How Many Litres Do You Need in a Backpack? A Size Guide
A bag's litre rating measures usable interior volume rather than the outer box, and ASTM F2153 explains why two bags both labelled 20 L hold different amounts.
For a daily commute with a laptop, 15 to 20 litres is the working range. A day without a laptop needs 3 to 5 litres, one or two nights needs 15 to 20 again, a week 35 or more. But the label does less work than you think. A litre rating describes usable interior volume, not the outer box, and no law obliges a brand to measure it the way the brand beside it does. There is a published test method, ASTM F2153, written so that "manufacturers and consumers may have a consistent means to compare pack volumes", and it is voluntary. That is why two bags both labelled 20 L can hold visibly different amounts. Thirty seconds of arithmetic tells you more than the litre figure does.
In this article
- What does the litre number on a bag actually measure?
- Is there an official standard for backpack capacity?
- How do you check a litre claim in thirty seconds?
- How many litres do you need for a daily office commute?
- Why does shape beat litres for a work bag?
- How many litres for a day without a laptop?
- Frequently asked questions
What does the litre number on a bag actually measure?
A litre is 1,000 cubic centimetres, which is why the figure feels authoritative: it sounds like a measurement, not a marketing number. But a bag is not a box. It has padding, seams, a curved back panel and a sleeve that eats depth, so the rated volume is normally smaller than the outer box the bag occupies.
When a rated figure is larger than that box, something is being counted you cannot always use: an expansion panel, a roll top at its limit, or every external pocket. The question is not "how many litres is this bag" but "which litres are these".
Is there an official standard for backpack capacity?
Yes, and almost no buying guide mentions it. ASTM International publishes ASTM F2153, Standard Test Method for Measurement of Backpack Capacity, in force as F2153-07(2018). Its scope is to standardise "an unextended and extended capacity for backpacks and related bags", which include rucksacks, frame packs and duffel bags.
Note the pair of words unextended and extended. One bag has two honest capacities: what it holds packed normally, and what it holds once every expansion is used. A brand quoting the extended figure and a brand quoting the unextended one are both telling the truth, and their labels are not comparable. That is the single biggest reason one 20 L bag swallows another.
ASTM states its purpose as providing "a means whereby manufacturers and consumers may have a consistent means to compare pack volumes". A standard written to create consistency exists because consistency was missing, and this one is voluntary. Compare litres inside one range; compare centimetres across ranges.
How do you check a litre claim in thirty seconds?
Multiply the three external dimensions in centimetres and divide by 1,000. That gives the outer box volume, the ceiling, since no bag holds more than the space it occupies.
A rating between roughly 70 and 85 per cent of that box is what an unextended capacity looks like; the missing portion is walls, padding, taper and sleeve. A rating matching or exceeding the box has counted extended capacity, external pockets, or both. Here is the test across the four hybeks Drift bags.
| Bag | Dimensions | Outer box | Rated | Share of box | A4 lies flat? |
|---|---|---|---|---|---|
| Drift Backpack | 43 x 29 x 17 cm | 21.2 L | 17 L | 80 per cent | Yes, with room over |
| Drift Tote | 45 x 30 x 8 cm | 10.8 L | 8.6 L | 80 per cent | Yes, with room over |
| Drift Sling | 28 x 15 x 10 cm | 4.2 L | 3.4 L | 81 per cent | No, 28 cm is under A4 |
| Drift Kit | 15 x 10 x 2 cm | 0.30 L | 0.25 L | 83 per cent | No, organiser only |
Outer box is the three measured dimensions multiplied together. The A4 column applies the ISO 216 trimmed size of 210 x 297 mm to each bag's largest face.
Four sizes, the same ratio each time: that consistency is the signal. Run the sum on any bag you are considering. If a 25 L pack measures 45 x 30 x 15 cm, its outer box is 20.3 litres, so the claim counts something you reach only when the bag is expanded.
How many litres do you need for a daily office commute?
Between 15 and 20 litres, for almost everybody. A 14 inch laptop and charger occupy roughly 2 to 3 litres once sleeve padding is counted. A one litre bottle is exactly one litre of volume and one kilogram of weight. An umbrella is half a litre, a folded layer for office air conditioning 1 to 2, a notebook, organiser and lunch box another 2 to 3. Total: 7 to 10 litres.
So why not a 10 litre bag? Because a bag packed to its rating is unpleasant to use: zips strain, the shape goes round, and the day something extra appears it has nowhere to go. At 60 to 70 per cent full a bag opens flat and absorbs the unplanned item. Above roughly 22 litres, the extra capacity mostly becomes a bag that hangs lower and catches on people in a crowded metro coach.
17 L, 43 x 29 x 17 cm, 560 g empty. A dedicated compartment for a laptop up to 15 inch, a rear-access compartment, a front organiser panel and padded breathable straps. Water-repellent nylon.
Why does shape beat litres for a work bag?
Because the things you carry to work are flat and rigid, and flat rigid objects care about two dimensions, not volume.
The clearest example is paper. ISO 216:2007, the international standard for trimmed paper sizes, sets A4 at 210 x 297 mm. That figure is fixed everywhere, which makes it the most reliable ruler you own. Any bag whose largest face is under 29.7 cm one way and 21.0 cm the other will not take an A4 file lying flat, however many litres it claims. A 3.4 litre sling at 28 x 15 cm cannot; an 8.6 litre tote at 45 x 30 cm holds one with 15 cm to spare. The decisive difference is 2 cm of height, not the volume.
The same logic governs laptops, where screen inches do not map cleanly to centimetres: a 16 inch machine can be narrower than a 15.6 inch one. We worked through the published footprints of eight laptops people buy in India in what size laptop bag do I need.
Litres also say nothing about how the volume is distributed. Eight litres across a 45 x 30 cm face behaves nothing like eight litres in a deep 30 x 20 x 13 cm block. Same number, opposite bags.
8.6 L, 45 x 30 x 8 cm, 465 g empty. Two separate compartments with a padded suspended sleeve for a laptop up to 16 inch, plus a front slip pocket and dual handles. Water-repellent nylon.
How many litres for a day without a laptop?
Three to five litres, and the gap between that and a 17 litre pack is the most under-used saving in most people's week. Phone, wallet, keys, earbuds, a power bank, a cable and sunglasses come to under two litres; add a half litre bottle and you are at roughly 2.5. A 3.4 litre bag carries that at 175 g empty rather than 560 g.
Below that sits the size people underestimate: a quarter of a litre. A 15 x 10 x 2 cm organiser holds a cable, a card, earbuds and a key tracker, which is exactly what you dig for at a ticket gate. Giving them one address turns a 17 litre search into a 15 x 10 cm one.
3.4 L, 28 x 15 x 10 cm, 175 g empty. A quick-access front zip pocket, a padded back panel and an adjustable strap. Water-repellent nylon.
Frequently asked questions
Is a 17 litre backpack big enough for daily office use?
Yes, for a laptop commute. A typical day of contents comes to 7 to 10 litres, leaving a 17 litre pack under three quarters full. That headroom is what lets it open flat and absorb an unplanned item.
Why do two bags with the same litre rating hold different amounts?
Because the rating is voluntary and the method rarely disclosed. ASTM F2153 recognises two figures, an unextended and an extended capacity, and a brand may quote either. One may also count every external pocket while another counts only the main bay.
How do I calculate a backpack's volume in litres myself?
Multiply height by width by depth in centimetres and divide by 1,000. A 43 x 29 x 17 cm bag gives 21,199 cubic centimetres, or 21.2 litres. That is the outer box; usable interior is typically 70 to 85 per cent of it.
How many litres do I need for a two day trip?
Fifteen to 20 litres covers one or two nights with a laptop and a change of clothes, if you wear your bulkiest layer. Three or four nights moves you to 25 to 35 litres. On flights the weight cap bites before volume, as we showed in what fits in a 7 kg cabin bag.
Will an A4 file fit in my bag?
Check the bag's largest face against 21.0 x 29.7 cm, the ISO 216 trimmed size for A4, allowing a centimetre or two for the interior walls. A 45 x 30 cm tote and a 43 x 29 cm backpack clear it. A 28 x 15 cm sling does not, because 28 cm is under the sheet's 29.7 cm side.
Does a bigger bag mean a heavier commute?
Usually, for two reasons. The empty bag weighs more, 560 g against 175 g across the range above. And capacity gets filled: a compartment that exists acquires contents.
Measure the bag, then trust the number
Drift is four sizes with one measuring method behind all of them: 0.25, 3.4, 8.6 and 17 litres, in water-repellent nylon in Morning Fog and Midnight Transit.
Shop the Drift collection →Free shipping on every order across India, no minimum. 10 days to return it.