How to read blueprints
Start with the title block, learn the sheet prefixes and the common symbols, then use the scale and the dimension strings to find real sizes.
Read the title block first
Every sheet has a title block, the information panel in the lower-right corner. Before reading anything else, read it: it says what the sheet shows, who drew it, which scale to use, and whether you have the current revision.
What each field means
Sheet number gives the discipline and the sheet's place within it. A1.01 is architecture, sheet 1 of the floor plan series; S2.03 is structural, sheet 3 of the framing series.
Scale is the ratio between the drawing and the building. Every measurement taken from the sheet uses it, and sheets in one set often use different scales, so check each sheet.
Revision and date tell you whether this is the current drawing. Sheets on active projects are revised often; confirm you have the latest set before measuring or ordering materials.
Sheet types
A construction set is organised by discipline, each with a letter prefix. Knowing the prefix gets you to the right sheet quickly.
Disciplines and their sheets
| Prefix | Discipline | What the sheets show | Typical series |
|---|---|---|---|
| A | Architectural | Floor plans, exterior elevations, building and wall sections, interior elevations, details. Usually the primary reference for layout. | A0 general and cover, A1 floor plans, A2 elevations, A3 sections, A4 details |
| S | Structural | Foundation plans, framing plans, structural sections, connection details. | S1 foundation, S2 framing, S3 details |
| C | Civil | Site plan, grading, utilities, erosion control: the building's relationship to the land. | C1 existing conditions, C2 site plan, C3 grading, C4 utilities |
| M, P, E | Mechanical, plumbing, electrical | HVAC ductwork and equipment, plumbing supply and drain, electrical distribution and lighting. FP is fire protection. | M, P, E and FP series |
| L | Landscape | Planting, irrigation, hardscape layout and details; plant schedules list species, size and quantity. | L1 layout, L2 planting, L3 irrigation, L4 details |
Common symbols
These conventions are shared across US construction drawings and appear on nearly every residential and commercial floor plan.
Floor plan symbols
| Symbol | What it looks like | What it means |
|---|---|---|
| Exterior wall | Thick pair of lines, often filled or hatched | Outer walls, typically 6 to 8 in thick in the building. Thicker on the plan than interior partitions. |
| Interior partition | Thinner pair of parallel lines | Interior walls, 3.5 in for 2 × 4 studs or 5.5 in for 2 × 6. The thickness tells you the framing. |
| Door | Thin line with a quarter-circle arc | The line is the door leaf, the arc its swing. The gap in the wall is the rough opening. |
| Window | Three thin parallel lines in a wall break | The centre line is the glass, the outer lines the frame. A tag refers to the window schedule. |
| Room tag | Circle or box with a number or name | Refers to the room schedule: name, area and finishes. |
| Section cut | Line with a triangle or arrow and a reference number | Points in the direction of view; the number says which sheet carries the section. |
| Hidden line | Dashed line | Something above the cut plane, such as a beam or upper cabinet, or below it, such as a footing. |
| Column grid | Circle with a letter or number on a long line | Grid lines run across the plan and locate every structural element. |
Check the legend. Every set has a general notes and legend sheet, usually at the front of the A series, that defines the symbols and abbreviations used on that project. Conventions vary a little between firms.
Reading scale and measuring
Scale is the ratio between the drawing and the building. At 1/4" = 1', a quarter inch on paper is one foot. Each sheet states its scale in the title block, and sheets in one set commonly differ.
Common scales on ARCH D 24 × 36 sheets
| Scale | Typical use |
|---|---|
| 1/4" = 1'-0" | Residential floor plans; the standard |
| 1/8" = 1'-0" | Large commercial floor plans |
| 3/16" = 1'-0" | Medium commercial floor plans |
| 1" = 20' | Site plans (engineer's scale) |
| 3/4" = 1'-0" | Wall sections |
| 1-1/2" = 1'-0" | Enlarged details |
Using a scale ruler
An architect's scale is a triangular ruler with a different scale on each edge. To measure a 1/4" = 1' drawing, find the face marked 1/4, put the zero at one point and read the feet at the other; the numbers are already in feet. Without a ruler, measure in inches and multiply by 4 to get feet at 1/4" = 1', or by 8 at 1/8" = 1'. The scale guide has every scale and a converter.
Written dimensions win. If a dimension string is on the drawing, use that number. Measure with a ruler only when nothing is written, and treat the result as an estimate. A sheet that was printed at the wrong size still shows the right numbers but measures wrong, which is why the print size has to match the page size; see the size guide.
Dimension strings
Dimensions are shown as strings: horizontal or vertical lines with tick marks or arrows at each end and the measurement written above, in feet and inches. 12'-6" is twelve feet six inches.
Strings are layered. The innermost string gives individual elements: door openings, window openings, wall segments. The middle string gives room-to-room dimensions. The outer string gives the overall building dimension. Check that the inner dimensions add up to the outer one; when they do not, there is a drafting error to raise before building.
Architectural dimensions run to one of two reference points: face of stud, the structural wall face used for framing, or face of finish, the finished surface used for interior dimensions and clearances. The general notes say which convention the set uses. If it is unclear, ask the architect before framing.
Blueprint reading questions
How do you read blueprints for beginners?
Start with the title block in the lower-right corner: it says what the sheet shows and which scale to use. Orient yourself with the north arrow on the floor plan. Learn the basic symbols: thick lines are walls, an arc on a wall break is a door, parallel lines in a wall break are windows. Work from general to specific: understand the overall plan before studying details and sections.
What do the symbols on blueprints mean?
Thick solid lines are walls, with exterior walls thicker than interior partitions. A door is a thin line with an arc for the swing. A window is a set of parallel lines in a wall break. A circle with a number is a room tag pointing to the room schedule. A triangle or arrow with a number is a section cut pointing to the view on another sheet. Dashed lines show things above or below the cut plane. The legend sheet in each set defines the symbols used on that project.
What is a blueprint scale and how do I use it?
Scale is the ratio between the drawing and the building. 1/4" = 1' means one quarter inch on paper is one foot in the building. Use an architect's scale ruler: find the face that matches the sheet's scale, put zero at one point and read feet at the other. Without a ruler, at 1/4" = 1' measure in inches and multiply by 4 to get feet. Use written dimension strings over scaled measurements whenever they exist.
What are the different types of blueprint sheets?
Sets are organised by discipline. A sheets are architectural: floor plans, elevations, sections, details. S sheets are structural: foundations and framing. C sheets are civil: site, grading, utilities. M, P and E sheets are mechanical, plumbing and electrical. L sheets are landscape. Within a discipline the sheets are numbered in series, so A1.01 is the first architectural floor plan sheet.
How do I find room dimensions on a blueprint?
Look for dimension strings: lines with tick marks at each end and the measurement written above, in feet and inches such as 12'-6". Strings are layered: the inner string gives openings and wall segments, the middle string room-to-room dimensions, the outer string the overall building. If a dimension is not written, measure with a scale ruler and treat the result as an estimate, not a confirmed dimension.
What is a title block on a blueprint?
The title block is the information panel in the lower-right corner of every sheet. It carries the project name and address, the architect or engineer, the drawing title, the sheet number, the scale, and the date and revision. Read it first: it tells you what you are looking at, which scale to use, and whether you have the current version.
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