Bolt Pattern Calculator

About the Bolt Pattern Calculator

This bolt pattern calculator determines the pitch circle diameter (PCD) of any motor, wheel, or flange pattern from a single bolt-to-bolt measurement. Supports 4, 5, 6, and 8 bolt configurations. Use it when matching a NEMA frame motor to a custom mounting bracket, cross-referencing IEC and NEMA frames, or verifying that a replacement motor will fit existing mounting holes without re-drilling.

The calculator handles all standard bolt patterns including the trickier 5-lug measurement that requires sin(π/5) trigonometry to derive PCD from non-adjacent bolt distance.

How to calculate bolt circle diameter

The general formula for any bolt count:

PCD = Distance ÷ sin(π ÷ N)
where N = number of bolts, Distance = between adjacent or alternate bolts depending on N

For 4-lug patterns, the PCD equals the distance directly across opposite bolts. For 5-lug, measure between two non-adjacent bolts (skip one) and divide by sin(π/5) ≈ 0.5878. For 6-lug, distance across opposite bolts equals the PCD. For 8-lug, distance across opposite bolts equals the PCD.

Worked example

A 5-lug motor with a 4-inch measurement between two non-adjacent bolts (skipping one) has PCD = 4 ÷ 0.5878 = 6.81 inches. This matches a NEMA 254T frame mounting pattern. For a quick sanity check: PCD should always be larger than the measured distance (since you’re measuring a chord, not the diameter).

Common NEMA frame bolt patterns

• NEMA 56 frame: 4-lug, 4.5″ PCD — common for fractional HP motors (1/4–3/4 HP)
• NEMA 143T / 145T: 4-lug, 5.5″ PCD — 1.5–3 HP general purpose
• NEMA 182T / 184T: 4-lug, 5.625″ PCD — 3–7.5 HP
• NEMA 213T / 215T: 4-lug, 7.5″ PCD — 7.5–15 HP
• NEMA 254T / 256T: 4-lug, 8.5″ PCD — 15–30 HP
• NEMA 284T / 286T: 4-lug, 9.5″ PCD — 25–40 HP
• NEMA 324T / 326T: 4-lug, 10″ PCD — 40–60 HP
• NEMA 364T / 365T: 4-lug, 11.25″ PCD — 60–100 HP

Always verify against the motor manufacturer’s drawing — some special-purpose motors deviate from standard NEMA dimensions.

When to use this calculator

• Matching a replacement motor to existing mounting holes without re-drilling
• Cross-referencing NEMA and IEC frame sizes for international motor swaps
• Designing custom motor mounting brackets for OEM equipment
• Verifying flange compatibility for C-face and D-flange motors with pumps and gearboxes
• Confirming pulley or coupler bore patterns for direct-drive applications
• Reverse-engineering bolt patterns when the original motor manufacturer is unknown
• Quick checks during equipment installations and maintenance

Frequently Asked Questions

How do you measure a bolt pattern?

Count the bolts and measure center-to-center across the circle. For 4-lug: across two opposite bolts (the diameter). For 5-lug: between two bolts that are 2 positions apart (a chord). For 6-lug: across opposite bolts. For 8-lug: across opposite bolts.

What is PCD?

Pitch Circle Diameter — the diameter of the imaginary circle through the centers of all bolt holes. Standard NEMA frames have specific PCDs. Matching the PCD is critical when sourcing a replacement motor; matching the bolt count alone isn’t enough.

How do I calculate 5-lug pattern PCD?

PCD = Distance ÷ 0.5878, where Distance is the center-to-center measurement between two bolts that are 2 positions apart (skip one). The 0.5878 factor is sin(π/5) — trigonometry from the 5-sided geometry.

What are common NEMA bolt patterns?

NEMA 56: 4.5″ PCD. 143T/145T: 5.5″. 182T/184T: 5.625″. 213T/215T: 7.5″. 254T/256T: 8.5″. Always match the motor base PCD to the mounting bracket spec — the bolt count alone isn’t enough.

Are bolt patterns interchangeable between manufacturers?

Yes when both motors follow the same NEMA frame standard. Baldor, WEG, Marathon, and US Motors all build to NEMA MG-1, so a 184T frame from any of them is dimensionally interchangeable. IEC frame patterns differ and are not directly interchangeable with NEMA.

What is C-face vs D-flange mounting?

C-face has a male pilot diameter that mates with a female pump or gearbox. D-flange has tapped bolt holes for direct flange mounting. Both use specific PCDs called out in NEMA MG-1. Verify your driven equipment’s mating spec before ordering.

Can I match an IEC motor to a NEMA mounting pattern?

Not directly — IEC frames use metric bolt patterns and different shaft heights. Adapter plates exist for common conversions (e.g., IEC 132 to NEMA 213T) but require careful measurement of bolt circle, shaft height, and shaft diameter.

NEMA Frame Bolt Pattern Reference

Common NEMA motor frame mounting hole patterns (foot mounting). Dimensions are the rectangular bolt-hole spacing (2F x distance) and bolt size. Always confirm against the manufacturer drawing before drilling.

Bolt Pattern Calculator FAQs

How do you measure a bolt pattern?

For an even number of bolts, measure straight across from the center of one bolt to the center of the bolt directly opposite. For a 5-bolt pattern, measure from the center of one bolt to the outer edge of the bolt that is two positions away, or use this calculator to find the true bolt circle diameter (PCD).

What is bolt circle diameter (PCD)?

The pitch circle diameter (PCD) is the diameter of the imaginary circle passing through the center of every mounting bolt. It is the key dimension for matching a motor or flange to a mating part.

How do I calculate a 5-bolt pattern?

Measure the distance between two adjacent bolt centers (S), then multiply by 1.701 to get the bolt circle diameter. Enter your measurements in the calculator above for an exact result.

Related calculators and motor categories

Sourcing a replacement motor with a specific bolt pattern? Browse General Purpose motors, IEC frame motors, or Farm Duty motors from Baldor, WEG, Marathon, and U.S. Motors.

Related calculators: Motor Torque Calculator · Speed to RPM Calculator · kW to HP Calculator · Motor Efficiency Calculator.