Bolt Circle Calculator
Enter a bolt circle diameter, the number of holes, and a start angle to get the exact X and Y coordinate of every hole measured from the circle centre. Switch between millimetres and inches, choose clockwise or counter-clockwise layout, and optionally offset the centre. Use the reverse-solve mode to find the bolt circle diameter from the measured distance between two adjacent bolts.
What is a bolt circle?
A bolt circle (also called a bolt circle diameter, BCD, or pitch circle diameter, PCD) is an imaginary circle that passes through the centre of every bolt hole in a pattern. Engineers use bolt circles to define flange connections, wheel hubs, engine flanges, clamping rings, and thousands of other applications where a set of fasteners must be positioned symmetrically around a common centre. The two key parameters are the circle diameter and the number of holes; together they fully define the geometry of an evenly spaced circular pattern.
How to calculate bolt hole coordinates
Each hole is located at a known angle from the positive X-axis (the 3 o'clock direction). Hole 1 sits at the start angle you choose, and each subsequent hole is spaced by 360 divided by the number of holes. The X and Y coordinates of hole i (0-indexed) are: X = X_centre + R x cos(start_angle + i x 360/n) and Y = Y_centre + R x sin(start_angle + i x 360/n), where R is the bolt circle radius (half the diameter). All angles are in degrees. A counter-clockwise layout is the mathematical convention; clockwise is also offered because many engineering drawings and automotive specifications number bolts clockwise.
Reverse-solving the bolt circle diameter
If you need to match an existing bolt pattern without a drawing, measure the centre-to-centre distance between two adjacent bolts (the chord) and enter it together with the number of holes in reverse mode. The formula is: BCD = chord / sin(180 / n). For a 5-bolt pattern with an adjacent spacing of 70 mm, the BCD is 70 / sin(36 deg) = 70 / 0.5878 = 119.1 mm, which corresponds to the common 5x120 wheel specification. For non-adjacent measurements you can also apply: BCD = chord / sin(k x 180 / n), where k is the number of hole gaps spanned.
Adjacent chord and edge-to-edge gap
The adjacent chord is the straight-line distance between the centres of two neighbouring holes: chord = 2 x R x sin(PI / n). This is the dimension you would measure with calipers between bolt centres on an existing part. The edge-to-edge gap is the clear distance between the drilled walls of neighbouring holes: gap = chord - hole_diameter. If the gap is zero or negative the holes would merge, so this figure acts as a quick interference check. Both values update automatically as you change the bolt circle diameter, number of holes, or hole size.
Common bolt circle patterns
| Application | Number of holes | Common PCD (mm) | Notes |
|---|---|---|---|
| Compact car wheel | 4 | 100 | Very common 4x100 pattern |
| Mid-size car wheel | 5 | 112 | Common on VAG, Mercedes |
| Truck / SUV wheel | 6 | 139.7 | 6x139.7 Chevrolet/GM/Toyota |
| PN16 DN50 flange | 4 | 125 | ISO 7005-1 flange standard |
| PN16 DN100 flange | 8 | 180 | ISO 7005-1 flange standard |
| SAE 2-bolt flange | 2 | 50.8 | SAE J518 code 61/62 |
| Rotary table (small) | 3 | 76.2 | 3-slot clamping ring |
| Engine crankshaft | 6 | 115 | Typical 6-cylinder flywheel |
Typical bolt circle diameters used in automotive wheel hubs, pipe flanges, and machinery. PCD = pitch circle diameter.
Frequently asked questions
What is the difference between BCD and PCD?
Bolt circle diameter (BCD) and pitch circle diameter (PCD) are exactly the same thing. BCD is the term used mostly in mechanical and aerospace engineering; PCD is common in the automotive and pipe-flange worlds. Both refer to the diameter of the imaginary circle that passes through the centre of every bolt or stud in the pattern.
How do I measure the BCD of an existing bolt pattern?
For a 4-bolt or 6-bolt pattern (even number), measure directly across the circle from the centre of one hole to the centre of the hole directly opposite - that is the BCD. For a 5-bolt or 3-bolt pattern (odd number) you cannot measure directly across, so instead measure the distance between two adjacent hole centres, then use: BCD = adjacent spacing / sin(180 / number of holes). Use the reverse mode in this calculator to do that conversion automatically.
What does the start angle mean?
The start angle places hole 1 at a specific position on the circle, measured counter-clockwise from the positive X-axis (the 3 o'clock direction). A start angle of 0 puts hole 1 at 3 o'clock. A start angle of 90 puts it at 12 o'clock (top). Most engineering drawings use 0 or 90 as the reference position, but you can enter any value to match an existing drawing or fixture orientation.
When should I use clockwise vs counter-clockwise direction?
Counter-clockwise is the mathematical convention and is the default for most CAD software. Use clockwise when a drawing or standard specifically numbers bolts in the clockwise direction, which is common in some automotive and aerospace specifications. The coordinates themselves are the same set of points; only the numbering order changes.
Can this calculator handle non-evenly spaced holes?
No - this tool assumes all holes are evenly spaced around the circle, which covers the vast majority of bolt patterns. For unequally spaced holes (sometimes used in aerospace shear flanges or indexing plates), you would need to calculate each hole angle individually from the drawing and apply the X = R x cos(angle), Y = R x sin(angle) formula for each one.
What is the edge-to-edge gap and why does it matter?
The edge-to-edge gap is the clear wall-to-wall distance between neighbouring drilled holes. If this number is too small, the holes can break into each other during drilling, leaving a weak or unusable part. As a practical rule, aim for a minimum gap of at least half the hole diameter to ensure adequate material between fastener holes. If the gap is negative in this calculator it means the holes as specified would overlap.