Bore to Stroke Ratio Calculator
Calculate engine bore-to-stroke ratio and displacement
Table of Contents
How to Use
- Choose metric (mm) or imperial (inches) units that match your measurements.
- Enter the cylinder bore diameter and piston stroke length for your engine.
- Optionally add the number of cylinders to compute total displacement.
- Select "Calculate Ratio" to view your bore-to-stroke ratio, engine classification, and displacement results.
What the Bore-to-Stroke Ratio Tells You
The bore-to-stroke ratio compares the cylinder's diameter (bore) to the piston's travel distance (stroke). It is a quick way to understand how an engine breathes, revs, and delivers torque across the RPM range.
- Ratios above 1.0 indicate a larger bore than stroke (oversquare).
- Ratios below 1.0 highlight a longer stroke than bore (undersquare).
- Ratios near 1.0 describe a balanced, square engine design.
Interpreting Engine Types
Different ratios suit different performance goals. Understanding each type helps tuners and builders choose cam profiles, gearing, and fueling strategies.
- Oversquare engines have larger valves and shorter strokes, supporting higher RPM and strong top-end power.
- Undersquare engines prioritize torque and efficiency, ideal for towing, endurance, or fuel-conscious builds.
- Square engines strike a balance and are popular in street vehicles that need versatility.
How Displacement is Calculated
When you provide cylinder count, the calculator uses the industry-standard formula π/4 × bore² × stroke × cylinders. Unit conversions keep the result consistent whether you work in millimeters or inches.
| Input | Metric Calculation | Imperial Calculation |
|---|---|---|
| Bore | Use millimeters directly | Converted to millimeters before squaring |
| Stroke | Use millimeters directly | Converted to millimeters before multiplying |
| Cylinders | Multiplies single-cylinder volume by cylinder count | Same approach after converting to metric |
| Output | Returns cubic centimeters (cc) | Returns cubic inches (in³) |
Performance Considerations
- Pair the ratio with compression, camshaft profiles, and airflow data for complete tuning decisions.
- Higher RPM oversquare engines often require stronger valve springs and lightweight rotating assemblies.
- Undersquare builds benefit from robust crankshafts and connecting rods to manage the longer stroke forces.
- Track your displacement when planning class legality, emissions compliance, or forced-induction upgrades.
Frequently Asked Questions
- What ratio should I target for my engine?
- Street cars often use square or mildly oversquare designs for flexible power. High-revving race engines lean oversquare, while towing and endurance engines trend undersquare. Match the ratio to your powerband and reliability goals.
- Does changing pistons or cylinders alter the ratio?
- Yes. Any change to bore (such as overboring a cylinder) or stroke (such as installing a stroker crank) directly alters the ratio. Recalculate after machining or rebuilds to verify the new specification.
- Why does displacement require cylinder count?
- Displacement is the combined swept volume of every cylinder. The calculator multiplies the single-cylinder volume by the number of cylinders you provide, so entering the count ensures accurate totals.
- Can I compare metric and imperial engines?
- Absolutely. The tool seamlessly converts between millimeters and inches, allowing you to input legacy specs and compare them with modern builds without manual unit conversions.