What Size Backpack Do I Need? UK Guide By Trip Length & Gear

Ask any outdoor shop assistant what size backpack you need and you’ll hear “it depends”. That’s because litre ratings mean nothing without context: a 20L can be perfect for a summer Munro but hopeless on a wet January round; a 65L is brilliant for DofE yet overkill for a city break. Trip length, activity, UK weather, how much safety kit you carry, airline rules and, crucially, how the pack fits your body all change the answer. No wonder people end up under‑packed or lugging half a house.

Here’s the simple way through the noise: start with your trip length and activity, then adjust for season and conditions. Match that to a proven capacity band, check fit on your torso and hips, and pick features suited to Britain’s rain, wind and mixed terrain.

In this UK-focused guide you’ll get a litre-by-trip chart, practical tweaks for summer, shoulder seasons and winter, and clear explanations of each size from 0–15L to 65L+. We’ll also cover fit for different bodies, wet‑weather essentials, smart packing and carry‑on limits, plus copy‑paste kit lists. Let’s nail your size and build your pack.

Step 1. Clarify your trip length and activity

Before you ask “what size backpack do I need?”, pin down the basics. How long you’re out and what you’re doing drives capacity more than anything else. Be honest about pace, season and how self‑sufficient you need to be — a fast summer day on the Downs isn’t the same as a winter Cairngorm.

• Duration: half day, full day, 1–2 nights, 3+ nights
• Activity: run/ride, hillwalk, scramble, mountaineer
• Shelter/cook: hut/B&B vs tent, stove, fuel
• Season/weather: summer, shoulder season, UK winter
• Resupply: water/food on route or carry all

Step 2. Choose capacity by trip length (quick UK-focused chart)

Use this quick chart to translate your trip length into a solid litre range. It’s tuned for UK hill days, wild camps and DofE-style outings. Pick the band that fits your plan now, then tweak for season and weather in the next step.

Step 3. Adjust for season and UK weather

Season is the second big lever in answering “what size backpack do I need?”. For the same trip length, British winter and shoulder-season conditions typically demand more kit than high summer, and wet forecasts change how you pack and access essentials.

• UK winter (hill days or light overnights): Go up a capacity band. You’ll carry extra insulation, full waterproofs, spare gloves/hats and often technical kit like crampons or a small storm shelter (as many brands note for winter use). Look for better padding and a supportive hip belt.
• Shoulder season: Keep waterproofs and a warm mid-layer handy; choose a pack with quick-access pockets so you’re not unpacking in a squall.
• Hot spells: You can often stay in the same size but trade spare layers for more water. Hydration sleeves/ports help.
• Rain is routine: Use a pack liner or dry bags, consider a rain cover, and stash waterproofs high and accessible. Compression straps help cinch smaller summer loads.

Step 4. 0–15L: fast and light for runs, rides and short hikes

If you’re asking “what size backpack do I need” for quick outings or high‑tempo days, 0–15L is the sweet spot. As brands note, these close‑fitting, streamlined packs are built for running, mountain biking and shorter hikes, often with hydration sleeves/ports and stabilising harnesses to stop bounce. Go to the smaller end for pure runs; the larger end suits short scrambles or summer hill loops where you want a shell and a few extras.

• Carries: water/hydration bladder, snacks, phone/map, minimal first aid
• Layers: packable windproof or lightweight waterproof
• Safety: head torch and foil bivvy/emergency shelter
• Bike days: multitool, tube, mini pump stashed securely

Step 5. 15–30L: best all-round daypacks for UK hills and city

If you’re wondering “what size backpack do I need” for most full-day UK outings, 15–30L is the sweet spot. This range comfortably swallows waterproofs, a warm layer, food, 1–2L of water and a small safety kit, with room for maps and gloves. It’s also versatile enough to double for commuting or city breaks, provided you like a tidy, stable carry rather than a bulky pack.

• 15–20L: Short summer hill days, fast walkers, bike commutes, light loads.
• 20–25L: Classic UK day hike setup in mixed weather; best all‑round pick.
• 25–30L: Heavier layers, camera/group kit, shoulder‑season comfort without cramming.
• Look for: breathable back panel, a supportive hip belt, hydration sleeve/side pockets.
• Quick access: big front shove‑it pocket for a wet shell; top/hip pockets for snacks/hat.
• Control: compression straps and pole holders to stabilise uneven loads.
• Wet plan: integrated rain cover or DWR fabric plus a liner/dry bags for essentials.

Step 6. 30–45L: overnighters, winter days and hut-to-hut

If you’re planning a light overnighter, a UK winter hill day, or an Alps/Scottish bothy trip, 30–45L is the reliable answer to “what size backpack do I need?”. As brands highlight, this band covers 1–2 days or winter day use, adding padding and a proper hip belt for comfort, plus space for a sleep system or winter safety kit without feeling unwieldy.

• Overnighters (spring–autumn, 30–40L): sleeping bag, lightweight mat, tarp/tent or bivvy, stove/fuel, food, warm layer, waterproofs.
• UK winter day (35–45L): full waterproofs, insulated layers, spare gloves/hats, food/water, small shelter; space for crampons when needed.
• Hut-to-hut/bothy (30–35L): no tent/mat required, so carry clothes, liner, wash kit, snacks, and foul‑weather gear comfortably.
• Go bigger in this band (40–45L): bulkier sleeping bags, cold/wet forecasts, or when carrying group/technical kit; many 45L packs add streamlined gear attachments for mountaineering.
• What to look for: supportive hip belt, stable back system, compression straps, quick‑access pockets for shell/gloves, hydration sleeve, and durable fabric with lash points for poles/tools.

Step 7. 45–65L: multi-day backpacking and DofE expeditions

For UK multi-day backpacking (typically 3–5 nights) and DofE/Scouts expeditions, 45–65L is the dependable choice. This range gives you the volume to stay self‑sufficient—shelter, sleep system, cooking kit and food—without tipping into expedition bulk. Packs here add proper padding, support and often adjustable back systems to keep heavier loads comfortable over distance. If you’re asking “what size backpack do I need” for a mixed‑weather Pennine Way section or DofE Silver/Gold, start in this band; go to the upper end for poor weather or shared/group kit.

• Carries: tent, sleeping bag/mat, stove/fuel, 3–5 days’ food, waterproofs, insulation, safety kit.
• Fit: supportive hip belt, adjustable back length, load lifters, stable harness.
• Organisation: generous side/stretch pockets, hip belt pockets, multiple access points.
• Weather/control: pack liner or dry bags plus rain cover; compression straps to stabilise part‑loads.

Step 8. 65L+: when you really need the extra space

Only pick 65L+ if your answer to “what size backpack do I need” is driven by true bulk: 7+ day self‑sufficient trips, winter expeditions with big insulation and extra fuel, remote routes without resupply, or carrying group/family kit. These are serious rucksacks with supportive frames—but they’re heavy and, when full, can top 20kg and feel unwieldy.

• Best for: 7+ days, winter expeditions, group gear.
• Plan for: checked luggage, robust hip belt and adjustability.
• Control: use a liner/dry bags, compress carefully, trim non‑essentials.

Step 9. Get the fit right: torso length, hip belt and harness

A well-fitted pack often feels a size lighter. Once you’ve narrowed “what size backpack do I need” by litres, nail the fit so the load rides your hips, not your shoulders. Measure your torso, match the back length, then fine‑tune the hip belt, shoulder straps and load lifters for a close, stable carry.

1. Measure torso: from the C7 vertebra to the iliac crest; choose the matching back length.
2. Set the hip belt: centre it over your hip bones and tighten so most weight transfers to your legs.
3. Snug the shoulder straps: wrap the shoulders without gaps; don’t over‑tighten.
4. Dial load lifters: aim for roughly a 45° angle to pull the top in.
5. Clip sternum strap and side stabilisers, then use compression straps to tidy the load.

• Quick checks: tingling fingers = shoulder straps too tight.
• Big gap at shoulders or lower back = wrong back length.
• Belt slipping onto your belly = retighten or adjust torso length.

Step 10. Women’s fit and different body types

Pack fit is about your body, not the label. Women’s‑specific rucksacks typically have shorter torso options, S‑curve shoulder straps that clear the chest, and hip belts shaped for narrower waists/curvier hips. Men’s/unisex packs often suit longer torsos or broader shoulders. Many brands now offer extended/plus fits and adjustable back systems—try both, loaded, to refine “what size backpack do I need?” into “what shape fits me best”.

• Torso match: Shorter torsos often fit women’s or small/unisex backs better.
• Hip belt wrap: Padded wings should hug the hip bones without gapping or riding up.
• Shoulder comfort: S‑curve straps help chest clearance; broader shoulders may prefer straighter straps.
• Sternum strap range: Should secure without compressing the chest.
• Extended fit options: Look for longer belts/extra strap length and adjustable back lengths to fine‑tune.

Step 11. Must-have features for the UK (rain and organisation)

Whatever size you choose, UK conditions reward simple weatherproofing and fast access. Build a rain plan first, then prioritise pockets that keep waterproofs, spare gloves and water instantly reachable. The right features make a 20–30L daypack effortless in showers and help larger 45–65L loads stay tidy, stable and quick to use in changeable weather.

• Waterproofing strategy: pack liner + small dry bags; add a rain cover.
• Quick access: big front shove‑it pocket for your wet shell.
• Small essentials: lid/hip‑belt pockets for snacks, map/compass, phone, head torch.
• Hydration: bladder sleeve/port or stretchy side pockets for bottles.
• Stability: compression straps to cinch, balance and shrink part‑loads.
• Carry points: trekking pole and tool attachments for hill/winter days.
• Comfort: breathable back, padded hip belt and load lifters for longer carries.

Step 12. Pack smart: weight distribution and access

No matter what size backpack you need, smart packing makes it feel lighter and more stable. Aim to keep the load close to your spine, stop it shifting as you move, and keep weather and safety kit instantly reachable. Use a pack liner plus a few small dry bags to stay organised and dry in UK rain.

• Bottom: soft items (sleeping bag, spare clothes) to cushion the base.
• Middle, tight to the back: dense/heavy kit (food, water, stove, tent body).
• Top/front pockets: waterproofs, warm layer, hat, gloves, head torch.
• Side pockets: water bottles balanced left/right; avoid one‑sided sway.
• Hip belt pockets: snacks, phone, compass; map where you can grab it.
• External carry sparingly: poles/tools only, strapped tight and centred.
• Finish by cinching compression straps to lock the load in place.

Step 13. Travel and carry-on: sizes that fly on UK/EU airlines

If your question is “what size backpack do I need for carry‑on?”, most UK/EU travellers land between 30–40L. Many one‑baggers report 30L sliding under the seat, while 40L travel packs (e.g., Farpoint/Fairview 40) commonly fit overhead. Some brands even suggest 40–50L front‑loading packs for airline limits, but policies vary—always check your carrier’s size and weight rules.

• 30L: Often fits under‑seat; great for minimalist weekends.
• 35–40L: Classic cabin size; clamshell/front‑loading makes packing easy.
• 40–46L: Sometimes fine, but airline‑dependent—verify first.
• Tips: Pack to shape, use compression, avoid dangling gear, weigh your bag before you go.

Step 14. Example kit lists with litre estimates you can copy

Use these copy‑paste kit lists to translate plans into litres. They mirror the size bands above and typical UK conditions. Add or remove items, and bump up a band for winter or group gear. If you’re still asking “what size backpack do I need?”, match your load to the closest list.

• 10–15L | Half‑day run/ride/summer hill: 1L water or bladder, windproof or light shell, snacks, phone/map, small first aid, head torch, foil emergency bivvy.
• 20–25L | Full‑day UK mixed weather: waterproof jacket/trousers, warm mid‑layer, hat/gloves, 1–2L water, lunch, small first aid/repair kit, map/compass, head torch, pack liner/dry bags, poles optional.
• 35–40L | Spring–autumn overnighter: lightweight tent/tarp or bivvy, 2–3 season sleeping bag, mat, stove/fuel/pot, food (1–2 days), spare layer, waterproofs, 1.5–2L water, storm shelter.
• 50–60L | 3–5 nights backpacking/DofE: 3‑season tent, sleeping bag/mat, stove/fuel, 3–5 days’ food, extra insulation, spare socks/underwear, waterproofs, repair/first aid kit, map/compass, liner + dry bags; increase water as route dictates.

Step 15. Common mistakes to avoid and care tips

Even with the right answer to “what size backpack do I need?”, small mistakes can make loads feel heavier and gear wear faster. Avoid these and add simple care so your pack carries better and lasts longer.

• Size for the trip: not “just in case”.
• Fit it properly: hip belt on hips; don’t overtighten shoulders (numb fingers).
• Have a rain plan: liner + dry bags; shell on top, essentials in pockets.
• Control the load: use compression; strap poles/tools tight and centred.
• Simple care: brush off mud, spot‑clean, air‑dry; reproof DWR; check/patch wear.

Conclusion

You’ve now got a simple, repeatable way to answer “what size backpack do I need?”: define trip length and activity, adjust for UK season and weather, choose the right litre band, then dial in fit and features. Use the kit lists to sanity‑check your load and the packing tips to keep it stable, dry and comfortable.

Ready to sort your kit? Explore UK‑ready backpacks and outdoor essentials, plus free delivery over £50 and friendly support at take a hike uk. Pick your size with confidence and get out the door.