Units

How units work in Elementarium — SI inside, your unit system on screen — and the handful of conversions that cause most wrong answers in structural models.

Getting started 5 min read

Units are the most common cause of a result that is wrong by a factor of a thousand.

How the app handles them

Internally, everything is SI:

Quantity	Internal unit
Force	N
Length	m
Moment	N·m
Stress	Pa
Density	kg/m³

Every value you type is converted to SI on entry, and back to your project's unit system for display, tables and the report. Two consequences:

The Unit presets popover listing SI m·kN (kN, m, MPa, selected), SI mm·N (N, mm, MPa), US kip·ft (kip, ft, ksi), US lb·ft (lb, ft, psi) and Custom, with a Project settings link to open the units editor — The unit presets, opened from the unit system at the right of the status bar: SI · m·kN, SI · mm·N, US · kip·ft and US · lb·ft, then Custom, with a Project settings link to set each quantity's unit individually.

Changing the display units never changes the model. Switch a project from kN and m to N and mm and every number redisplays; nothing is re-entered or rounded.
The report says what unit every quantity is in. The project chapter carries a legend for exactly this reason — see Exporting a report.

The unit system is chosen when the project is created and can be changed afterwards from the status bar, where the current system is shown at the right-hand end, or under Project settings › Units. Four presets cover every quantity, from forces to section moduli to temperature:

Preset	Force · length · stress	Line load	Surface load	Sections
SI · m·kN	kN · m · MPa	kN/m	kN/m²	mm
SI · mm·N	N · mm · MPa	N/mm	N/mm²	mm
US · kip·ft	kip · ft · ksi	klf	psf	in
US · lb·ft	lb · ft · psi	plf	psf	in

Change any single quantity in the project settings and the system becomes Custom. A new project starts in the units of your browser's region, US customary for an en-US browser and SI otherwise, and the quickstart sets it in the first step.

Where the factors of a thousand come from

Quantity	The trap
Section properties	$I$ in mm⁴ is $1 0^{12}$ times $I$ in m⁴. An IPE 400 has $I_{y} \approx 23130 cm^{4} = 231.3 \times 1 0^{6} mm^{4} = 2.313 \times 1 0^{- 4} m^{4}$
Young's modulus	Steel is $210000 MPa = 210 GPa = 2.1 \times 1 0^{11} Pa$ . Entering 210 with the wrong unit gives a structure a thousand times too soft
Distributed load	`kN/m` on a member; `kN/m²` on a surface. A slab load applied to a beam needs the tributary width multiplied in
Surface results	`kNm/m` and `kN/m`, per unit width. A slab moment of 45 is 45 kNm per metre of strip, not 45 kNm
Bedding modulus	`kN/m³` for $k_{s}$ ; a line support takes $k_{s} b$ in `kN/m` per metre, which is `kN/m²` — see Elastic foundations
Density	`kg/m³` for mass, but `kN/m³` for self-weight. Steel is 7 850 kg/m³ ≈ 77 kN/m³; a density typed as 7.85 (t/m³) or 78.5 (kN/m³) gives a structure a thousand or a hundred times too light
Rotational springs	Moment per radian (`kNm/rad`), not per degree

Sections and materials are defined once and reused, so a mistake in one is repeated everywhere the definition is used. When a whole model is off by the same factor, check them first — see Sections and materials.

Per unit width, once more

The most common unit error in slab and wall design is silent: a surface result in kNm/m read as if it were kNm. The number looks plausible either way, and reinforcement sized from it is out by the strip width.

Angles and directions

Rotational results and rotational stiffnesses are in radians: a nodal rotation of 0.002 is 0.002 rad, and a rotational spring is so many kNm per radian. If a rotational spring seems to do nothing, check that its stiffness was not worked out per degree:

1 kNm/deg \approx 57.3 kNm/rad

Direction is not a unit, but it fails the same way. Gravity acts in negative global Z. A downward load is a negative value in the global system, or a positive magnitude with a downward direction, depending on how it is applied. A load in a member's local direction acts perpendicular to that member, which on a sloping rafter is not vertical — see Load cases.

Imperial and mixed practice

An AISC design or an ASCE 7 combination set does not require US units; the check reads SI internally like everything else and displays whatever the project uses. For the projects that work in US customary units, they are first-class:

Every printed unit follows the project: tables, the properties panel, the section and material editors, load summaries, captured figures and the report.
Line loads are klf or plf, never kip/ft, so a line load cannot be read as a kip-ft moment.
Feet and inches. Fields accept 5'6", 5'-6 1/2", 6 1/2", mixed fractions, k for kip and sums such as 10ft + 6in. Project settings › Units › Write feet as can show lengths as feet-inches, to the nearest 1/16", with a value that does not round exactly marked ≈.
The AISC Shapes Database v16.0 is a section library of its own, 1 329 W, M, S, HP, C, MC, L, HSS and pipe shapes stored in US units, and A992 is the default steel of a US project. Tees and double angles are not in it yet.
Conversions are exact, to the NIST definition of the pound-force, and the model is stored in SI either way, so switching between systems changes nothing but the display.

The section editor's free-form outline is still drawn in millimetres. Where a project mixes sources (a European section table in a US job, a supplier's data sheet in psi), convert at the point of entry and note the conversion in the report. Don't carry two systems in your head.

Useful factors, to four figures:

From	To	Multiply by
kip	kN	4.448
kip·ft	kNm	1.356
ksi	MPa	6.895
psf	kN/m²	0.04788
in	mm	25.40
in⁴	cm⁴	41.62
kip/in	kN/m	175.1

The three checks

Your first analysis works through these on a portal frame.

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