Scaleconvert / the method
The same shape.
A different size.
A scale ratio compares matching lengths in the same unit. Scaleconvert makes that unit conversion explicit before applying the ratio, so metres on a real object can become millimetres on your model without a mental detour.
Read 1:N in the right direction
At 1:50, one unit on a model represents 50 of the same units on the real object. A 1 mm model feature therefore represents 50 mm in reality. The ratio is about dimensions, so it works with any consistent length unit.
Real → model: model length = real length ÷ N
Model → real: real length = model length × N
The direction selector changes the interpretation of every dimension row. The input and output unit selectors are independent of that direction. You can use metres to millimetres, inches to centimetres, or any other supported pair.
Example 1: real metres to model millimetres
Suppose a model represents a rectangular object 6 m long, 3.5 m wide and 2.4 m high, at 1:50. First express the real lengths as 6,000, 3,500 and 2,400 mm. Dividing each by 50 gives a model dimension list of 120 mm, 70 mm and 48 mm.
In the calculator, choose Real → model, N = 50, input metres and output millimetres. Add one labelled row for each dimension. The units apply to the whole list; a row named “width in inches” does not change its unit automatically.
Example 2: measure a model and work back
A model is drawn at 1:87. A measured feature is 18 mm long. Choose Model → real, set N = 87, input millimetres and output metres. The real length is 18 × 87 = 1,566 mm, or 1.566 m.
Measurement error scales too. An uncertain model measurement does not become more precise simply because the conversion returns several decimal places. The tool converts the value you enter; it cannot check the model, ruler or original drawing.
Example 3: decimal inches and exact unit factors
One inch is exactly 25.4 mm, and one foot is 12 inches, or exactly 304.8 mm. A real length of 12 inches at 1:48 becomes 0.25 inches on the model, equal to 6.35 mm. Enter input inches and output millimetres to convert both unit and scale in one step.
Imperial inputs here are decimal inches or decimal feet. Enter half an inch as 0.5, rather than 1/2. Enter 5 feet 6 inches as 5.5 feet, or 66 inches. Do not enter “5.6 feet” expecting the decimal part to mean six inches; 0.6 feet is 7.2 inches.
What happens at 1:1 or below
A scale of 1:1 is full size, so only the unit conversion changes the number. A positive denominator below one represents enlargement in the Real → model direction: at 1:0.5, a 10 mm real dimension becomes 20 mm on the model. Zero is rejected because division by zero has no defined result.
Rounding and exact arithmetic
Input dimensions and the scale denominator accept up to six decimal places. Internally, decimal inputs and unit factors are kept as integer fractions. This avoids turning a decimal boundary into a different value through binary floating-point arithmetic.
Displayed results are rounded to at most six decimal places in the selected output unit. The ≈ symbol identifies a result that needs display rounding. For example, 1 mm at 1:3 displays as approximately 0.333333 mm; its exact value is one third of a millimetre. Whole-number-looking outputs are not rounded to a whole cutting allowance.
Keep the labels with the numbers
Copy the converted dimension list or print it after calculation. The list includes direction, scale, units and the rounding explanation. Input rows can be labelled for the parts you are planning. The calculator does not automatically allow for blade kerf, assembly clearance, material thickness, shrinkage or a printer's scaling settings.
This tool converts lengths only. To derive an area ratio you would square the linear scale factor, and a volume ratio uses its cube; those are different calculations and are not silently applied to a row.
Your values remain in this browser tab and are not automatically saved or submitted. Refreshing restores the example. Convert your dimension list →