Measuring floor plans is an important part of understanding the building’s actual size.
Measuring floor plans involves understanding how the drawing relates to the actual building. This is done by using a drawing scale. For example a floor plan drawn at 1/4″ = 1′-0″ means 1 foot on the drawing is represented by 1/4 inch on a ruler, and 4 feet is represented by 1 inch. Metric scale is proportional to actual size: At 1:100 scale 1 centimeter = 1 meter.
Since buildings can be big or small their plans can be drawn at different scales in order to fit the full plan of the building on a given size of paper. There are also different types of drawings besides plans that are drawn at different scales. Let’s take a look at drawing scales, drawings, and how they are used.
Common Plan and Drawing Scales
Lengths are typically measured by one of two systems.
The Imperial System is one based upon feet and inches and is used primarily in the United States and a few other countries.
The Metric System is based upon the meter and its increments of centimeters and millimeters.
This article will reference the Imperial system as the primary focus.
Most floor plans are drawn at either 1/8 inch scale for really large homes or 1/4 inch scale for more typical and smaller homes. This allows their plans to fit on standard paper sheet sizes such as 18 inches by 24 inches (18″ x 24″), 24 inches by 36 inches (24″ x 36″), or 30 inches by 42 inches (30″ x 42″).
Comparable metric plan scales are 1:100 and 1:50 respectively. There are also comparably-sized drawing sheets. Review the table below to get a sense of the different scales used and the drawings they are used for.
| Common Imperial Drawing Scales | Common Metric Drawing Scales | Types of Drawings |
| 1″ = 40′-0″ | 1:500 | Location Plans |
| 1″ = 20-0″‘ | 1:250 | Site Plans |
| 1/16″ = 1′-0″ | 1:200 | Site Plans |
| 1/8″ = 1′-0″ | 1:100 | Site Plans, Floor Plans, Building Sections |
| 1/4″ = 1′-0″ | 1:50 | Floor Plans, Building Sections, Enlarged Floor Plans, Room Elevations |
| 1/2″ = 1′-0″ | 1:25 | Enlarged Floor Plans, Room Elevations |
| 3/4″ = 1′-0″ | 1:20 | Wall Sections, Room Elevations |
| 1 1/2″ = 1′-0″ | 1:10 | Details |
| 3″ = 1′-0″ | 1:5 | Details |
| Common Sheet Sizes – Imperial Inches | Common Sheet sizes – Metric Millimeters |
| 9 x 12 | 229 x 305 |
| 12 x 18 | 305 x 457 |
| 18 x 24 | 457 x 610 |
| 24 x 36 | 610 x 914 |
| 30 x 42 | 762 x 1,070 |
| 36 x 48 | 914 x 1,220 |
Since 1/8 inch scale represents 8 feet within 1 inch and 1/4 inch represents 4 feet per inch the 1/4 inch scale allows you to show more detail within the plan. For instance, a 1/8″ plan might not be able to show plan dimensions for each step on a stair or for the placement of fixtures in a bathroom; however, you might could show them on a 1/4″ drawing.
When more information is needed within a portion of a plan, larger plans covering a limited area are drawn. Stairs and bathrooms on a 1/8 scale plan might be enlarged on a different sheet and drawn at 1/4 inch or 1/2 inch scale depending on the complexity of information needed to be shown.
To show drawings for much smaller areas, such as showing how a window is to be built into a wall, drawings called details are created at even larger increments. This window detail could be drawn at a scale of 3/4 inch, where 1 foot of actual length is shown as 3/4 of an inch of length on the drawing.
Because the drawing scale increments go from small (e.g. 1/8″) to large (e.g 3/4″), the range of scales are described as small scale drawings (plans) to large scale drawings (details).
Standard rulers can be used to measure plans and drawings; however, that can become challenging trying to count by all the different increments when measuring. To solve this issue the architecture and engineering professions developed unique rulers called ‘scales’ that indicate lengths of measurement for the different scale increments.
Let’s review how rulers and scales work and how they are used to determine sizes in different types of drawings.
Rulers versus scales. What’s the difference?
The standard ruler using Imperial units (feet and inches) is 12 inches in length and has markings representing fractions within each inch. Determining the lengths to draw lines on a plan is fairly easy: 1/16 inch scale gives us 16 feet for every inch, 1/8 scale gives us 8 feet for every inch, and so forth. However, reading longer lengths becomes challenging since the ruler only shows numbers from 1 to 12 denoting each full inch.
The architectural ruler is called an architectural scale and changes the way lengths are shown compared to a standard ruler. Each drawing scale has its own ruler showing the number of feet along its length at that scale. This allows the user of the scale to easily know the number of feet along its length.
An engineering scale is a ruler that uses increments of 10 for each inch rather than fractions of an inch. A drawing scale of 1 inch equals 10 feet is shown as 1:10, where each increment equates to 1 foot. These rulers show numbered increments of 10 foot measurement. Most engineering drawing scales have rulers that show numbers that drop the last zero. For example a length of 20 feet on a 1:20 scale will show the number 2 at that location on the ruler.
Drawings using the metric scale have special sets of rulers as well, also called scales. A ruler for a given drawing scale will show the number of meters of measured length.
Measuring site plans
Site plans cover the largest area in a set of house plans. Engineering drawing scales are most often used to draw them. 1 inch on the drawing can represent 10 feet (1:10), 20 feet (1:20), 30 feet (1:30), etc.
In some cases site drawings can be drawn at smaller fractions-of-an-inch scale, where 1/16 inch on the drawing equates to 1 foot of actual size, and 1/8 inch equates to 1 foot actual size. These scales are typically used to draw plans for smaller sites.
Metric scales continue to use proportional scales relating to a meter, where 1 centimeter on the drawing can represent 1 meter (1:100), 2 meters (1:200), 3 meters (1:300), etc.
Measuring floor plans
Floor plans are drawn to accurate lengths beyond the foot. Since rooms are rarely sized exactly to a foot increment the plans need to be drawn to the inch.
Architectural rulers allow you to easily measure these inches. On each ruler the start of its drawing scale starts at zero with foot measurements increasing to the right of the zero. These scales also provide a short section to the left of the zero that shows a 1 foot length with increments marked at each inch allowing you to draw to foot-and-inch lengths.
Since the metric system is based upon increments of ten it has a methods to measure beyond the meter itself. Since most lengths will fall short of or beyond a meter the metric rulers allow for measurements in centimeters or millimeters. Fractions are not involved.
A metric length measurement for plans and details will typically be shown in millimeters. The millimeter increment allows for easy conversion to centimeters and meters by dividing the number of millimeters by 100 to get centimeters or a thousand to get meters. A room that is 6532 millimeters in length is also 6 meters, 53 centimeters, and 2 millimeters in length. However it is more effective to show it in millimeters on a plan.
Measuring details
Details are drawn at a variety of scales. Fractions of an inch become important once the scale increment gets large enough. Details drawn at 1/2″ scale or larger have rulers that show the foot length portion divided into inches and fractions of an inch. The larger the drawing scale the greater number of fractions shown on the ruler.
A ruler at 1/2 inch scale shows half-inch fractions. A ruler at 1 inch scale shows quarter-inch fractions, while a 3 inch scale ruler shows eighth-inch fractions.
Larger drawing scales allow for ever-increasing accuracy of measurement. Building materials are rarely created to an exact inch or specific centimeter. Being able to accurately measure to the fraction of an inch or to the mm becomes important in representing those materials in large-scale detail drawings.
Summary
It’s beneficial to purchase an appropriate measurement scale. These scales typically come with two or more drawing scales on them. You can easily find these on the internet by typing in ‘architectural scales’, ‘engineering scales’, or ‘metric drawing scales’ into your browser.
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