A section properties calculator: draw or assemble a cross-section and read its area, centroid, moment of inertia (Ix, Iy), principal axes, radii of gyration, and elastic + plastic section moduli — updated live as you draw. Place a standard steel section, or draw plates and cut holes. It draws both centers — the elastic centroid and the plastic neutral axis — which coincide only on a doubly symmetric shape. It also handles composite sections by the transformed-section method.
Place any of 2,600+ published steel sections across six standards — AS4100, AISC, EN 10365, BS 4, JIS and IS 808.
Draw plates, polylines and arcs, cut holes, and merge touching plates into one shape — so a hand-built section can take a root fillet.
Area, centroid, Ix and Iy, product of inertia, principal axes and angle, radii of gyration, elastic and plastic moduli.
Two centers, both drawn: the elastic centroid and the plastic neutral axis — which are the same point only on a doubly symmetric section.
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How Each Tool Works
Every command in the toolbar, what it is for, and how to use it — with before-and-after drawings produced by the tools themselves. Open a section, or search for a tool by name or by what you are trying to do.
How to read these commands
x,y
an exact coordinate — type it instead of clicking. Example: 0,0
@dx,dy
relative to the LAST point, so it is a size rather than a place. Example: @200,12 is 200 wide by 12 thick
@len<angle
a distance at an angle from the last point. Example: @150<30 is 150 at 30°
<name>
a value you must give
[name]
a value you may leave out — the default is shown after the =
[A|B]
pick one of these; the letter alone is enough
…
as many more as you like — Enter finishes, Esc abandons
The four shapes every cross-section is built from, plus the dimension tool. Everything you draw is a region: a closed outline the engine integrates over. You never have to close a shape by hand — each command closes its own.
RectangleREC
Draws a rectangle from two opposite corners. It is the workhorse — every plate, flange, web and slab is one.
TypeREC x,y x,y
Click the ▭ tool, or type REC and press Enter.
Click the first corner, or type an exact coordinate like 0,0.
Click the opposite corner, or type a RELATIVE size like @200,12 — 200 wide by 12 thick.
Tip. Build up a plate girder as three rectangles: bottom flange, web, top flange. Draw each at its true position and the properties are correct with no further work.
CircleC
Draws a circle from its center and a radius — a bar, a pin, a bolt hole, or a round hollow section.
TypeC x,y <radius> [D|3P]
Type C, or click the ○ tool.
Pick the center.
Type the radius, or click a point on the circumference.
Watch out. The prompt asks for the RADIUS, not the diameter. A 20 mm bar is radius 10.
ArcA
Draws a circular arc through three points, for a curved edge that is part of a larger outline.
TypeA x,y x,y <angle> [3P]
When to use it. Rarely needed on its own — most curves in a steel section are fillets between straight edges, and FILLET makes those. Reach for ARC when you are tracing a shape that is genuinely curved, such as a haunch or a tapered nose.
Type A.
Pick the start point.
Pick a point the arc passes through.
Pick the end point.
PolylinePL
Draws a run of connected straight segments and closes it into one region — the tool for any shape that is not a rectangle or a circle.
TypePL x,y x,y … [C]
When to use it. An angle, a tee, a channel, a cold-formed lipped section, or a profile you are tracing from a drawing.
Type PL.
Pick each vertex in turn. Relative entry helps: @100,0 goes 100 mm right, @0,-12 goes 12 mm down.
Press Enter to close the outline back to the first point.
Watch out. A polyline that does not close is not a region and cannot be integrated. Pressing Enter closes it for you — do not draw the last segment back to the start by hand, or you will leave a zero-length edge.
Tip. Turn ORTHO on before tracing a section with square corners. Every segment then comes out truly horizontal or vertical, which matters because a web 0.3° off vertical changes Iy.
DimensionDAL
Adds a dimension line between two points you pick.
TypeDAL x,y x,y
When to use it. For the report. A dimension is an ANNOTATION — it is drawn on the figure and it can never change a computed property, which is why you can add as many as the drawing needs without worrying about the numbers.
Type DI.
Pick the two points to measure between.
Tip. Drag a placed dimension to move it clear of the geometry; Alt-drag rotates it. Double-click resets it to where the tool put it.
These act on what is SELECTED. Click an entity to select it, drag a box to take several, or press Escape to let go. If nothing is selected, the modify commands that need a selection say so rather than guessing.
FilletF
Replaces a sharp corner with a circular arc of the radius you type, tangent to both edges.
TypeF <radius> x,y
When to use it. Every rolled steel section has a root radius where the web meets the flange, and it carries real area. Fillet the re-entrant corners of a built-up section and the properties stop being slightly conservative.
Type F.
Type the radius.
Click the corner to round.
Before
After
The top-right corner is replaced by a 30 mm arc that stays tangent to both edges — the rolling radius a real plate has.
Watch out. A radius larger than either edge can accommodate is refused — the arc would run past the end of the edge it is supposed to be tangent to. Use a smaller radius, or lengthen the edge first.
ChamferCHA
Cuts a corner off flat, a set distance back along each edge.
TypeCHA <distance> x,y
When to use it. A flame-cut or machined corner, and the usual way a plate edge is prepared for welding.
Type CHA.
Type the distance.
Click the corner to cut.
Before
After
The same corner cut flat 30 mm back along each edge — a chamfer, not a radius.
MoveM
Moves the selection from a base point to a second point.
TypeM x,y x,y
When to use it. Positioning a flange relative to a web. Note that MOVING a region changes the centroid and the second moment of the whole section — that is not a side effect, it is the point.
Click the shape on the drawing to select it (shift-click to add more).
Type M.
Pick the base point.
Pick where it goes, or type a relative offset like @0,300.
CopyCO
The same gesture as MOVE, but the original stays where it is.
TypeCO x,y x,y
Click the shape on the drawing to select it (shift-click to add more).
Type CO.
Pick the base point, then the destination.
Before
After
COPY leaves the original where it was and adds the new one. MOVE is the same gesture without the original — the shape simply arrives at the new place.
RotateRO
Turns the selection about a base point you pick, by an angle you type.
TypeRO x,y <angle>
When to use it. A raking member, a skewed plate, or checking a section about a non-principal axis.
Click the shape on the drawing to select it (shift-click to add more).
Type RO.
Pick the base point — the pivot.
Type the angle in degrees; positive is anticlockwise.
Before
After
Rotated 30° about its own bottom-left corner. Pick the base point first — rotating about the wrong point moves the shape as well as turning it.
Watch out. The base point is the pivot, not the origin. Picking the wrong one turns the shape AND moves it somewhere unexpected.
Tip. You rarely need to rotate a section to find its worst axis. Turn on the principal axes instead — the tool computes θp and draws them, which is the same answer without touching the geometry.
MirrorMI
Reflects the selection about a line you pick, keeping the original — so you get a symmetric pair from one drawn half.
TypeMI x,y x,y
When to use it. Back-to-back angles, a pair of channels, the second flange of a symmetric girder, or any section you would rather draw half of. It is the single biggest time-saver in the tool.
Click the shape or shapes on the drawing to select them (shift-click to add more).
Type MI.
Pick two points that define the mirror LINE — for a vertical axis, pick two points directly above one another.
The reflected copy is added; the original stays.
Before
After
The angle is mirrored about a vertical axis at x = 150 and the original is kept, giving the back-to-back pair a double-angle strut is made from.
Tip. Turn ORTHO on before picking the two axis points. Otherwise the axis is a degree or two off vertical and the "symmetric" section that results has a small Ixy and a rotated set of principal axes — a subtle wrong answer rather than an obvious one.
ArrayAR
Copies the selection into a rectangular grid — a number of columns and rows at a spacing you give.
TypeAR <cols> <rows> <dx> <dy>
When to use it. Bolt holes, stiffeners, a run of identical voids. Six holes in one command instead of six copies.
Click the shape on the drawing to select it (shift-click to add more).
Type AR.
Type the number of columns, then rows.
Type the column spacing, then the row spacing.
Before
After
One hole arrayed 2 columns × 3 rows at 80 × 30 mm spacing. The holes are VOID, so the area they remove is taken off the properties — and every one of them sits fully inside the plate, which is what makes that true.
EditPR
Changes a drawn entity by typing an exact value, instead of dragging it.
TypePR <R|C|W|H> <value>
When to use it. When the drawing is nearly right and the number has to be exact — "that plate is 300 long, not 297".
Click the shape on the drawing to select it (shift-click to add more).
Type ED.
Choose what to change. Only the fields that entity actually HAS are offered — a rectangle is never offered a radius.
Type the new value.
Watch out. Center means the center of the bounding BOX, not the centroid. On an angle or a channel those are different points, and it is the box center that moves to where you type.
Tip. The same four fields sit in the Selected panel under the drawing, as boxes you can type straight into. Same code, same result — use whichever is closer to your hand.
EraseE
Deletes the selected entities. Undo brings them back.
TypeE
Every region is either SOLID (it adds material) or VOID (it removes it). The mode is set BEFORE you draw and applies to the next shape, which is how a hole, a duct opening or a hollow box gets drawn.
Solid
The default. A shape drawn in solid mode adds its area, and its own second moment about the section axes, to the totals.
Void
A shape drawn in void mode SUBTRACTS. Its area comes off, and so does its contribution to I — correctly weighted by how far it sits from the centroid.
When to use it. A bolt hole, a service penetration, or the inside of a hollow section: draw the outside solid, then the inside as a void.
Switch the mode to VOID before drawing.
Draw the shape exactly as you would a solid one.
Switch back to SOLID for the next piece of material.
Before
After
A solid plate, then the same plate with a circle drawn in VOID mode. The void subtracts its area and its second moment — that is how a bolt hole, a duct opening or the inside of a hollow box is drawn.
Watch out. A void that is not fully inside a solid removes area that was never there, and the properties become meaningless. Keep every void within the material it is cutting.
Tip. This is how you check the cost of a penetration: note Ix, add the void, and read Ix again. The difference is what the hole cost you, and it is far larger for a hole in the flange than the same hole at the neutral axis.
A section made of two materials cannot be added up directly, because a square millimeter of concrete does not carry what a square millimeter of steel carries. The transformed-section method scales each material by its stiffness ratio before summing — and that ratio is the only number you have to supply.
Modular ratio (n)
The stiffness of this material divided by the stiffness of the reference material: n = E_this / E_reference. The first material in the list is the reference, so its n is exactly 1.
When to use it. A composite steel-and-concrete deck, a timber flitch beam with steel plates, or an aluminum section checked against a steel one.
Add a second material. It arrives as concrete on steel, the commonest case.
Pick a ratio from the menu, or type your own.
Select the shapes made of that material and press Assign.
Watch out. Codes quote this ratio THE OTHER WAY UP. AS 3600, Eurocode 2 and AISC all write n = E_steel / E_concrete ≈ 7 to 15; this box wants E_concrete / E_steel ≈ 0.143, which is 1/7. Type 7 here and you have claimed your concrete is seven times stiffer than steel. The value beside the box shows the reciprocal live, so the two readings are visible at once.
Tip. Use n ≈ 1/7 for short-term loading and n ≈ 1/15 for sustained load, where creep has softened the concrete. Running the section both ways takes ten seconds and brackets the real behavior — that is the standard check, not an optional refinement.
Assign
Makes every selected shape the active material. Until you assign, everything is the reference material.
Select the shapes.
Choose the material in the dropdown.
Press Assign.
Rather than draw a rolled section, place one: 2,600+ sections across six standards (AS/NZS, AISC, EN, BS, JIS and IS), each with its published dimensions. Placed sections draw at their real size and can be modified, cut or combined like anything you drew yourself.
Place a section
Chooses a standard, a type and a size, and draws that section at the origin.
Pick the standard.
Pick the type — universal beam, channel, angle, hollow section.
Pick the size.
Tip. Place a rolled section, then add a cover plate as a rectangle on top. That is the classic strengthening check, and the properties update as soon as the plate lands.
These do not change the drawing — they change where your clicks LAND. Every one of them exists so a picked point is exact rather than nearly right, which matters because the properties are computed from the geometry, not from what you meant.
OSNAP — object snap
Makes a click land exactly on a real feature: an endpoint, a midpoint, a center, an intersection.
When to use it. Always on. It is the difference between a web that meets the flange and a web that ends 0.4 mm short, leaving a sliver the engine faithfully integrates.
Tip. Press F3 to toggle it mid-command. Choose WHICH features it catches in Drafting Settings — turning off the ones you do not want stops the snap fighting you on a crowded drawing.
ORTHO
Locks the next point to horizontal or vertical.
When to use it. Tracing any section with square corners, and — importantly — picking a MIRROR axis that is truly vertical.
Tip. F8 toggles it. ORTHO is POLAR fixed at 90°; turning one on turns the other off.
POLAR tracking
Locks the direction to fixed angular steps, so a raking edge comes out at exactly 30° rather than 29.7°.
DYN — dynamic input
Shows the prompt and the live length and angle at the crosshair, so you can type an exact value without looking away from the drawing.
GRID
A background rule for judging size. It never affects where a point lands — that is OSNAP's job.
PNA — plastic neutral axes
Draws the equal-AREA axes, which split the section into two halves of equal area.
When to use it. The plastic neutral axis governs the plastic section modulus and therefore the plastic moment capacity. It coincides with the centroidal axis only on a doubly symmetric section — on a tee, a channel or an angle the two are visibly apart, and seeing that is the fastest way to understand why the plastic and elastic moduli differ. (⚠ the letters for those two are reversed between AS 4100 and AISC — the panel prints whichever your design code uses.)