Bolt Circle Calculator

Find the spacing for evenly-spaced bolts on a flange or wheel hub.

How the Math Works

The Bolt Circle Calculator uses two key formulas to determine bolt spacing and distances. First, the angular spacing between each bolt is calculated by dividing 360 degrees by the total number of bolts (angle = 360 ÷ bolts). This ensures even distribution around the circle. Second, the chord length (straight-line distance between adjacent bolt holes) is derived using the formula chord = BCD × sin(180 ÷ bolts), where BCD is the bolt circle diameter. This formula stems from trigonometry: the chord length in a circle is equal to twice the radius multiplied by the sine of half the central angle, simplified here using the diameter (BCD) and the angle between bolts.

Practical Applications

This calculator is essential for engineers, manufacturers, and DIY enthusiasts designing components with bolt holes arranged in a circular pattern, such as flanges, wheels, or machinery parts. By inputting the bolt count and bolt circle diameter (BCD), users quickly determine the precise angular spacing and chord length needed for accurate drilling or machining. For example, a 6-bolt pattern with a 100mm BCD yields 60-degree spacing and 50mm chord lengths, ensuring symmetrical hole placement critical for structural integrity and alignment in automotive, aerospace, or industrial applications.

Day-to-Day Use

In everyday projects, this tool simplifies tasks requiring symmetrical hole placement, such as assembling bicycle wheels, installing ceiling fans, or building custom furniture. Knowing the exact spacing prevents misalignment and ensures secure fastening. Hobbyists and craftsmen can use it to create balanced designs, like decorative metalwork or wooden frames, where precision and aesthetics matter. Even in home maintenance, understanding bolt spacing helps when replacing parts or ensuring proper fitment of circular components like pulleys or drive belts, reducing guesswork and improving efficiency.

Worked example

6 bolts on a 5 in BCD → 60° apart, 2.5 in chord.

FAQ