Engineering & Trades Glossary
Plain-language definitions of engineering, construction, and trades terms — each linked to the calculators, charts, and guides that use them.
Every trade has words that gatekeep its math. This glossary defines them the way the calculators use them — each term in plain language, each definition traceable to the same published standards and manufacturer data the tools are verified against.
Entries link straight to the calculators that do the arithmetic, the reference charts that hold the tabulated values, and the guides that walk through the decisions.
A
Air Changes per Hour (ACH)
Ventilation rate as room volumes of air moved per hour: ACH = 60 × CFM ÷ volume in ft³ — or simply m³/h divided by m³ in metric.
American Wire Gauge (AWG)
The North-American conductor sizing system, where smaller numbers mean bigger wires and sizes above 4/0 switch to thousands of circular mils (kcmil).
Ampacity
The maximum current, in amperes, that an insulated conductor can carry under stated conditions without exceeding its insulation temperature rating.
Angle of Internal Friction
The soil strength parameter φ that earth-pressure theory runs on: Rankine’s Ka = tan²(45 − φ/2) and Kp = tan²(45 + φ/2) both flow from it.
Apparent Power (kVA)
The volts-times-amps a supply must deliver, S = P/PF — the kVA figure transformers and generators are rated by, independent of load power factor.
AS568 Dash Number
The U.S. three-digit O-ring size code: the first digit sets the cross-section series, the rest step through inside diameters — a -214 is 0.139″ × 0.984″ ID.
B
Bearing Capacity
The pressure a foundation soil can carry: ultimate capacity from qu = c·Nc + q·Nq + 0.5·γ·B·Nγ, divided by a factor of safety for design.
Bend Allowance
The arc of flat material a bend consumes, measured along the neutral axis: BA = θ·(π/180)·(R + K·T) — the key number in a flat-pattern layout.
Bending Moment
The internal moment a beam section must resist at a given point, found from static equilibrium and plotted as the bending-moment diagram (BMD).
Board Foot
The lumber trade’s volume unit: 1 nominal inch thick × 12 nominal inches wide × 1 foot long, so BF = thickness × width × length ÷ 12.
Bolt Preload
The clamping force a tightened bolt develops — typically 75% of proof load, reached through torque via the short-form relation T = K·F·d.
Bolt Property Class
The ISO 898-1 strength marking on metric bolts: 8.8 encodes roughly 800 MPa tensile strength with yield at 80% of it; classes run 4.6 to 12.9.
Bond Beam
A horizontally grouted course in a block wall — one course-height band of fully grouted masonry, taken off separately from the vertical-core grout.
Box Fill
The NEC 314.16 accounting of everything inside an electrical box — conductors, clamps, yokes, grounds — as volume allowances the box must equal or exceed.
BTU (British Thermal Unit)
The US customary heat unit — 1 BTU (IT) = 1,055.056 J per NIST SP 811 — with BTU/h as the rating unit for heating and cooling equipment.
Buckling
Sudden sideways instability of a compressed member at the Euler critical load Pcr = π²EI/(KL)² — a stiffness failure, not a strength failure.
C
CFM (Cubic Feet per Minute)
The US volumetric airflow unit for fans and blowers — 1 CFM = 4.719474×10⁻⁴ m³/s per NIST SP 811, about 1.699 m³/h or 0.4719 L/s.
Chain Pitch
The repeating link dimension of a roller chain, encoded in its ANSI number: the tens digits are the pitch in eighths of an inch, so #40 chain is 1/2 in.
Chip Load
Feed per tooth — the thickness of material each cutting edge removes per revolution, the bite size that milling feed rates are built from.
Coefficient of Performance (COP)
Heat delivered per unit of electrical input for a heat pump — a COP of 4.4 means a 10 kW load draws only 2.27 kW, versus COP 1 for resistance heat.
Coefficient of Thermal Expansion
The strain a material develops per kelvin, α — carbon steel grows about 12×10⁻⁶ of its length per degree, driving ΔL = α·L·ΔT movement checks.
Common Rafter
The straight sloped member running from wall plate to ridge — laid out from run and pitch, and span-checked on its horizontal projection, not its length.
Compound Miter
A cut that swings the miter table and tilts the blade at once — how crown molding is cut lying flat, e.g. 31.62° miter with 33.86° bevel for a square corner.
Compressive Strength (f′c / fck)
The specified 28-day strength of concrete under compression — f′c from cylinder tests in US practice, fck cylinder/cube pairs in Eurocode classes.
Concrete Cover
The clear concrete thickness between a rebar surface and the face of the member — it reduces effective depth and directly drives crack spacing.
Concrete Masonry Unit (CMU)
The hollow concrete block walls are laid from — conventional two-core units in 6, 8, 10, and 12 in widths whose cells take reinforcement and grout.
Conduit Fill
The fraction of a raceway’s internal area occupied by cables — capped at 40% for more than two conductors in conduit per NEC Chapter 9.
Continuous Load (NEC)
An NEC load expected to run at maximum current for three hours or more — conductors and standard breakers must be sized at 125% of it.
Cord (Firewood)
The legal firewood unit per NIST Handbook 130: the wood contained in 128 cubic feet when ranked and well stowed — the only basis sellers may quote.
Cut and Fill
Balancing excavation against embankment along an alignment: integrate the cut and fill areas between survey stations, then net them for surplus or borrow.
Cutting Speed (SFM)
The surface speed between cutting edge and workpiece, in surface feet per minute or m/min — the tooling number spindle RPM is computed from.
D
Dead Load
The permanent, fixed weight a structure always carries — written G in Eurocode and D in ASCE 7 — factored differently from variable loads in design.
Deflection
How far a loaded member bends from its unloaded position — a serviceability quantity checked against span-over-limit ratios such as L/360.
Degree Day (HDD / CDD)
A climate severity tally: each day contributes max(0, Tbase − Tmean) heating degree-days, summing how far and how long it stays cold (or hot, for CDD).
Density
Mass per unit volume, ρ — water near 1,000 kg/m³, structural steel 7,850, aluminum 2,700, normal-weight concrete about 2,400 (≈150 lb/ft³).
Development Length
The embedment a deformed rebar needs in concrete to develop its full yield strength before it can be counted on in design — ld per ACI 318-19 §25.4.
Dew Point
The temperature at which air becomes saturated when cooled at constant pressure and moisture content — set by the actual vapour pressure alone.
E
Effective Throat
The narrowest section of a fillet weld, running from the root diagonally out to the face — 0.707 × leg for a 45° fillet — where all strength checks live.
Electrode Classification (E6010, E7018)
The AWS A5.1 stick electrode code — E6010, E7018 — whose digits encode minimum tensile strength, welding position, and coating with its usable current.
Equipment Grounding Conductor (EGC)
The fault-clearing conductor run with every circuit — sized from the overcurrent device per NEC Table 250.122 so faults trip the breaker fast.
F
Factor of Safety
The ratio of resisting capacity to driving demand — FoS = 1 is limiting equilibrium, and design practice requires margins like 1.3–1.5 or 3.
Fault Current
The available short-circuit current at a point in an electrical system — it sets the interrupting (AIC) ratings breakers and fuses must carry.
Feed Rate
How fast tool and work advance past each other — RPM × chip load × flutes for milling, RPM × feed per revolution for drilling and turning, in in/min or mm/min.
Fillet Weld
The triangle of weld metal laid into the corner where two plates meet — sized by its leg on the drawing, but failing across its smaller diagonal throat.
Fixture Unit (DFU / WSFU)
A plumbing load unit assigned per fixture — drainage (DFU) and water supply (WSFU) totals convert to design flow and pipe size via the Hunter curve.
Flow Coefficient (Cv / Kv)
A valve’s capacity rolled into one number: Cv is the US gpm of 60 °F water passed at a 1 psi drop; the metric Kv uses m³/h at 1 bar, Kv = 0.865·Cv.
Free Air (Compressed-Air Systems)
Air volume expressed at atmospheric conditions — the units tool demand is measured in, converting what a receiver stores into what the tools consume.
Friction Loss (Head Loss)
The energy flowing fluid surrenders to pipe-wall friction, expressed as lost head — computed by Darcy-Weisbach or, for water, Hazen-Williams.
G
Galvanic Corrosion
Accelerated corrosion when dissimilar metals share a damp joint — the more anodic metal is consumed protecting the other, as zinc is against stainless.
Grounding Electrode Conductor (GEC)
The conductor tying a service’s grounded conductor to the earth electrode system, sized from the service-entrance conductors per NEC Table 250.66.
H
Header
The built-up beam over a window, door, or garage opening in a bearing wall, carrying the load the missing studs would have taken — sized per IRC R602.7.
Heat Input (Welding)
Arc energy per length of weld — HI = 60 × V × I / (1000 × travel speed) in kJ/in or kJ/mm — the number welding procedure specifications limit.
I
K
K-Factor (Sheet Metal)
Where the neutral axis sits in a bend, as a fraction of sheet thickness from the inside face — 0.44 is the air-bent mild-steel baseline, never above 0.50.
Key and Keyway
The square or rectangular bar and matching slots that lock a hub to a shaft, sized from shaft diameter per ASME B17.1 and checked in shear and bearing.
L
Latent Heat
The moisture-removal side of a cooling load — from occupant respiration and outdoor-air humidity — paid in condensed water rather than temperature change.
Ledger Board
The horizontal board bolted to the house band joist that carries one edge of an attached deck — its fastener schedule is set by IRC Table R507.9.1.3(1).
Live Load
The variable, movable load from occupancy and use — people, furniture, stored goods — carrying the largest factors in code load combinations.
Lumber Grade
The certified strength floor stamped on a board — No. 2, Select Structural — that span tables assume, since a 2x10 is a shape, not a strength.
Luminous Efficacy
Lumens of light per watt of electricity — the measure that separates halogen at 15–25 lm/W from fluorescent and from LED at 80–150 lm/W.
M
Manning's Roughness Coefficient (n)
The empirical boundary-roughness factor in Manning's equation — n = 0.013 for finished concrete, higher for rougher linings, and flow falls as n rises.
Material Removal Rate (MRR)
The volume of material a cut removes per minute — feed × depth × width for milling — the figure compared against the machine’s spindle-power capability.
Modulus of Elasticity (E)
A material stiffness constant relating stress to elastic strain — the E in beam deflection and column buckling formulas, in MPa or GPa.
Moment of Inertia (Second Moment of Area)
The geometric measure of how a cross-section’s area is spread about an axis (I, in mm⁴ or in⁴) — the shape’s contribution to bending stiffness.
N
Net Positive Suction Head (NPSH)
The suction-side head a pump installation supplies above the liquid’s vapor pressure — NPSHa must exceed the pump’s NPSHr or it cavitates.
Nominal Pipe Size (NPS)
The naming convention for pipe: each NPS fixes one outside diameter regardless of wall schedule — a “2-inch” pipe is always 2.375 in OD.
Nominal vs Actual Lumber Size
A 2x4 measures 1-1/2 × 3-1/2 in: nominal names the rough-sawn green size, and PS 20 fixes the smaller actual dressed size after drying and surfacing.
O
O-Ring Squeeze
The percentage an O-ring cross-section is compressed by its gland — (W − L)/W × 100 — roughly 15–30% for static radial seals per the Parker handbook.
Offset Multiplier
The memorized trade constant that turns an offset height into distance between bends — 5.76, 2.61, 2.0, 1.41, 1.15 — each just 1/sin(angle), rounded.
Ohm’s Law
The defining relation of a resistive circuit, V = I × R — paired with P = V × I it turns any two of volts, amps, ohms, and watts into the other two.
On-Center (OC) Spacing
Framing layout measured centerline to centerline of repeating members — 16 or 24 inches on center for studs, 12/16/24 for deck joists.
Overcurrent Protective Device (OCPD)
The breaker or fuse guarding a circuit — selected per NEC 240.4, driving both the conductor pairing and the equipment grounding conductor size.
P
Pilot Hole
The hole drilled before driving a screw or lag so dense wood removes material instead of splitting — sized to the fastener, larger in hardwood than softwood.
Pipe Pressure Rating
The internal pressure a pipe is rated to carry — hoop-stress arithmetic, P = 2·S·t/(D − t), derated for temperature and threading and exclusive of surge.
Pipe Schedule
The wall-thickness series of a pipe (Sch 40, 80, 160): higher schedule means a thicker wall and smaller bore inside the same fixed outside diameter.
Pitch Diameter
The thread diameter where groove and ridge are equally wide — the surface where flanks engage, loads transfer, and go/no-go gauging happens.
Power Factor
The ratio of real power (kW) to apparent power (kVA) in an AC system — low PF means more current, more losses, and often a utility penalty.
Proctor Density (Maximum Dry Density)
The maximum dry unit weight a soil reaches in the laboratory Proctor test — the yardstick field compaction is graded against (ASTM D698 / D1557).
Proof Load
The tension a bolt must carry without permanent set — tensile stress area times proof strength — the anchor torque charts hang their 75% preload target on.
Punching Shear
Two-way shear where a column tries to punch through a flat slab — checked on a control perimeter at d/2 from the column face per ACI 318.
R
R-Value
A layer’s thermal resistance — R per inch times thickness — where bigger means better insulation; the reciprocal of the assembly’s U-value.
Reynolds Number
The dimensionless ratio of inertial to viscous forces, Re = ρvD/μ — it decides whether pipe flow is laminar (below 2300) or turbulent (above 4000).
Rise and Run
The vertical and horizontal legs of a slope: stair design divides total rise into uniform risers and treads, checked against code and comfort rules.
Rolling Offset
A pipe offset that moves over and up at the same time — the true offset is the box diagonal √(rise² + roll²), then the usual travel math applies.
Roof Pitch
A roof’s steepness stated as rise per 12 inches of run (e.g. 6/12) — convertible to degrees and percent, and classified from flat to steep.
Runoff Coefficient
The fraction of rainfall that becomes direct runoff (C, 0–1) — the land-cover factor in the rational method peak-flow formula Q = C·i·A.
S
Section Modulus
A cross-section property, S = I/c, that converts a bending moment directly into extreme-fibre bending stress; the primary number for shortlisting beams.
Sensible Heat
The part of a cooling load that changes air temperature — envelope, solar, lighting, equipment, and people — as opposed to the moisture-removal part.
Shear Force
The net transverse force acting on a beam cross-section, taken positive when the resultant to the left of the cut acts upward; plotted as the SFD.
Sheet Metal Gauge
A material-specific thickness number where higher means thinner: 16 gauge is 0.0598 in steel, 0.0635 galvanized, 0.0625 stainless, 0.0508 aluminum.
Shielding Gas
The gas envelope that protects a molten weld puddle from air — bottled CO₂/argon mixes in MIG, inert argon in TIG, flux-generated in stick welding.
Slump
The workability specification for fresh concrete, stated in mm (e.g. 75–100 mm) — one of the design inputs that sets mixing water in ACI 211.1.
Specific Heat Capacity
The heat that raises a unit mass of fluid one degree — cp ≈ 4.186 kJ/kg·K for water — linking thermal load to flow through Q = m·cp·ΔT.
Spring Angle
The angle crown molding leans off the wall — 38° for standard US 52/38 stock, 45° and 52° for other profiles — the input every compound cut starts from.
Square (Roofing)
The roofing trade’s unit of area: 1 square = 100 sq ft of roof surface, the basis shingle bundles, coverage, and estimates are all quoted in.
Static Pressure
The pressure a fan or pump must develop against a duct or pipe system’s resistance — straight-run friction plus fitting losses plus static head.
Surcharge Load
Extra load sitting on retained or founding soil — a uniform surcharge q adds a rectangular wall-pressure block K·q·H acting at mid-height.
Swell Factor
How much excavated soil bulks past its bank volume — 100 yd³ in place at 25% swell loads out as roughly 125 loose cubic yards of spoil.
T
Take-Up (Conduit Bending)
The bender deduct for a 90° stub-up — the length the bend itself consumes — subtracted from the stub height to place the mark: 5″ for 1/2″ EMT, 6″ for 3/4″.
Tap Drill Size
The hole diameter drilled before tapping an internal thread, from the handbook rule drill = major − 1.29904 × pitch × %engagement ÷ 100.
Tensile Strength (Ultimate)
The ultimate stress a material is rated for — the figure metric bolt classes encode: an 8.8 bolt carries roughly 800 MPa tensile, yielding at 80% of it.
Tensile Stress Area
The effective cross-section a threaded fastener breaks across — smaller than the shank, larger than the minor-diameter area — that bolt ratings are built on.
Thermal Bridging
Extra heat loss where a junction or conductive element bypasses the insulation — quantified per metre as the psi-value under EN ISO 10211.
Thread Engagement Length
The depth of thread actually carrying load in a tapped joint — sized so the bolt breaks before either thread strips, per the FED-STD-H28 formulas.
Thread Pitch
The spacing of a metric thread in millimetres — the second number in M10×1.5 — with inch threads quoting its reciprocal as threads per inch.
Threads Per Inch (TPI)
How inch threads state their pitch: the count of thread crests per inch — a 1/4-20 UNC bolt has 20, and its pitch is the reciprocal, 1/20 inch.
Tolerance Class (ISO 286)
An ISO 286 designation such as H7 or p6 that fixes a pair of limit deviations for a nominal size, so a fit callout fully determines clearance or interference.
Ton of Refrigeration
The cooling rate that melts one US short ton of ice in 24 hours — standardized as exactly 12,000 BTU/h = 3,516.853 W, about 3.517 kW.
Torque
A moment — force times lever arm — in N·m or ft·lb (1 ft·lbf = 1.355818 N·m), tied to shaft power through P = T × ω.
Total Dynamic Head
The total energy a pump must add to the fluid at design flow — static lift plus friction losses plus velocity head, expressed in metres of head.
Tributary Area
The patch of floor or deck whose load drains to one supporting member — tributary width times spacing — used to compute post and footing loads.
U
U-Value (Thermal Transmittance)
The heat flowing through each unit of area per degree of temperature difference (W/m²K) — the reciprocal of R, where smaller means a better assembly.
Underlayment
The rolled layer that goes down before the finish surface — under shingles on a roof deck, under planks on a subfloor — bought by nominal roll coverage.
V
Viscosity (Dynamic & Kinematic)
A fluid’s internal resistance to flow, in two forms: dynamic μ (Pa·s) and kinematic ν = μ/ρ — the property pair behind Reynolds number and friction.
Voltage Drop
The voltage lost along a conductor carrying current — kept within the NEC 3% branch / 5% total recommendation to protect equipment performance.
W
Waste Factor
The extra material added to a takeoff for offcuts the layout cannot reuse — 10% for straight flooring lays, 15% diagonal, 20% herringbone.
Water Hammer
The transient pressure surge when flow in a pressurised pipeline stops suddenly — bounded by the Joukowsky equation dP = ρ·a·Δv for rapid closure.
Water-Cement Ratio
The mass of mixing water divided by the mass of cement — the single number that most controls concrete strength; lower w/c means stronger concrete.
Welded Wire Reinforcement (WWR / Mesh)
Factory-welded wire sheets or rolls per ASTM A1064 — slab steel whose job is holding shrinkage cracks tight between joints, not carrying design moment.
Welding Polarity (DCEP / DCEN / AC)
Which terminal the electrode runs on — DCEP, DCEN, or AC alternating both every half-cycle — deciding penetration, heat split, and which rods will run.
Wet-Bulb Temperature
The temperature air reaches through evaporative cooling — always between the dew point and the dry-bulb, meeting them only at 100% relative humidity.