Piping take-off glossary

Plain definitions of the words used in piping take-off, Navisworks and estimating, from bill of materials to zero-length pipe. 46 terms, each with its own link.

MTO, BOM and BoQ: the difference

An MTO is the measured list of materials for the whole job: pipe in metres or feet, everything else counted.
A BOM is the parts list for one assembly or drawing, such as the table printed on an isometric.
A BoQ is the list of priced work items a contractor tenders against. The MTO feeds its pipework items; see how NRM2 measures pipework.

Take-off and estimating

Material take-off (MTO): The list of materials a job needs, measured from drawings or a model: pipe in metres or feet, fittings, flanges and valves counted. Estimators price from it and procurement buys from it. See: A free piping MTO template.
Take-off (quantity take-off): Measuring quantities from drawings or a model. A piping take-off produces the MTO. See: Take-off from a Navisworks model with Metriq MTO.
Bulk MTO: An early take-off from preliminary design, used for budget prices and long-lead orders. It usually carries an allowance for what is not yet designed.
Detailed MTO: A take-off from the finished design or model, line by line, used for the firm price and for buying.
Bill of materials (BOM): A list of the parts needed to build something. In piping, the BOM printed on an isometric lists the parts for that drawing; the project MTO adds them up across the job.
Bill of quantities (BoQ): A list of work items and quantities, usually measured to a standard method, that contractors price at tender. For building works in the UK, the RICS rules for measuring a BoQ are NRM 2 (Detailed measurement for building works). A piping MTO feeds the pipework items of a BoQ. See: Getting an MTO into a pricing tool.
Line-based MTO: An MTO with one row per line, item and size, rather than one total per item. It can be checked against isometrics and priced by the lines a revision changed. See: Comparing quantities between revisions.
Cut length: The length of a straight piece of pipe between fittings, as it is cut in the shop. The net pipe length is the sum of cut lengths; waste and cutting allowances are added on top when pricing.
Waste allowance: A percentage added to the net pipe length when pricing, to cover offcuts and cutting losses. It belongs in the price, not in the measured take-off.

Piping components

Fitting: A part that changes the direction, size or branching of a pipe: elbows, tees, reducers, caps and couplings. Counted in each. Factory-made steel butt-welding fittings are covered by ASME B16.9. See: Counting fittings and valves in Navisworks.
Elbow (bend): A fitting that changes the direction of a pipe, usually by 90° or 45°.
Tee: A fitting with a branch. In an equal tee the branch is the same size as the run; in a reducing tee it is smaller, so the tee carries two sizes.
Reducer: A fitting that joins two pipe sizes. A concentric reducer keeps both centrelines in line; an eccentric reducer keeps one side flat, often to keep the top or bottom of a pipe level.
Cap: A fitting that closes the end of a pipe permanently.
Flange: A ring at the end of a pipe, fitting or valve that is bolted to a matching flange, making a joint that can be taken apart. Flanges are counted by size, type and rating. Steel flanges rated by class, up to NPS 24, are covered by ASME B16.5; steel flanges designated by PN, by BS EN 1092-1.
Weld neck flange: A flange with a long tapered hub that is butt-welded to the pipe. Common on higher-pressure and process lines.
Slip-on flange: A flange that slides over the pipe and is fillet-welded in place.
Blind flange: A flange with no bore, used to close off a pipe end or a nozzle.
Gasket: The seal between two flanges in a flanged joint.
Bolt set: The studs or bolts and nuts for one flanged joint, sized by the flange.
Valve: A part that starts, stops or controls flow: gate, globe, ball, butterfly and check valves, among others. Counted in each, by type, size and rating.
Spool: A section of pipe with its fittings and flanges, made in the shop and installed as one piece.

Sizes and specifications

Nominal size (NB, NPS): The size name of a pipe, not its actual inside or outside diameter. Nominal pipe size (NPS) is given in inches; nominal bore (NB) is the same idea under a British name.
DN: The metric nominal size, for example DN 50, which corresponds to NPS 2. Like NPS, it is a name for the size, not a measured diameter. DN is defined in ISO 6708, published in the UK as BS EN ISO 6708.
Outside diameter (OD): The actual outside diameter of a pipe. For a given nominal size the OD is fixed, and the wall thickness changes with the schedule.
Schedule: The wall thickness designation of a pipe for a given nominal size, for example Schedule 40 or Schedule 80. Wrought steel pipe dimensions are standardised in ASME B36.10M; stainless steel pipe has its own standard, ASME B36.19M, whose schedule numbers carry the suffix S.
Pressure rating (class, PN): The pressure a flange or valve is rated for: Class 150, 300 and so on in the imperial system, PN 16, PN 40 and so on in the metric one. For steel flanges, class ratings for flanges up to NPS 24 come from ASME B16.5 and PN designations from BS EN 1092-1: two separate standards.
Piping spec (piping class): The document that sets which components, materials and ratings are allowed for a service. Two lines of the same size can use different parts because their specs differ.
Line number: The identifier of a pipeline. It usually carries the service, a sequence number and often the size and spec, and it is how an MTO is split by line.
Line list: The table of every line in a project, with its service, size, spec and design conditions.

Drawings and models

Isometric (iso): A drawing of one line, or part of one, in isometric view, with dimensions and usually a bill of materials. Isos are produced from the design model for fabrication. See: Isometric readers and model take-off compared.
P&ID: Piping and instrumentation diagram: a schematic of the process lines, valves and instruments. It is not to scale, so it can give valve and instrument counts but not lengths.
AutoCAD Plant 3D: Autodesk’s plant design software. Its models can be exported to Navisworks with the part properties, such as class, size, length and line number. See: Plant 3D piping MTO when you only have the NWD.
Revit: Autodesk’s BIM software, used for building services piping. In Navisworks its piping objects sit in categories such as Pipes, Pipe Fittings and Pipe Accessories.
DWF: Design Web Format, an Autodesk file format for publishing designs. Navisworks can read it, and some piping models arrive that way.
Federated model (federation): Several source models appended into one Navisworks file, typically one per discipline or package, so they can be reviewed together.
Plain mesh: Geometry exported without its properties: the shape of a pipe or a valve with no class, size, length or line attached. It looks right in 3D, but there is nothing to count by, so a take-off has to be measured by hand. See: Alternatives to Navisworks Quantification.
Duplicate objects: The same object present twice in a federation, for example because a source model was appended twice. Copies sit on top of each other, look normal in 3D and inflate every total. See: Duplicate pipes in a federated model.

Navisworks

Navisworks: Autodesk’s model review software, in three editions: Manage, Simulate and Freedom, the free viewer. It combines models from different tools into one file for review.
NWD, NWC and NWF: An NWD is a single published file that contains the model data and is what is usually sent out. An NWC is the cache Navisworks writes when it opens a native model such as a Revit or DWG file. An NWF is a working file that links to its source files instead of containing them.
Navisworks Quantification: The take-off tool in Navisworks Manage and Simulate. It measures what an item catalogue and property mapping define and exports to Excel. See: Navisworks Quantification compared.
Item Catalogue: In Quantification, the list of take-off items you define (what is measured, and in which units) and organise into a WBS. It is built before any quantity comes out and is usually kept per model family. See: Navisworks Quantification compared.
Property mapping: In Quantification, the link between a model property and a take-off field, for example which property holds a pipe’s length.
Search set and selection set: A search set is a saved Navisworks search that selects every object matching its conditions each time it runs, so it picks up changes in a new revision. A selection set keeps the same objects it was saved with. See: Search sets for a repeatable export.
Selection Tree: The Navisworks panel that lists the model as a hierarchy: files, layers or categories, objects and their geometry.
Zero-length pipe: A pipe that reads 0 m in a Navisworks take-off because no length property was published or mapped. Quantification reads properties; it does not measure the solid. See: Why Navisworks shows zero length for pipes.

For the full picture, start with how to get a piping MTO from a Navisworks model or browse all piping take-off guides. To see the terms in a real workbook, download the sample take-off.

Sources

Links checked in October 2026. Menu names and pages can move between releases.