Scale Calculator
Convert between real dimensions and scaled drawing or model dimensions for blueprints, site plans and scale models.
How to use the Scale Calculator
- Choose the convert.
- Choose the scale.
- Enter the custom scale 1 :.
- Enter the real length and pick its unit.
- Enter the scaled length and pick its unit.
- Press Calculate. The result updates automatically as you type, and you can switch the result unit at any time.
About the Scale Calculator
US architectural drawings use fractional-inch scales: floor plans are usually 1/4″ = 1′-0″ (1:48), larger buildings 1/8″ = 1′-0″, and details 1½″ or 3″ = 1′-0″. Civil and site plans use engineering scales such as 1″ = 20′ or 1″ = 50′. Metric drawings use plain ratios like 1:50 and 1:100, and hobby models use ratios such as 1:87 (HO trains) or 1:24.
Always check the scale printed in the title block, and remember that a print that has been reduced or enlarged is no longer at its labeled scale; measure a known dimension first to confirm.
Formula
Scaled length = Real length ÷ n · Real length = Scaled length × n
For architectural scales, n = 12 ÷ (inches per foot); e.g. 1/4″ = 1′ → n = 12 ÷ 0.25 = 48
Engineering scales are 1″ = X feet, so n = X × 12. Work in the same unit on both sides; the calculator converts units automatically.
Example calculation
Using the default values:
- Convert = Real size → scaled size
- Scale = 1/4″ = 1′-0″ (1:48)
- Real length = 20 ft
the calculator returns:
- Result: 5″ (127 mm)
- Result length: 5 in
- Scale factor: 1:48
Frequently asked questions
How long is 20 feet at 1/4 inch scale?
At 1/4″ = 1′-0″, every foot is 1/4 in, so 20 ft is drawn 5 in long.
What does 1/8 inch scale mean?
1/8″ on the drawing represents 1 ft in reality, a ratio of 1:96.
How do I convert a model scale like 1:87?
Divide the real size by 87 to get the model size, or multiply a model measurement by 87 to get the real size.
Last updated: · Category: Construction calculators