Hong Life CalculatorsUnderstanding the bag volume calculator — concept and principle
This calculator works out the capacity of a backpack, suitcase or duffel bag in litres from its dimensions, and the sum of length + width + height that many airlines use for size limits. It helps compare bags and check cabin baggage rules.
How it is calculated
Box-shaped bag: volume (L) = length × width × depth (cm) ÷ 1,000
Cylinder (duffel): volume (L) = π × diameter² ÷ 4 × length ÷ 1,000
Cubic inches = cm³ ÷ 16.387; US gallons = litres ÷ 3.785
Sum of dimensions = length + width + depth (for a cylinder, diameter counts twice)
The principle
A litre is 1,000 cubic centimetres, so multiplying the three dimensions in centimetres and dividing by 1,000 gives litres. A duffel is roughly a cylinder: its circular cross-section times its length gives its volume. Real capacity is a little less than this because of padding, frames and rounded corners.
Worked example
Default inputs: box-shaped, 55 × 35 × 20 cm.
- Volume = 55 × 35 × 20 = 38,500 cm³ = 38.5 L.
- Cubic inches = 38,500 ÷ 16.387 = 2,349; US gallons = 38.5 ÷ 3.785 = 10.17.
- Sum of dimensions = 55 + 35 + 20 = 110 cm = 43.3 in.
| Length or height (cm) | Capacity (L) |
|---|---|
| 13.8 | 9.6 |
| 27.5 | 19.3 |
| 41.2 | 28.9 |
| 55 | 38.5 |
| 68.8 | 48.1 |
| 82.5 | 57.8 |
| 110 | 77 |
How to read the result
Daypacks are often 15–30 L, weekend bags 30–50 L and large travel packs 50–75 L or more. Many airlines limit cabin bags to around 55 × 40 × 20 cm or a total of about 115 cm, but rules vary, so check your airline. External dimensions, including wheels and handles, are what airlines measure.
Common mistakes
- Using inside dimensions to check airline limits, which use outside dimensions.
- Assuming the calculated litres equal usable packing space.
- Forgetting that soft bags can bulge beyond their nominal size.
Frequently asked questions
How many litres is a carry-on bag?
A typical 55 × 35 × 20 cm cabin bag is about 38–40 L.
How do I measure a duffel?
Measure its length and the diameter of the round end.
Why does my 40 L backpack hold less?
Padding, pockets and frame reduce usable space, and makers measure differently.
Sources: BIPM, The International System of Units (SI Brochure), 9th ed., 2019 (litre = 1 dm³) · NIST Special Publication 811, Guide for the Use of the International System of Units (conversion factors)