Your fitness watch says the day’s hike burned 350 calories an hour. Your body, three days into a backpacking trip, says otherwise — by dinner you’re eyeing the peanut butter jar with intent. The watch isn’t broken. It’s just answering the wrong question: it estimated walking, and you weren’t walking. You were carrying.
The formula built for carrying
In 1977, researchers at the US Army Research Institute of Environmental Medicine — Pandolf, Givoni, and Goldman — published an equation for predicting the energy cost of soldiers marching under load. Nearly fifty years later it remains the standard model, because it accounts for exactly the things backpacking adds to a walk:
- Load weight — and not linearly: cost scales with the square of the load-to-body-weight ratio. The second 10 kg hurts more than the first.
- Grade — every percent of climb multiplies through your total moving weight, body plus pack.
- Surface — measured terrain factors: dirt costs 10% more than pavement, sand 80% more, soft snow more than double.
Real numbers
For a 75 kg (165 lb) hiker at a typical 4 km/h (2.5 mph) trail pace:
| Scenario | Burn rate |
|---|---|
| Level pavement, no pack | ≈ 310 kcal/h |
| Dirt trail, 12 kg pack, 5% grade | ≈ 470 kcal/h |
| Dirt trail, 20 kg pack, 10% grade | ≈ 700 kcal/h |
Over a six-hour hiking day, that top scenario clears 4,000 kcal of walking alone — before your body’s baseline needs. This is why “I’ll just bring some granola bars” fails by day two, and why thru-hikers talk about food with the seriousness other athletes reserve for training plans.
Turning calories into food weight
The practical question isn’t calories — it’s grams in the food bag. Typical dried trail food averages about 4,000 kcal per kilogram (110 kcal/oz). The arithmetic that follows:
- 6-hour day at 470 kcal/h → ≈ 2,800 kcal of hiking
- Plus resting metabolism for the other 18 hours → ≈ 1,300 kcal
- ≈ 4,100 kcal total → just over 1 kg (2.2 lb) of food for the day
Want the same math with your body weight, your pack, your grade? The backpacking calorie calculator runs the full Pandolf equation and converts the answer to food weight — and if you draw your route in the route planner, the grade and duration arrive pre-filled.
Where the model is honest about its limits
Pandolf was validated for level and uphill walking; steep descents are messier (your muscles brake rather than lift), so tools built on it — including ours — count downhill as level ground. Over a normal up-and-down route the error mostly washes out; on an all-descent day, you’ll burn somewhat more than predicted. The formula also doesn’t know about cold (shivering costs energy) or altitude — pad your food plan on winter and high-elevation trips.
Feed the machine properly and the machine carries you farther. That’s the whole trade.