Introduction
Every steel tube weight — every entry in every A500 table, and every number the calculator returns — comes from one physical fact: steel weighs 490 pounds per cubic foot (7,850 kg/m³). The rest is geometry. A tube is a length of steel whose cross-section is a ring: outside dimensions minus inside dimensions, times length, times density. Once you see that, every formula on this page is the same idea wearing different clothes.
The Steel Tube Weight Calculator runs these formulas for you — enter size, wall, and length, and it returns pounds and kilograms. This guide shows the full derivation: where the constants come from, the square, rectangular, and round formulas, and two worked examples cross-checked in imperial and metric.
The Master Equation
Weight = density × cross-section area × length. That’s the whole theory. Steel density is 490 lb/ft³ (7,850 kg/m³) — the same figure behind every published tube weight table. The only work is computing the cross-section area — the steel in the ring between the outside and inside faces.
Tube dimensions come in inches, so area comes out in square inches. To convert: 1 ft³ = 1,728 in³, and weight per foot = area (in²) × 490 ÷ 144. The constant 490 ÷ 144 = 3.4028 is the number you’ll see repeatedly: lb/ft = area in in² × 3.4028.
Formula 1: Square Tube
The thin-wall approximation treats the cross-section as four flat walls:
Area ≈ 4 × (side − wall) × wall (all in inches)
lb/ft = Area × 3.4028
The (side − wall) term accounts for the corners being counted once instead of twice. Worked example — 2×2×1/4 in: area = 4 × (2 − 0.25) × 0.25 = 4 × 1.75 × 0.25 = 1.75 in²; lb/ft = 1.75 × 3.4028 = 5.95 lb/ft.
Compare with the published A500 value of 5.59 lb/ft: the approximation runs about 6% high here because real corners are rounded, not sharp, and the mill tolerance on wall thickness averages slightly under nominal. The formula is excellent for odd sizes no table covers; published values win wherever they exist.
Formula 2: Rectangular Tube
Same idea with two different sides. Using depth (d) and width (w):
Area ≈ 2 × (d + w − 2 × wall) × wall
lb/ft = Area × 3.4028
Worked example — 4×2×1/4 in: area = 2 × (4 + 2 − 0.5) × 0.25 = 2 × 5.5 × 0.25 = 2.75 in²; lb/ft = 2.75 × 3.4028 = 9.36 lb/ft, versus the published 8.81 lb/ft — again a few percent high, same reason. Note the formula treats both axes the same; real bending strength differs by axis, which is a structural design question, not a weight question. For choosing between grades and finishes once the math is done, see the material guide.
Formula 3: Round Tube
Round tube uses the circumference-based approximation — the same formula pipe weight tables use:
lb/ft = 10.68 × (OD − wall) × wall
The constant 10.68 is π × 3.4028: the circumference of the centerline of the wall (π × (OD − wall)) times the density constant. Worked example — 2 in OD × 0.120 in wall: 10.68 × (2 − 0.120) × 0.120 = 10.68 × 1.88 × 0.120 = 10.68 × 0.2256 = 2.41 lb/ft; × 1.4882 = 3.59 kg/m. Cross-check: 2.41 × 0.45359237 = 1.093 kg/ft; × 3.28084 ft/m = 3.59 kg/m — agreement both ways, confirming the constant and the conversions.
Unit Conversions
Three conversions cover everything:
| Conversion | Factor |
|---|---|
| lb/ft → kg/m | × 1.4882 |
| kg/m → lb/ft | × 0.67197 |
| in² → lb/ft | × 3.4028 (490 ÷ 144) |
Gauge to decimal inches for thin tube: 16 ga ≈ 0.065 in, 14 ga ≈ 0.083 in, 11 ga ≈ 0.120 in — convert before plugging into any formula. And a useful mental check: 1 in² of steel cross-section weighs 3.4028 lb per foot of length, so a quick area estimate immediately tells you the ballpark lb/ft. If the calculator returns something wildly different from your mental estimate, recheck the wall input first — it’s the culprit in the large majority of bad outputs. The common mistakes guide covers what happens when gauge, fractions, and decimals get mixed in one estimate.
Why Published Tables Beat the Approximation
Published A500 weights come from the actual mill geometry — corner radii, real wall tolerances, the weld seam — while the thin-wall formulas assume sharp corners and nominal walls. The gap is typically 2–8%, with the formula usually reading high. Practical rule:
- Quoting, ordering, freight: use published lb/ft values.
- Odd or imported sizes with no table entry: use the formula, then round your order up — the formula’s slight optimism is on the safe side for purchasing.
- Cross-checking the calculator: either method should land within a few percent; a bigger gap means a wrong input, usually wall thickness in the wrong units.
The FAQ answers the short version of these questions if you don’t need the derivation.
Related Guides
- Complete Guide — the full overview with the common-size reference table.
- Material Guide — grades, finishes, and picking the right tube.
- Common Mistakes — the unit and input errors that break estimates.
- FAQ — quick answers without the derivation.
- Estimating Guide — from cut list to order.
Get Your Number
The formulas are simple — density times area times length — but hand-calculating a 30-piece cut list is where errors creep in. The Steel Tube Weight Calculator runs the same math instantly and in both unit systems. Use the formulas above to sanity-check its output on one or two sizes, then let the tool handle the full list.
Disclaimer: Calculator results are planning estimates based on the dimensions you enter. Actual tube weights vary slightly by mill, coating, and manufacturing tolerance. Verify weights and availability with your supplier, and consult a licensed professional or your local building authority for structural or code-related decisions.
