Basic dimension
A number in a rectangle is a basic dimension: a theoretically exact value with no tolerance of its own. The tolerance for the feature it locates or shapes is in a feature control frame.
Easily confused with
How to read it
Cogorno’s guide puts the definition in one line: a theoretically exact dimension is called a basic dimension. The notes from Illinois’s ME 170 course say the box marks it, and that basic dimensions are considered exact. They locate the true position of a feature from its datums (National Physical Laboratory; Cogorno) and define the true profile that a profile tolerance is measured from (Cogorno). The variation allowed comes from a Position (⌖), profile or Angularity (∠) frame; the boxed number itself has no plus-or-minus, and Berez’s UNC Charlotte notes call basic dimensions theoretically exact.
Also searched as: basic dimension gd&t, basic dimension symbol, boxed dimension.
This explains what a symbol specifies on a drawing. It isn't inspection or design guidance; the drawing and the edition of the standard it cites take precedence.
Look-alikes
A number in parentheses is a reference dimension, given for information only (Illinois ME 170 notes; Cogorno lists it as “Dimension, Reference (60)”). The box of a basic dimension holds one number; a feature control frame is longer and divided into compartments. An ordinary dimension has a tolerance of its own: Cogorno’s guide says every dimension needs one except those marked reference, maximum, minimum or stock.
Differences by standard
ISO calls the same boxed value a theoretically exact dimension, TED (Keyence).
In CAD models
On model-based definitions, NIST and UNC Charlotte’s seminar notes say, drawings are often minimally dimensioned: basic dimensions aren’t shown but are queried from the CAD data, and the drawing carries a note referring to that data for them.
Read next
- Next in GD&T symbolsMaximum material condition (Ⓜ)Ⓜ, a capital M in a circle, is the maximum material condition modifier. After a tolerance, it means the tolerance applies when the feature holds the most material, the smallest hole or the largest pin, and grows as the feature departs from that size.
- An ordinary dimension carries its own tolerance; a boxed basic dimension has none.Dimension lineA line carrying a measurement: the distance between the two points its ends touch.
More GD&T symbols
The same shape elsewhere
Browse symbols drawn with letters and numbers or squares and rectangles.
Sources
- Fundamentals of Geometric Dimensioning and Tolerancing (GD&T), Part I, Jaime Berez, seminar adapted from an undergraduate mechanical engineering course, version 2.1.0, 2023, Center for Precision Metrology, University of North Carolina at Charlotte (Zenodo), Slide 35
- Geometric Dimensioning and Tolerancing for Mechanical Design: Instructors’ Guide, Gene R. Cogorno, guide to the McGraw-Hill textbook, rev. 10/12/2020, Technical Training Consultants, Chapters 3 and 11, review answers
- Engineering Drawing Notes, Unit 8: Terms and Symbols (Part C, Geometric Dimensioning & Tolerancing), ME 170, fall 2019, Grainger College of Engineering, University of Illinois Urbana-Champaign, Unit 8, Basic dimension
- Fundamental Good Practice in the Design and Interpretation of Engineering Drawings for Measurement Processes, Measurement Good Practice Guide No. 79, David Flack and Keith Bevan, 2005, National Physical Laboratory (UK), Chapter 4, Tolerances of location
- GD&T Symbols, GD&T Fundamentals, KEYENCE America, Associated Symbols
- Fundamentals of Geometric Dimensioning and Tolerancing (GD&T), Part II, Jaime Berez (UNC Charlotte) and Maxwell Praniewicz (NIST), seminar slides, National Institute of Standards and Technology, Slides 17 and 18
Researched, edited and approved by Rory Hansen before publishing. Last reviewed October 9, 2026. How we research
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