Dimensioning a technical drawing refers to the process of adding measurements and numerical values to indicate the size, location, and specifications of various elements in the drawing. These measurements provide important information for understanding the object's size, proportions, and how its components relate to each other.

Dimensioning is crucial in technical drawing for several reasons:

Overall, dimensioning is essential for clear communication, precise manufacturing, efficient assembly, and quality control. It ensures that technical drawings effectively convey the necessary information, leading to accurate and successful realization of the designed object.




There are a few different dimensioning formats that are used to dimension a drawing.  The reason there are different types is based on who the drawing is intended for. If you are sending your part out to be machined and tolerance is important or if you are drawing a architectural drawing you may want to use a different dimensioning format to ensure that the information is clearly displayed. I am only going to go through two of these styles now but know that there are more. 

Chain Dimensioning

In chain dimensioning, dimensions are applied sequentially from one feature to the next, forming a chain-like sequence. Each dimension is referenced to the preceding one, which creates a linear progression of measurements. This method is useful when dimensions are related or when there is a logical sequence of features. However, it can become complex and confusing if there are many interconnected dimensions.

This method of dimensioning can lead to tolerance issues the further down the part you go, with variances in machining being compounded each dimension down the line. To many this is the easiest dimension style to draw though because you are measuring actual feature values. 

Baseline Dimensioning

The second type of dimensioning style is baseline dimensioning. With baseline dimensioning you will continuously pull your dimensions off of the same baseline. This is helpful when machining parts because variations due to tolerance will not be compounded since each dimension is independent of the other dimensions in your drawing. 

Datum Dimensioning

Datum dimensioning, also known as datum reference dimensioning, relies on a set of reference points, lines, or planes called datums to establish a coordinate system for dimensioning and tolerancing. Dimensions are typically referenced to these datums, providing a consistent and standardized frame of reference for measurement. Datum dimensioning is commonly used in geometric dimensioning and tolerancing (GD&T) to specify the location and orientation of features relative to a datum reference frame.

Other Dimensioning Types

Coordinate Dimensioning: In coordinate dimensioning, each feature's location is defined by Cartesian coordinates (X, Y, Z), usually referenced to a common origin or a specific datum. This method is particularly useful for specifying the precise location of features in three-dimensional space, especially in computer-aided design (CAD) environments.

Ordinate Dimensioning: Ordinate dimensioning involves establishing a zero reference point (usually at the intersection of two datum lines) and then dimensioning features using the distance from this reference point along orthogonal lines (X and Y directions). It's often used for linear or rectangular features, providing a clear and consistent method for dimensioning.

Direct Dimensioning: Direct dimensioning, also known as aligned dimensioning, involves placing dimensions directly on the object, aligned with the features being dimensioned. This method is straightforward and intuitive, as dimensions are positioned adjacent to the features they describe. Direct dimensioning is commonly used for simple parts with few features or for dimensioning in 3D CAD models.


Geometric Dimensioning and Tolerancing (GD&T) is a symbolic language used on engineering drawings and 3D CAD models to specify the geometric tolerances and permissible variations in form, size, orientation, and location of features. It provides a precise and standardized way to communicate the design intent to manufacturers and ensures that the parts produced conform to the designer's requirements.

GD&T is widely used in industries such as automotive, aerospace, and manufacturing, where precise dimensional control is essential for ensuring the interchangeability and functionality of mechanical parts. It offers several advantages over traditional dimensioning and tolerancing methods, including clearer communication of design intent, improved manufacturing efficiency, and better product quality and reliability.

Key Components of GD&T

Symbols: GD&T uses a set of symbols, such as squares, circles, triangles, and lines, along with letters and numbers, to define the tolerance zones and geometric characteristics.

Feature Control Frames (FCF): These are the basic building blocks of GD&T. A feature control frame consists of geometric characteristic symbols, tolerance values, and modifiers that specify the acceptable limits of a feature's variation.

Datum Reference Frames (DRF): Datums are reference points, lines, or planes on a part used to establish a coordinate system for dimensional measurements. A datum reference frame consists of datum features and datum planes that establish a set of reference points for dimensioning and tolerancing.

Modifiers: GD&T uses various modifiers to further refine the tolerance zones and control the relationship between features. Common modifiers include maximum material condition (MMC), least material condition (LMC), and regardless of feature size (RFS).

Form Controls: These specify the shape of a feature, such as flatness, straightness, circularity, and cylindricity.

Orientation Controls: These control the orientation of features relative to a specified datum, such as parallelism, perpendicularity, and angularity.

Position Controls: These specify the allowable deviation of a feature's location from its theoretical position, taking into account both the size and orientation tolerances.

Profile Controls: Profile tolerance specifies the allowable deviation of a surface from its true profile within a specified boundary. It is often used to control the overall shape of complex features.