, the descent correction, and how PackCal blends in heart rate when GPS drops, is on the calorie math page.
PackCal runs this equation live in a single data field you add to a Hike, Walk, or Ruck: speed from GPS, grade from a rolling barometric buffer, terrain from a picker, and an unloaded baseline computed in parallel so the pack bonus shows on the watch and lands in Garmin Connect. When GPS drops, it falls back to your watch's own estimate so the count never stalls. Indoors, an Auto mode scales that estimate by your load. It is $1.99 on the Connect IQ store, with no account and no subscription.