Bolt Circle Calculator
Plan your materials specifications project with our free bolt circle calculator. Get precise measurements, material lists, and budgets.
Formula
X = R x cos(angle), Y = R x sin(angle)
Each hole position is calculated from the bolt circle radius R and the angular position. The angle for each hole equals the start angle plus the hole index times (360 divided by the number of holes). The chord distance between adjacent holes is 2R times sin(pi/n).
Worked Examples
Example 1: 8-Hole Pipe Flange
Problem: Calculate hole positions for 8 equally spaced holes on a 12-inch bolt circle starting at 0 degrees.
Solution: Angular spacing = 360/8 = 45 degrees\nRadius = 6 inches\nHole 1: X=6.000, Y=0.000 at 0 deg\nHole 2: X=4.243, Y=4.243 at 45 deg\nChord = 2 x 6 x sin(22.5) = 4.5922 in
Result: 8 holes at 45-degree spacing with 4.5922-inch chord distance
Example 2: 5-Lug Wheel Pattern
Problem: Find the bolt positions for a 5-lug pattern on a 4.5-inch (114.3mm) bolt circle.
Solution: Angular spacing = 360/5 = 72 degrees\nRadius = 2.25 inches\nChord = 2 x 2.25 x sin(36) = 2.6450 in\nArc = pi x 4.5 / 5 = 2.8274 in
Result: 5 holes at 72-degree spacing with 2.645-inch chord distance
Frequently Asked Questions
What is a bolt circle and how is it defined?
A bolt circle, also called a bolt circle diameter (BCD) or pitch circle diameter (PCD), is an imaginary circle that passes through the center of each bolt hole in a circular pattern. It is defined by the diameter of this circle and the number of equally spaced holes. Bolt circles are commonly found on pipe flanges, wheel hubs, base plates, and machinery mounting patterns. The BCD is measured from the center of one hole across the pattern through the center to the opposite hole.
How do I measure a bolt circle diameter on an existing part?
For an even number of holes, measure directly from the center of one hole to the center of the opposite hole. For an odd number of holes, measure from the center of one hole to the far edge of the opposite space between holes, then use the formula BCD = distance / cos(90/n) where n is the number of holes. Alternatively, measure the chord between two adjacent holes and calculate BCD = chord / sin(180/n). Using calipers for precision is recommended.
What is the difference between bolt circle diameter and bolt spacing?
Bolt circle diameter is the diameter of the imaginary circle passing through all bolt centers. Bolt spacing refers to the distance between adjacent bolts, which can be measured as either a chord (straight line) or an arc (along the circle). Chord spacing equals 2R times sin(180/n) and arc spacing equals pi times BCD divided by n. For flanges and standards compliance, the BCD is the primary specification, while spacing is a derived value used for checking clearances.
How accurate are the results from Bolt Circle Calculator?
All calculations use established mathematical formulas and are performed with high-precision arithmetic. Results are accurate to the precision shown. For critical decisions in finance, medicine, or engineering, always verify results with a qualified professional.
Can I use Bolt Circle Calculator on a mobile device?
Yes. All calculators on NovaCalculator are fully responsive and work on smartphones, tablets, and desktops. The layout adapts automatically to your screen size.
Does Bolt Circle Calculator work offline?
Once the page is loaded, the calculation logic runs entirely in your browser. If you have already opened the page, most calculators will continue to work even if your internet connection is lost, since no server requests are needed for computation.