Measure backpack capacity by finding the volume of each usable compartment, converting those volumes to liters, and adding them together. For a practical home estimate, fill compartments with containers of known volume. For a rough calculation, measure compartment dimensions and calculate volume. Backpack capacity is not always measured the same way by every brand, so treat your result as an estimate and compare packs using the same method.
What backpack capacity in liters means
A liter measures volume: one liter equals 1,000 cubic centimeters, or about 61 cubic inches. A backpack marked 40 liters is intended to hold roughly that much volume, but the label may reflect the main compartment alone or include certain pockets. There is no guarantee that every maker measures every model identically. External lash points, straps, and gear attached outside the bag generally do not count as enclosed storage, but check the maker’s stated method when available.
Capacity describes space, not carrying comfort or load limit. It does not tell you how much weight the pack can safely or comfortably carry, whether your gear will fit through its opening, or how well its shape suits bulky items. A measured number helps compare storage volume, but fit, design, and intended use matter too.
Method 1: Measure compartments with known-volume containers
This method usually gives the most useful household estimate because it accounts for irregular shapes better than a simple length-times-width-times-height calculation. Use clean, dry containers or bags with clearly stated capacity. Avoid loose materials that compress or spill, such as rice or packing beads.
- Empty the pack. Open every compartment you plan to count. Leave internal dividers in their normal position.
- Choose what to include. Decide whether you are measuring only the main compartment or all usable pockets. Record the choice so you can compare packs fairly.
- Fill one compartment at a time. Add known-volume containers without forcing them, stretching fabric, or pushing against zippers. Stop when the space is reasonably full but closures still operate normally.
- Record the volume. Add the container capacities used in that compartment. Repeat for each pocket you chose to include.
- Add compartment totals. Sum the liters for the main compartment and counted pockets. Keep the main-compartment figure separate from the all-compartments estimate.
For example, if the main compartment holds containers totaling 28 liters and counted pockets hold another 4 liters, your estimate is about 32 liters under that method. Do not count a pocket twice because it opens into the main compartment. If a sleeve shares the same physical space as the main compartment, count the space once.
Method 2: Calculate volume from dimensions
Dimension-based math works best for boxy compartments. Measure the usable interior length, width, and depth, not the outside dimensions of the whole pack. Convert all measurements to the same unit before calculating. For centimeters, multiply length × width × depth, then divide by 1,000 to get liters. For inches, multiply the dimensions in inches, then divide by 61.024 to get liters.
Example: an approximately rectangular compartment measuring 50 cm × 30 cm × 20 cm has a calculated volume of 30,000 cubic centimeters, or 30 liters. This is only a geometric estimate. A backpack tapers, curves, contains seams and padding, and may not hold a rectangular block of that size. Measuring outside dimensions usually overstates usable capacity.
For a non-rectangular compartment, split its shape into simpler sections, estimate each section, and add the results. This remains approximate. If you need a practical capacity figure for packing, the known-container method is generally more informative.
Practical checks before comparing packs
- Keep the method consistent. Compare main compartment with main compartment, or include the same types of pockets on both packs.
- Check the opening. A large stated volume may still be awkward for a sleeping bag, food container, or other bulky item if the opening is narrow.
- Test your actual gear. Pack the items you expect to carry, including food, water, and layers. Confirm closures work and frequently used items remain accessible.
- Account for compression. Compression straps can reduce pack dimensions, but they do not create storage volume. Avoid overstuffing or forcing zippers closed.
- Separate internal and external storage. Strapped-on equipment adds carrying bulk, not enclosed liters. Note it separately rather than adding it to the bag’s capacity.
- Check the maker’s description. Look for whether listed capacity includes side pockets, lid pockets, or other compartments. When the method is not stated, use the number as a comparison guide, not an exact standardized measurement.
Capacity is only one part of choosing hiking gear. See this guide to hiking daypacks for related pack considerations, or review hydration backpack features if water access matters on your trips. For trekking equipment carried outside a pack, this overview of trekking poles covers another common part of trail loadout.
Common measurement mistakes
- Using outside dimensions. Fabric, padding, frame components, and curved shapes reduce usable interior space.
- Counting shared space twice. An internal sleeve may occupy part of the main compartment rather than adding a separate full volume.
- Filling beyond normal use. Stretching fabric or forcing a closure can produce a number that is not practical for packing.
- Mixing units. Convert measurements before multiplying. Centimeters and inches cannot be combined in one calculation.
- Assuming every pocket counts in the label. Makers may use different inclusion rules, so check product documentation when available.
- Confusing liters with weight. Two packs with equal volume can carry different loads comfortably. Volume alone gives no load rating.
Limitations of a home measurement
A household estimate cannot establish a manufacturer’s official capacity or reproduce an undisclosed test method. Container shape, soft fabric, internal structure, and your chosen fill level all affect the result. A dimension calculation adds further error for tapered and curved compartments. Use your measurement to understand approximate usable space and compare your own packing needs; rely on the maker’s documentation for its stated capacity and intended use.
Optional supporting equipment:
Pros
- 40L storage capacity
- Included rain cover
- Adjustable hip and chest belts
- Wet-clothing separation pocket
- Hydration bladder compatibility
Cons
- Limited published load-rating details
- Water-resistant fabric needs rain-cover support
- Large size may exceed short-hike needs
Specifications
| Color | Khaki |
|---|---|
| Size | 40l |
| Dimensions | 8.86 x 12.99 x 23.62 inches |
| Product Group | Apparel |
| Binding | Luggage |
Verify the current product variation before purchase; specifications may change.
Detailed Review
What stands out in the currently available product data:
- 40L capacity holds clothing, food, hydration gear, and day-trip essentials
- Water-resistant 210D ripstop nylon helps handle light moisture and abrasion
- Included rain cover adds protection during changing weather
- Padded, breathable back and shoulder straps improve carrying comfort
- Adjustable hip and chest belts help stabilize loaded pack weight
- Wet pocket separates damp clothing from dry gear
Best For / Avoid If
Best for buyers prioritizing
- 40L storage capacity
- Included rain cover
- Adjustable hip and chest belts
Consider another option if
- Limited published load-rating details
- Water-resistant fabric needs rain-cover support
- Large size may exceed short-hike needs
Frequently asked questions
Can I measure a backpack with water?
Do not pour water into the backpack. It can soak materials, affect coatings, or damage contents and components. Use dry containers with known volume instead. Follow the care instructions supplied for your particular pack.
Should I include side pockets in the total?
You may include them in your own total, but state that you did. Some capacity descriptions include pockets and others may not. For a fair comparison, use the same counting rule for every pack.
Why does my measurement differ from the label?
Your method may count different compartments, use a different fill level, or handle tapered spaces differently. The maker’s capacity method may also differ from a home estimate. Check the product documentation and treat small differences as expected rather than proof of a defect.
Is a larger liter capacity always better?
No. Choose enough usable space for the gear and trip, while considering pack fit, organization, and load. Extra volume can tempt you to carry unnecessary items; a smaller pack may not fit bulky essentials. Test your intended load before relying on a pack for a trip.
