Short answer
Food weight equals the planned energy divided by the energy in each 100 g of food, then adjusted for packaging. With 2,800 kcal per day and food at 450 kcal per 100 g, the calculation is 2,800 ÷ 450 × 100 = 622 g per day. Adding 5% packaging gives 653 g per day, or 1.96 kg for three days. Use your own calorie target, since the model does not estimate energy needs.
Food weight is the planned energy divided by the energy in each gram of food, adjusted for packaging. Two inputs decide the result: the calories you plan to eat each day, and the energy density of the food you pack. For 2,800 kcal per day at 450 kcal per 100 g, the food is 622 g per day before packaging and 653 g per day after 5% packaging.
Energy per gram sets the weight
The FAO chapter on food energy conversion factors describes the Atwater general factor system. It uses one value for each energy-yielding component: 17 kJ per gram (4.0 kcal) for protein, 37 kJ per gram (9.0 kcal) for fat and 17 kJ per gram (4.0 kcal) for carbohydrate. The chapter notes that the system has been widely used, in part because of its simplicity, and it also describes more extensive systems that refine these values.
A gram of fat therefore supplies more than twice the energy of a gram of protein or carbohydrate, since 9.0 ÷ 4.0 = 2.25. Foods rich in fat are denser in energy, and foods rich in water are less dense. Energy density is the number that the food tool uses, so the calculation does not need each component separately.
The calculation, step by step
food per day (g) = planned kcal per day ÷ energy density (kcal per 100 g) × 100 × (1 + packaging)
Energy density is entered per 100 g. Dividing the calorie target by it gives hundreds of grams, and the × 100 turns that back into grams. Packaging is entered as a percentage of the food mass, and the tool adds it to the food itself.
Worked example: three days at 2,800 kcal per day
The plan is one person, 2,800 kcal per day for three days, food at 450 kcal per 100 g, and 5% packaging.
food before packaging = 2,800 ÷ 450 × 100 = 622.2 g per day with packaging = 622.2 × 1.05 = 653.3 g per day three days = 653.3 × 3 = 1,960 g = 1.96 kg
How energy density changes the weight
The same plan, at three energy densities, shows how much the food choice matters. The numbers below use 2,800 kcal per day, three days and 5% packaging.
| Energy density | Food per day, before packaging | Food per day, with packaging | Three days, with packaging |
|---|---|---|---|
| 350 kcal per 100 g | 800 g | 840 g | 2.52 kg |
| 450 kcal per 100 g | 622 g | 653 g | 1.96 kg |
| 500 kcal per 100 g | 560 g | 588 g | 1.76 kg |
Moving from 350 to 500 kcal per 100 g saves 2.52 − 1.76 = 0.76 kg over three days. That is 30% of the heavier total, 0.76 ÷ 2.52 × 100. The saving is for this plan alone, and it depends on whether the denser food is one the walker will actually eat.
Why dried food is lighter per calorie
Energy density depends on water content. Water adds mass and no energy, so a food with more water supplies fewer kilocalories per gram. Dried foods carry more energy per gram for that reason. The density you enter should describe the food as packed, not as it would be after cooking or rehydrating.
Some packs quote energy per ounce. The conversion to the food tool’s unit uses 1 oz = 28.35 g:
100 kcal per oz ÷ 28.35 g = 3.53 kcal per g = 353 kcal per 100 g 125 kcal per oz ÷ 28.35 g = 4.41 kcal per g = 441 kcal per 100 g
Counting meals instead of days
The formula assumes every day has the same target. In practice the last day may need less food if the trip ends at lunch, and the first day may carry a little more. Work out grams per meal instead, then add up the meals you will eat. A hypothetical 700 kcal lunch at 450 kcal per 100 g, with 5% packaging, weighs 700 ÷ 450 × 100 × 1.05 = 163 g.
Snacks count too. A snack is a meal with a smaller target, so the same arithmetic applies, and it is easy to miss when a plan is written per day.
Packing for a group
The food tool takes a people input from 1 to 12 and multiplies the per-person figure by the number of walkers. The per-person grams do not change with group size. What changes is the total, and who carries it. For four walkers on the three-day plan above, the total is 1.96 kg × 4 = 7.84 kg, and each walker carries their own share.
Plan each person’s food separately if their calorie targets differ. Using one target for the whole group would under-count a larger walker and over-count a smaller one. The tool reports the group total, so enter each person’s target as a separate run when the targets differ.
Mixing dense and wet foods
A pack rarely holds one kind of food. Dense dry foods and wetter foods combine into an average density, and the average is what the formula needs. Weight the average by grams, not by items. Take 500 g of food at 500 kcal per 100 g, which holds 2,500 kcal, and 500 g at 350 kcal per 100 g, which holds 1,750 kcal. Together they hold 4,250 kcal in 1,000 g, which is 425 kcal per 100 g.
The simple average of the two densities is also 425, but only because the masses are equal. With unequal masses, divide total kilocalories by total grams. Mixing in a wetter food lowers the density of the whole pack, so the total food weight rises for the same calories.
What the food model leaves out
The food tool converts kilocalories into grams. It does not check whether a food is nutritionally complete, how it tastes or whether it suits a person’s diet. Walking energy is a separate estimate: the Pandolf equation guide covers the energy of carrying a load, and the Loaded miles tool applies it to a walk.
Food also has to be prepared on the trip. The boiling time guide covers the water side of cooking, and base weight explained shows where the consumables sit in the pack total. The ten essentials list is a separate question about what to carry on any trip.
Common questions
How many grams of food per day is 2,800 kcal?
At 450 kcal per 100 g, 2,800 kcal is 622 g before packaging, and 653 g per day with 5% packaging. The result depends on the density entered, so 350 kcal per 100 g gives 840 g per day.
Does calories per ounce convert to kcal per 100 g?
Yes. One ounce is 28.35 g, so 100 kcal per ounce is 353 kcal per 100 g, and 125 kcal per ounce is 441 kcal per 100 g. Enter the per-100-g value in the food tool.
How much food should I bring?
This guide does not set a daily figure. Calorie needs vary with body size and activity, so enter your own target into the food tool, then check the total against the days and meals you plan.
Sources
Published 2 November 2026. Reviewed 2 November 2026. Sources support the figures quoted; they do not endorse Trailwright. Report a correction through the owner’s contact page.