Compression Ratio Calculator
Estimate static compression ratio and total displacement using common engine geometry measurements.
How to Use This Calculator
Measure bore and stroke
Use calipers or manufacturer specs for accurate cylinder diameter and crankshaft stroke.
Record chamber and piston volume
CC the combustion chamber and piston crown. Enter dish volumes as positive, domes as negative.
Add gasket and deck dimensions
Use installed gasket thickness and piston-to-deck measurements to capture remaining clearance volume.
Calculate and review results
Compare the compression ratio to fuel octane requirements and tuning goals.
Formula
CR = (Swept Volume + Clearance Volume) ÷ Clearance Volume
Swept Volume (per cylinder): π ÷ 4 × Bore² × Stroke
Clearance Volume: Chamber + Piston + Gasket + Deck (all in cc)
Example with 3.5" bore, 3.3" stroke, 60 cc chamber, flat-top piston, 0.04" gasket @ 3.6" bore, 0.01" deck:
Swept = 31.8 ci = 521.9 cc; Gasket = 6.8 cc; Deck = 4.0 cc → Clearance = 70.8 cc
CR = (521.9 + 70.8) ÷ 70.8 = 8.37 : 1
About the Compression Ratio Calculator
Static compression ratio sets the foundation for combustion efficiency, octane requirements, and power potential. Our calculator translates precise measurements into a reliable ratio so you can optimize cam selection, ignition timing, and fuel choice.
When to Use This Calculator
- Engine rebuilds: Verify compression after machining, decking, or piston swaps.
- Performance tuning: Determine if a planned camshaft or forced induction setup matches your compression.
- Fuel planning: Ensure your ratio suits the octane available in your region.
- Educational analysis: Teach students how geometry changes affect engine performance.
Why Use Our Calculator?
- ✅ Precision math: Accounts for gasket volume, deck height, and piston design.
- ✅ Instant displacement: Provides total engine displacement in both cc and cubic inches.
- ✅ Mobile-friendly: Capture measurements in the shop without spreadsheets.
- ✅ Free and flexible: Experiment with overbore or thinner gaskets in seconds.
Common Applications
Street builds: Dial in a safe pump-gas compression ratio.
Race engines: Optimize for E85 or race fuel by modeling aggressive domes.
Motorcycle tuning: Evaluate the impact of head milling or gasket swaps.
Tips for Best Results
- CC each chamber and piston to confirm actual volumes instead of relying on catalog numbers.
- Measure gasket thickness after torque to account for crush.
- Recalculate if you change piston deck height or use different rods.
- Remember that dynamic compression depends on cam timing—use a dynamic calculator for full analysis.
Frequently Asked Questions
What is the difference between static and dynamic compression?
Static compression ratio uses pure geometry at top and bottom dead center. Dynamic compression also considers intake valve closing timing from the camshaft, which changes effective compression under running conditions.
How do I enter piston dome or dish volume?
Enter dish volumes as positive numbers because they add clearance volume. Enter domes as negative numbers because they displace volume inside the chamber.
Why does gasket thickness matter?
Thicker gaskets increase clearance volume, lowering compression. Thin performance gaskets reduce clearance, raising compression. Always use actual installed thickness for accuracy.
What compression ratio can I run on pump gas?
Most naturally aspirated street engines stay between 9.0:1 and 10.5:1 on 91–93 octane. Higher ratios may require premium fuel, custom tuning, or alternative fuels like E85.