The twenty percent rule, and why it exists
Ask any experienced hiker how heavy a backpack should be and you will hear the same answer: about twenty percent of your body weight. The rule is not a law of physics, but it is a useful line drawn from decades of sore shoulders and ruined trips, because above it most people walk slower, tire faster, and risk knee and back injuries, while below it most people can walk all day in reasonable comfort. The calculator on this page applies that rule to your body weight, adds your food and water for the number of days you are out, and tells you whether your planned load sits inside the line or beyond it. It will not make your pack lighter, but it will tell you honestly whether you need to.
The rule needs interpreting, because body weight is not the same as fitness or frame. A fit, muscular walker can often carry twenty-five percent comfortably, while a smaller or less fit walker may struggle at fifteen percent, and anyone with knee problems should aim lower regardless of fitness. Children and teenagers should stay well below twenty percent, because growing joints are more vulnerable, and older walkers often find fifteen percent a kinder target. The calculator's figure is a starting point for judgement, not a permission slip: if twenty percent feels heavy on a training walk, it will feel heavier on day three of a real trip.
Base weight, food, water: the three parts of a pack
A pack's weight divides into three parts, and separating them is how you control it. Base weight is everything except food, water, and fuel: tent, sleeping bag, mat, clothes, stove, and toiletries, and it is the part you can reduce permanently by buying lighter gear or leaving things behind. Food weighs roughly seven hundred grams to a kilogram per person per day for normal hiking rations, and water weighs exactly one kilogram per litre, which makes water the heaviest single item most walkers carry. Fuel is small by comparison but still counts. The calculator uses one point eight kilograms per day as a combined food-and-water allowance, which assumes you resupply water along the route; if you must carry all your water for a dry stretch, the figure will be far higher and the calculator's estimate will be optimistic.
Water deserves special attention because it is the one weight you can manage during the day rather than at the packing stage. Carrying three litres for a stretch with a stream every hour is wasted effort, while carrying one litre for a dry six-hour stretch is dangerous, so the smart approach is to know your water sources in advance and carry only what the next leg needs. The same logic applies to food: heavy tins and glass jars are luxuries on a trail, and repackaging food into light bags, choosing calorie-dense foods like nuts, cheese, and dried meals, and planning resupplies can cut a kilogram or more without eating less. Every kilogram saved from food and water packaging is a kilogram that does not hurt your knees.
Where the weight hides
Most overloaded packs are not overloaded by one big item but by dozens of small ones, each justified on its own: a second book, a heavy knife, a full wash kit, spare clothes for every contingency, and a camera with three lenses. The cure is to lay out everything, weigh it, and ask of each item whether it earns its weight, and whether a lighter version exists. A common surprise is clothing: walkers pack for the worst weather they can imagine rather than the worst weather that is likely, and carry two fleeces when one plus a waterproof would do. Another is the pack itself, because a heavy, feature-laden rucksack can weigh two kilograms before anything goes in it, while a simple light pack of the same volume weighs half that.
Weighing gear is the single most effective habit a walker can adopt, because estimates are always optimistic. A kitchen scale, a luggage scale, or a spreadsheet with each item's weight turns vague guilt into decisions, and most walkers find they can cut two to three kilograms without losing anything they actually use. The biggest wins are usually the tent, sleeping bag, and pack, the so-called big three, where modern light versions can save a kilogram each, and the next wins are in clothing and cooking gear. After that, savings get smaller and more personal, and the right stopping point is when further cuts would reduce safety or comfort more than they help your knees.
Packing technique: weight distribution matters
How weight sits in a pack matters almost as much as how much there is. Heavy items belong close to your back and around shoulder-blade height, which keeps the load stable and close to your centre of gravity, while light, bulky items like a sleeping bag belong at the bottom and frequently needed items near the top or in pockets. A pack loaded with heavy gear at the bottom pulls backwards and strains the shoulders, while a pack loaded with heavy gear at the top feels tippy. Hip belts should carry most of the weight, roughly seventy to eighty percent on a well-fitted pack, which is why a properly adjusted hip belt matters more than any amount of padding on the shoulder straps, and why a pack that does not fit your back length will hurt no matter how light it is.
Fitting a pack is worth doing properly in a shop, with weight in it, rather than by guessing online. Back length, hip belt size, and strap adjustment all interact, and a pack that fits will distribute weight so well that a twenty percent load feels easy, while a pack that does not fit will make a fifteen percent load feel miserable. Adjust the hip belt first, then the shoulder straps, then the load lifters and sternum strap, and walk around the shop for ten minutes with weight in the pack before deciding. Women-specific and short-back versions exist for good reason, and using them is not a compromise but a better fit.
Training, pacing, and the multi-day reality
A pack that feels fine for an hour in the shop may feel very different after six hours on day three, because fatigue accumulates and muscles that were fresh start to complain. Training with the actual pack weight, on hills, for several hours, is the only honest test, and it should be done weeks before the trip, not the night before, so there is time to change gear or plans. Pacing matters too: walking slower on climbs, taking short breaks before you are exhausted, and eating and drinking little and often all reduce the effective burden of a heavy pack, while racing uphill with a heavy load is how knees get injured.
Multi-day trips add a wrinkle that day walks do not: your pack gets lighter as you eat food and use fuel, so the heaviest day is always the first, and the calculator's estimate is best read as a first-day figure. Planning the hardest terrain for later in the trip, when the pack is lighter, is a small kindness to yourself that experienced walkers practise routinely. Conversely, a trip that starts with a long dry stretch, where water must be carried in full, is heaviest at exactly the moment when you are freshest, which is a trade-off worth knowing about in advance rather than discovering on the trail.
When to break the rule
There are legitimate reasons to exceed twenty percent: winter trips need heavier sleeping bags and clothing, technical trips need ropes and safety gear, and some routes have no resupply, forcing more food weight. In those cases the rule is not broken but consciously exceeded, with training, a well-fitted pack, and a slower pace to compensate, and the calculator's warning is a prompt to check those compensations rather than a prohibition. What the rule should never excuse is carrying weight you do not need, because every unnecessary kilogram is paid for in tired legs, sore knees, and a trip that ends earlier than it should.
| Body weight | 20% target | Example base weight | 5 days food and water allowance |
|---|---|---|---|
| 60 kg | 12 kg | 8 kg | 9 kg (total 17 kg — over) |
| 70 kg | 14 kg | 10 kg | 9 kg (total 19 kg — over) |
| 80 kg | 16 kg | 8 kg | 9 kg (total 17 kg — over) |
| 90 kg | 18 kg | 10 kg | 9 kg (total 19 kg — over) |
- Aim for about 20% of body weight, less if you are young, older, or have knee problems.
- Separate base weight from food and water; base weight is where permanent savings live.
- Weigh every item; estimates are always optimistic.
- Put heavy items close to your back at shoulder-blade height.
- Train with real pack weight on hills before the trip.
The lightest pack is the one that still keeps you safe, warm, and fed, and the calculator above tells you how close you are. Pack it, weigh it, walk with it, and adjust before the trail adjusts you.