Iron weight in kilograms equals the cross-section area of the stock multiplied by its length and then by the metal's density — weight = volume × density, where volume = cross-section area × length. For a round iron bar the cross-section is π·r² (with r = diameter ÷ 2), for a square bar it is s², for a round pipe or tube it is the outer disk minus the bore π·(R² − r²), for a flat sheet or plate it is length × width × thickness, and for a hexagon bar it is (√3/2)·F² where F is the distance across flats. Convert every dimension to meters before multiplying, and use density in kilograms per cubic meter — for steel and most "iron" stock the engineering reference value is 7850 kg/m³, which puts a 20 mm steel round bar 1 m long at about 2.47 kg. Pure iron weighs about 7874 kg/m³ — within roughly one percent of the steel value — while cast iron sits between 6800 and 7800 kg/m³ depending on grade, so the 7850 kg/m³ steel number is a tight stand-in for most iron and mild-steel stock. The Metal Weight Calculator applies the right cross-section formula automatically the moment you pick a profile shape and a metal, so the math above never has to be done by hand.
The Core Formula: Weight = Volume × Density
The whole problem of finding iron weight in kg collapses into two multiplications. First, multiply the cross-section area by the length to get the volume of the piece in cubic meters. Second, multiply that volume by the metal's density in kilograms per cubic meter. The result is weight in kilograms, with no other conversion step required as long as every dimension is in meters and the density is in kg/m³.
That single law covers every stock shape — bar, pipe, sheet, hex — because each one only changes how the cross-section area is computed. The volume is always area × length, and the weight is always volume × density. The reason most people reach for a calculator is not the formula itself but the geometry: keeping the right cross-section equation in mind for a hollow pipe versus a solid round bar versus a hex piece is easy to mix up.
This is also why a single dedicated tool beats a stack of one-off spreadsheets. Once the profile shape is selected, the Metal Weight Calculator handles the cross-section formula, the unit conversion, and the density lookup in one step, returning weight, volume, cross-section area, and the density used in the answer.
Cross-Section Formulas by Profile Shape
Each profile has one cross-section equation. Picking the right one is the only step that takes any thought; once it's chosen, the volume and weight follow mechanically.
| Profile Shape | Cross-Section Area | Dimensions to Enter |
|---|---|---|
| Round bar | π · r² | diameter, length |
| Square bar | s² | side, length |
| Round pipe / tube | π · (R² − r²) | outer diameter, wall thickness, length |
| Rectangular sheet / plate | L × W × T | length, width, thickness |
| Hexagon bar | (√3 / 2) · F² | distance across flats, length |
For a sheet or plate the cross-section is straightforward: it is just length × width × thickness, with all three dimensions in the same units. For pipe, the bore is subtracted from the outer disk rather than added — a 50 mm OD pipe with a 5 mm wall has an inner diameter of 40 mm and a cross-section of π·(25² − 20²) mm², not π·(25² + 20²) mm².
A few notes that save real errors at the bench:
- For a round bar, the input is the diameter, not the radius. The tool divides by two internally.
- For a round pipe, the input is outer diameter and wall thickness. Inner diameter is not entered directly — it is derived as OD minus twice the wall.
- For a hexagon bar, the input is the distance across flats, which is the wrench size the bar is sold by, not the corner-to-corner distance or the side length.
How to Calculate Iron Weight in kg Step by Step
- Open the Metal Weight Calculator and pick the profile shape that matches your stock: round bar, square bar, round tube or pipe, rectangular sheet, or hexagon bar.
- Pick the metal from the material list — steel for general "iron" stock, stainless steel (304), aluminum, copper, brass, or titanium.
- Choose metric mode so dimensions are entered in millimeters and the result reads in kilograms. Imperial mode swaps to inches in and pounds out.
- Type the dimensions the selected shape asks for. A round bar needs diameter and length, a pipe needs outer diameter, wall thickness, and length, a sheet needs length, width, and thickness, and a hex bar needs the across-flats value plus length.
- Read the weight result. The tool also shows the cross-section area, the volume, and the density it used, so the math behind the number is visible rather than hidden.
- Change any input and watch the weight update immediately — there is no Run button, no upload, and no waiting, because the whole calculation runs in the browser.
For someone weighing a single piece before quoting or shipping, those six steps replace a hand calculation, a lookup table, or a spreadsheet cell. For someone pricing a cut list, repeating the steps for each line is faster than re-deriving the cross-section formula each time.
Worked Example: A 20 mm Steel Round Bar, 1 m Long
Walking through one full calculation by hand shows how the formula and the tool line up. Take a round iron bar 20 mm in diameter and 1000 mm long, in steel (density 7850 kg/m³).
Step 1 — radius. r = 20 mm ÷ 2 = 10 mm = 0.01 m.
Step 2 — cross-section area. A = π · r² = π · (0.01)² = π · 0.0001 ≈ 0.00031416 m².
Step 3 — volume. V = A · length = 0.00031416 · 1 = 0.00031416 m³.
Step 4 — weight. W = V · density = 0.00031416 · 7850 ≈ 2.47 kg.
Run the same numbers through the Metal Weight Calculator with round bar, steel, 20 mm, and 1000 mm, and the weight field returns about 2.47 kg — the same answer, with no pencil work. This is the canonical sanity check: if your hand math matches the tool on a 20 mm × 1 m round bar, it will match on every other shape the tool covers.
Density Values for Iron, Steel, and Common Metals
The cross-section formula decides how much space a piece of stock takes up; density decides how heavy that space is. Density is the only place where the metal itself enters the answer, so a 20 mm aluminum round bar 1 m long weighs about 0.85 kg while the same shape in steel weighs about 2.47 kg — close to three times heavier.
| Metal | Density (kg/m³) |
|---|---|
| Steel | 7850 |
| Stainless steel (304) | 8000 |
| Aluminum | 2700 |
| Copper | 8960 |
| Brass | 8500 |
| Titanium | 4500 |
Switching metals can flip a project's cost and weight budget overnight. A structural bracket sized in steel and re-cut in aluminum weighs about a third as much but loses stiffness proportional to Young's modulus rather than density, so the section often has to grow to recover strength. A heat exchanger or bus bar moves the other way: copper at 8960 kg/m³ is heavier than steel but conducts electricity far better, so the trade is in volume rather than mass. Titanium at 4500 kg/m³ is roughly half the weight of steel for the same shape and is the default in aerospace and medical parts where every kilogram matters.
For "iron" specifically, pure iron sits around 7874 kg/m³ and cast iron ranges from roughly 6800 to 7800 kg/m³ depending on grade. The calculator's 7850 kg/m³ steel value is a close stand-in for general structural iron and mild steel stock — typically within a percent or two — but a mill certificate beats any reference number when the order has to ship at an exact weight.
When the Result Is Close but Not Exact
Two things make a real piece of stock weigh slightly more or less than the calculator says. First, density is a typical engineering value: the actual alloy, grade, and temper shift it by a few percent in either direction. Second, real bars, pipes, and sheets carry mill tolerances — small but non-zero slop on every dimension that propagates through volume.
The tool returns the theoretical weight of a nominal piece. That is exactly the right number for quoting, estimating freight, sizing a rack, and pricing metal by the kilogram or pound, and it stays within roughly one to two percent for most common stock. For final invoicing on a heat-critical job, weigh the finished part rather than trusting any reference density.
For fabricators checking a cut list, machinists sizing a piece for a spindle, or hobbyists mailing a small bracket, running the size and material through the Metal Weight Calculator takes the guesswork out of iron weight in kg. Change the unit system with one click if your stock list is in inches instead of millimeters, switch the metal if a quote comes back in aluminum, and rerun every line of a cutting list almost instantly. The geometry stays right, the density changes, and the weight follows.