Estimate backpack capacity from dimensions or trip type and climate, with results in liters, cubic feet, and US gallons for pack sizing.
Related Calculators
- Short Or Long Torso Calculator
- Tent Area Calculator
- Net Weight Calculator
- Minimum Connection Time Calculator
- All Everyday Life and Education Calculators
Backpack Capacity Formula
Two modes, two formulas.
Dimensions to liters:
V (L) = L × W × H × s × 0.001
- V = pack capacity in liters
- L, W, H = length, width, height in centimeters
- s = shape factor (1.00 rectangular, 0.85 typical pack, 0.75 tapered)
- 0.001 converts cm³ to liters
If you measure in inches, convert first (1 in = 2.54 cm). The shape factor accounts for curved panels, rounded corners, and stuff that real packs aren't perfect boxes. Use 0.85 unless you know the pack is unusually boxy or unusually tapered.
Trip planner:
Recommended V = base range × climate factor
- base range = liter range tied to trip length
- climate factor = 0.9 warm, 1.0 three-season, 1.2 cold/winter
Cold-weather gear takes more space than it weighs. Down jackets, four-season sleeping bags, and insulated pads push the climate multiplier up.
Capacity Reference Tables
Use these to sanity-check a result or to pick a pack before you measure.
| Trip Type | Capacity | Typical Load |
|---|---|---|
| Commute / school | 10–20 L | Laptop, lunch, water |
| Day hike | 18–30 L | 10 essentials, food, layer |
| Overnight | 30–45 L | Sleeping bag, shelter, cookset |
| Weekend (2–3 nights) | 45–60 L | Full kit, three-season |
| Multi-day (3–5 nights) | 60–75 L | Thru-hike sweet spot |
| Extended (5–7 nights) | 70–85 L | Week of food + sleep system |
| Expedition / winter | 85–110 L | Bulky cold-weather gear |
| Volume | Cubic Inches | Cubic Feet |
|---|---|---|
| 20 L | 1,220 | 0.71 |
| 35 L | 2,135 | 1.24 |
| 50 L | 3,050 | 1.77 |
| 65 L | 3,966 | 2.30 |
| 80 L | 4,881 | 2.83 |
Example and FAQ
Example. A pack measures 55 cm tall, 30 cm wide, and 25 cm deep. Using the typical pack factor (0.85):
V = 55 × 30 × 25 × 0.85 × 0.001 = 35.1 L
That puts it in the day-hike to overnight range.
Why doesn't L × W × H match the manufacturer's stated liters? Brands measure the actual interior volume, often by filling the pack with small balls (ASTM method). Your tape measure captures the bounding box, which includes empty corners. The shape factor closes that gap.
Should I include external pockets? No. Manufacturers usually list main compartment volume and total volume separately. Measure the main bag, then add 3–8 L if you want to estimate total capacity with hip belt and lid pockets.
What if my pack is between two trip ranges? Size up if you carry a bulky sleeping bag or winter layers. Size down if you use compression sacks and ultralight gear.
