Units & sign conventions

PileCalc does not impose a unit system. In the app you simply pick Metric or US and every label, input, and result follows; when you drive the engine directly through the API you supply any self-consistent system and get answers back in it. Either way, the rules below prevent the most common class of mistake in any pile program.

Switching units in the app

The Metric / US switch lives in the app header, next to the theme toggle. Flip it and the whole workspace re-expresses itself — field labels, the values you've entered, the summary numbers, charts, and tables all convert instantly. Your preference is remembered across sessions.

You can't create a mixed-unit model

Under the hood PileCalc keeps every value in one canonical system (SI) and converts only at the moment of display. That means the engine is never handed a mix of, say, pcf and ksf — switching units can't corrupt your model, and results are always self-consistent. It also lets the US view show the familiar geotechnical units below rather than forcing one rigid system.

What each system shows:

Quantity	Metric	US
Length / depth / diameter	m	ft
Deflection / settlement	mm	in
Force / capacity	kN	kip
Moment	kN·m	kip·ft
Strength / pressure (c, qu, bearing)	kPa	ksf
Soil unit weight (γ)	kN/m³	pcf
Subgrade modulus (k)	kN/m³	pci
Steel modulus / yield (E, Fy)	kPa	ksi
Bending stiffness (EI)	kN·m²	kip·ft²

Unit-agnostic by design

Internally the engine carries no hidden unit conversions. If you supply forces in kN and lengths in m, then a stiffness must be in kN·m², a pressure in kN/m² (kPa), and a unit weight in kN/m³ — and deflections come back in m, moments in kN·m, and so on. Supply US customary instead and every output follows suit. This matters most when you call the API directly, where there is no switch to help you.

One system, end to end (when calling the API)

The single rule for direct API use: pick one self-consistent system and use it for every input. Mixing kPa cohesion with pcf unit weights, or feet of depth with metric stiffness, produces numbers that look reasonable and are wrong. (The app's switch protects you from this automatically.)

Staying consistent

Inside the app the switch keeps you consistent automatically. The rule only bites when you call the engine through the API, where you must supply a single self-consistent system yourself. The cleanest choices are SI (m, kN, kPa, kN·m, kN/m³, kN·m²) and a kip–foot English system (ft, kip, ksf, kip·ft, kcf, kip·ft²).

Why the app's US labels look friendlier than a strict English system

A strictly self-consistent English system would express unit weight in kcf (kip/ft³) and the subgrade modulus in kcf too. The app's US view instead shows the familiar pcf for soil unit weight, pci for subgrade modulus, and ksi for steel — because it converts each quantity individually around an SI core, so the displayed units never have to agree with one another.

The water-unit-weight trap

The single most common unit error in axial and drilled-shaft analyses is the water unit weight. Effective vertical stress — which drives both side friction and end bearing — is built by subtracting pore pressure from total stress:

σ′v = σv − u = Σ(γ·Δz) − γ_w·(z − z_w)

Effective vertical stress below the water table

If you work in SI but leave the water unit weight at its US-customary default of 62.4 (pcf), then γ_w dwarfs your kN/m³ soil weights, pore pressure exceeds total stress, and the effective stress collapses to zero — silently zeroing your tip resistance. In SI, the water unit weight must be 9.81 kN/m³.

Set γ_w to match your units

SI: γ_w = 9.81 kN/m³. US customary: γ_w = 62.4 pcf. In the app the Metric / US switch sets and converts this field for you (9.81 kN/m³ ⇄ 62.4 pcf), so the trap is really one for direct API calls — but it's still worth checking first whenever an end-bearing result looks impossibly low.

Sign conventions

The engine uses a single, consistent set of conventions:

Depth is measured downward from the ground surface (or the pile head), positive into the ground. Profiles in the app are drawn with depth increasing downward, the way you read a borehole log.
Lateral deflection is positive in the direction of the applied lateral load.
Axial load is compression-positive: a positive axial force pushes down the pile. Uplift capacity is reported as a separate, positive resistance.
Bending moment and shear follow the beam-column convention of the COM624P formulation. What matters for design is the magnitude and the depth of the peak, both of which the app labels directly.

Sign conventions differ between programs

LPILE and RSPile even use opposite signs for moment and soil reaction in places. When you compare against another tool, compare magnitudes and locations, not raw signs.

Depth & geometry

Soil layers are defined by their top and bottom depths and must tile the profile without gaps or overlaps. The bottom of the last layer should reach at least the pile tip. If a pile stands partly above grade (a pier, or a scoured pile), the free length above the ground surface is entered separately and adds cantilever deflection — see the pile inputs.