For rectangular boxes, the girth formula is simple: 2 × (width + height). For cylindrical packages — mailing tubes, poster tubes, shipping cylinders, the kind of containers you'd use for blueprints or rolled artwork — the formula changes. Most carriers use the circumference of the cylinder as the girth.
This sounds like a minor technicality but it routinely catches shippers off guard, both for better and worse.
The formula
For a cylinder with diameter D:
girth = π × diameter ≈ 3.14 × D
length + girth = length + π × D
When the tube formula helps you
For a tube with a 6-inch diameter, girth = 18.85 inches.
Compare that to a 6 × 6 square box: girth = 2 × (6 + 6) = 24 inches.
The tube has a girth 5 inches smaller than the equivalent square box. For a 60-inch-long tube, length plus girth is 60 + 18.85 = 78.85, versus 60 + 24 = 84 for a 6 × 6 × 60 box. The tube is more carrier-friendly by 5 inches — which can be the difference between standard rates and surcharge tier.
When the tube formula hurts you
For a tube with a 12-inch diameter, girth = 37.7 inches.
Compare to a 12 × 12 square box: girth = 2 × (12 + 12) = 48 inches.
The tube is still smaller, but not by much in absolute terms. And here's where it gets interesting: for some shapes, a flat rectangular box can be smaller than a tube. A 12 × 4 flat box has girth = 2 × (12 + 4) = 32 inches — smaller than the 12-inch tube's 37.7.
If you have a rolled item that could ship rolled (tube) or folded flat (rectangular box), the flat box is often the better carrier choice.
The "thickest cross-section" rule
Most carriers' published rules say girth is measured around the "thickest part" of the package — which for a cylinder means the circumference of its widest cross-section. For a regular cylinder, that's straightforward.
For an irregular cylinder (a tube with end caps that are wider than the main body), the carrier measures around the widest cross-section, which is the cap. This is rarely a meaningful difference, but for very large tubes with bulky caps, it can push you up a tier.
For a tapered tube (narrower at one end), the girth is at the wider end.
Where carriers disagree
Most US carriers use circumference (π × diameter) for cylindrical girth. A few international carriers use 2 × diameter as a simplification — which is more generous than circumference (since π × D > 2D when D > 0). Royal Mail and some European carriers have used the 2D approach historically.
In practice, US-based shippers can rely on π × D. If you're shipping internationally, check the specific carrier's rules for any tube-shaped package, since methodology varies.
Worked examples
Poster tube for a 36 × 24 inch print. Rolled into a 4-inch diameter tube, 38 inches long. Girth = 3.14 × 4 = 12.6 inches. Length plus girth = 38 + 12.6 = 50.6 inches. Easily under any carrier's standard tier.
Long thin pipe in a tube. 96 inches long, 3-inch diameter tube. Girth = 3.14 × 3 = 9.4 inches. Length plus girth = 96 + 9.4 = 105.4 inches. Just under USPS's 108-inch limit. If you'd added an end cap that made the effective diameter 4 inches, you'd be at 96 + 12.6 = 108.6 — over.
Wide tube with a small contents. 24 inches long, 14-inch diameter tube. Girth = 3.14 × 14 = 44 inches. Length plus girth = 24 + 44 = 68 inches. Reasonable for most carriers.
Use the girth calculator and select "Tube" mode for any cylindrical package. It applies the π × diameter formula automatically and checks against all major carrier thresholds at once.
Practical implication
If you have a long thin item (rod, paper roll, fishing pole), a tube is your best option — it minimizes girth, which is exactly what the formula penalizes. If you have a flat item (artwork that could roll or fold), either format works but a snug rectangular box is sometimes smaller. If you have something already cylindrical (a rolled rug, a coiled hose), the tube isn't really a choice — pack accordingly.