6.1 Isometric Drawings

An isometric drawing is a type of pictorial drawing that shows an object or space in three dimensions, with its width, depth and height drawn along three axes equally inclined to the picture plane. Because all three axes are treated equally, measurements can be taken directly along them, so isometric drawings are both easy to read and accurate enough to measure from. Interior designers use them to show furniture, joinery, room layouts and construction details clearly to clients, contractors and fabricators. Example: a designer explaining a built-in reception counter for a microfinance office draws it in isometric, so the client sees the front, top and side at once, and the joiner can read the sizes directly from the drawing.

6.1.1 Principles of isometric projection

Isometric projection is based on viewing an object so that its three principal edges, width, depth and height, appear equally foreshortened and are equally spaced around a central point. The word isometric means "equal measure", because the same scale applies along all three axes. These principles make it possible to produce a single drawing that shows three faces of an object in their true proportions. Example: an isometric of a kitchen island shows its front, side and worktop in one view, with all edges measurable.

Key principles

• Three axes: One vertical axis for height and two axes at 30° to the horizontal for width and depth.
• Equal angles: The three axes are 120° apart.
• Equal scale: The same scale is used along all three axes.
• Parallel lines stay parallel: Lines that are parallel on the object remain parallel in the drawing.
• No vanishing points: Objects do not get smaller with distance.
• Three faces visible: Usually the top, front and one side are shown.

6.1.2 Equal angles of 120 degrees between axes

In isometric drawing the three axes meet at a point and are spaced exactly 120° apart, so together they make a full 360°. This equal spacing is what gives isometric drawings their balanced, symmetrical appearance and allows every axis to be measured in the same way. In practice, the vertical axis is drawn straight up, and the two receding axes are drawn at 30° above the horizontal to the left and right. Example: when drawing a cube-shaped storage unit for a children's library, the designer sets the three corner edges at 120° apart to show it accurately.

Points about the 120° axes

• Vertical axis: Shows height and is always drawn vertical.
• Left receding axis: Drawn at 30° above horizontal to the left.
• Right receding axis: Drawn at 30° above horizontal to the right.
• Equal spacing: The angle between each pair of axes is 120°.
• Using a 30/60 set square: Placed on a T-square, it produces the 30° axes accurately.

Worked example: checking the axis angles

Sum of angles around the meeting point = 3 × 120°

Sum = 360°, which confirms the three axes are equally spaced around a full turn

6.1.3 Uniform scaling along isometric axes

Uniform scaling means the same scale is applied to measurements along all three isometric axes, so a length of 1 m on the object is drawn the same length whether it is a width, depth or height. In a true isometric projection, lengths are reduced to about 81.6% using an isometric scale, but in an isometric drawing, the version most used in practice, true lengths are used for simplicity. Example: in an isometric of a wardrobe for a staff housing project, the designer uses the same 1:20 scale for height, width and depth.

Isometric projection and isometric drawing compared

Worked example: isometric scale

Isometric length = true length × 0.816

A cabinet edge is 1,000 mm long.

Isometric length = 1,000 × 0.816

Isometric length = 816 mm in an isometric projection, while an isometric drawing shows it as 1,000 mm at the chosen scale

6.1.4 Parallel lines remain true to scale

In isometric drawings, lines that are parallel on the real object stay parallel in the drawing and, when they lie along or parallel to an isometric axis, keep their true scaled length. This makes it easy to draw repeated elements, such as shelves or panels, and to take measurements directly. Lines that are not parallel to an axis, called non-isometric lines, are not true length and must be found by plotting their end points. Example: in an isometric of a bookshelf for a teachers' resource centre, all the shelf edges are drawn parallel and to the same scaled length.

Rules for parallel and non-isometric lines

• Parallel edges stay parallel: Top and bottom edges of a box are drawn parallel.
• Isometric lines are measurable: Lengths along the axes are true to scale.
• Non-isometric lines are not measured directly: Sloping edges such as a chair back are found by locating their ends.
• Repetition is easy: Equal parallel elements are drawn the same length.
• Accuracy check: Lines that should be parallel can be checked with a set square.

6.1.5 No perspective distortion; true dimensions

Unlike perspective drawings, isometric drawings have no vanishing points, so objects do not appear smaller as they go back in space. This means dimensions along the axes are shown at their true scaled size, making isometric drawings suitable for measuring and for showing construction details. The trade-off is that isometric views look slightly less natural than perspectives. Example: a fabricator building a display stand for a mobile phone retailer uses an isometric drawing to read the true sizes of each part.

Effects of having no perspective distortion

• Measurable drawings: Sizes can be read directly along the axes.
• Equal sizes at any depth: Objects at the back are drawn the same size as those at the front.
• Clear construction information: Useful for joinery and assembly drawings.
• Simple to draw: No vanishing points or horizon lines are needed.
• Less realistic look: Large spaces can appear slightly distorted to the eye.

6.1.6 3D representation of width, height, and depth

Isometric drawings show all three dimensions, width, height and depth, in one view, which helps viewers understand the form and size of objects and spaces without needing several separate views. Each dimension is measured along its own axis: height on the vertical axis, width and depth on the two 30° axes. Example: a designer shows a client how a built-in desk unit fits a study nook by drawing it in isometric with its full width, depth and height.

Representing the three dimensions

• Width: Measured along one 30° axis, usually the left or right.
• Depth: Measured along the other 30° axis.
• Height: Measured along the vertical axis.
• Three faces: Top, front and side are visible together.
• Combined understanding: The viewer grasps the overall form quickly.

Worked example: drawing a cabinet in isometric

Drawn length = actual length ÷ scale

A cabinet for a veterinary clinic is 1,200 mm wide, 600 mm deep and 900 mm high, drawn at 1:10.

Width = 1,200 ÷ 10 = 120 mm along one 30° axis

Depth = 600 ÷ 10 = 60 mm along the other 30° axis

Height = 900 ÷ 10 = 90 mm along the vertical axis

6.1.7 Depth indicated by a 30-degree angle from horizontal

In isometric drawings, depth and width are shown by lines drawn at 30° from the horizontal, rising to the left and right. This angle is easily drawn with a 30/60 set square and gives a balanced view of both side faces of an object. Measurements are taken along these sloping lines, not horizontally. Example: drawing a rug on an isometric floor plan, the designer sets out its sides along the 30° lines so it sits correctly in the room.

Drawing the 30° lines

• Use a T-square and set square: The 30/60 set square on a T-square gives accurate 30° lines.
• Measure along the slope: Lengths are marked along the 30° lines, not horizontally.
• Keep both directions: Lines rise at 30° to both left and right.
• Use isometric grid paper: Pre-printed grids speed up freehand isometric sketches.
• Check consistency: All depth and width lines should be at exactly 30°.

Worked example: horizontal and vertical offsets of a 30° line

Horizontal offset = length × cos 30°

Vertical offset = length × sin 30°

A 100 mm line is drawn along a 30° axis.

Horizontal offset = 100 × 0.866 = 86.6 mm

Vertical offset = 100 × 0.5 = 50 mm

6.1.8 Start with basic geometric shapes for form

Complex objects are easier to draw in isometric if they are first broken down into simple geometric forms, such as boxes, prisms and cylinders, which are then added to or cut away. This approach keeps proportions correct and makes the drawing process logical. Example: an isometric of an armchair for a hotel lounge starts as one box for the seat base and arms, then the cushions and curved back are shaped within it.

Using basic shapes

• Rectangular prisms: Represent cabinets, tables, beds and rooms.
• Cylinders: Represent round tables, lamps and columns.
• Triangular prisms: Represent sloping parts such as stairs and roof forms.
• Adding forms: Combine shapes to build up complex objects.
• Subtracting forms: Cut away parts of a box to create recesses and openings.

6.1.9 Maintain clarity and readability; avoid excessive detail

An isometric drawing should communicate the form and key features of an object clearly, without so much detail that it becomes confusing. Designers include only the information needed for the drawing's purpose, using line weight and simple shading to help the viewer read the form. Example: an isometric presented to a SACCO's board to explain a new teller counter shows the main forms and materials clearly, leaving fixing details for separate drawings.

Guidelines for clear isometric drawings

• Include essential features only: Show the main forms and key details.
• Use line weight: Draw outer edges thicker and inner edges thinner.
• Omit hidden lines: Hidden lines are normally left out of isometric drawings.
• Use simple shading: Light tone on one face can help show form.
• Keep labels neat: Place labels and dimensions outside the main drawing where possible.

6.1.10 Consistent orientation of objects for easier interpretation

Consistent orientation means drawing objects in the same direction and from the same viewpoint, usually with the front face towards the viewer on the left or right, so that a set of drawings can be compared easily. Objects should be oriented to show their most important faces and features. Example: in a furniture catalogue for a hospital fit-out, all beds, cabinets and chairs are drawn with the front face on the left, so staff can compare items quickly.

Guidelines for orientation

• Show the most informative faces: Choose the view that shows key features.
• Keep the same orientation in a set: Use the same viewpoint for related drawings.
• Place the longest side along one axis: This gives a clear and balanced view.
• Avoid hiding features: Rotate the object if important details would be hidden.
• Match plans and elevations: Orient the isometric to agree with the orthographic views.