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Angular Size & Milliradian Distance Estimate

Estimate range from a known object height and its angular height using the small-angle milliradian relationship.

Calculator inputs

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Enter the known values in the units shown. Results update as you type.
Where results are editable, change one to solve backwards. Lock a value to hold it fixed.
Use the worked example to check the method. Reset restores the starting fields.

Inputs and results stay in this browser tab. Bookify does not upload or store the values you enter.

An object 2 m tall spanning 4 mrad is approximately 500 m away.

Distance ≈ 1000 × object height / angular height in mrad

Enter these known values and leave the other values blank.

This is the small-angle approximation, most accurate when angular height is much smaller than one radian.
Use the visible dimension perpendicular to the line of sight. Unknown object size, tilt and reading error directly affect the estimate.
Angular height must be positive. Milliradians mean 1/1000 radian; reticles using a different military-mil convention require conversion.
This geometric estimate does not account for terrain, atmospheric refraction or measurement uncertainty.

This is the small-angle approximation, most accurate when angular height is much smaller than one radian.

Bookify requires a positive angular span to avoid division by zero and negative range.