How to Calculate Compression Ratio
Seven steps. Seven measurements. One result that determines every piston, gasket, and cam decision. Grab your bore gauge and burette — let's do the math on a real engine.
Tool List — Everything You'll Need
You don't need a full machine shop to calculate compression ratio, but you do need specific tools with specific precision. A tape measure and a guess aren't going to cut it.
The Seven Steps
Step 1: Mic the Bore
Use a dial bore gauge. Measure at three depths (top, middle, bottom of cylinder) and two axes (parallel and perpendicular to crank). Record all six readings and use the LARGEST. Wear and taper make the bottom of a used cylinder wider than the top.
Step 2: Determine Stroke
Stroke = 2 × crankshaft throw. Measure throw directly: install a dial indicator on a crank journal, rotate the crank 180°, and double the distance. Or trust a known crank part number — most stroker cranks are clearly stamped.
Step 3: Burette the Chamber (most critical!)
1) Close both intake and exhaust valves — seal seats with light grease. 2) Seal the spark plug hole. 3) Lay a clear plexiglass plate over the chamber area, seal edges with grease. 4) Zero the burette, fill slowly through a small hole in the plexiglass until it reaches the plate. Record volume in cc. Do this THREE times and average.
Step 4: Get Piston Dome/Dish Volume
Call the manufacturer. Aftermarket pistons carry their cc rating on the box or spec sheet. Domed pistons REDUCE clearance volume (−cc). Dished pistons ADD it (+cc). Flat top = 0 cc. If unsure, CMM-verify or do a water-displacement test.
Step 5: Gasket Thickness — Compressed, Not Advertised
New MLS gaskets compress under torque-to-yield. A '0.040" advertised' gasket may settle to 0.036" in the engine. Mic a compressed sample between two flat surfaces torqued to spec, or trust the manufacturer's installed (compressed) thickness.
Step 6: Deck Clearance at TDC
1) Remove the head. 2) Bring piston to ABSOLUTE TDC — use a dial indicator on the piston face, don't trust timing marks. 3) Zero a second indicator on the block deck surface. 4) The difference is your deck clearance. Positive = piston below block. Negative = piston above (dangerous!).
Step 7: Run the Formula
CR = (Vd + Vc) / Vc Vd = (π/4) × bore² × stroke Vc = chamber_cc + piston_cc + gasket_cc + deck_cc Use consistent units — everything in inches → cu in, everything in mm → cc. Multiply by 16.387064 to convert between cu in and cc.
Complete Worked Example — Chevy SBC 350
Let's walk through a real engine build. A classic Chevy small-block 350 bored 0.030\" over with +4cc domed pistons and 0.041\" MLS gaskets. Every number below is real, measured on real parts.
Input Specs (all measured or verified)
Step-by-Step Calculation
Error Propagation — How Bad Measurements Add Up
Every measurement has error. Here's how common mistakes cascade into your final CR:
- Chamber cc off by 4 cc: ±0.5 CR error on this engine
- Bore measured 0.010\" too small: −0.3 CR error
- Using advertised (not compressed) gasket thickness: +0.15 CR error
- Missing piston dome sign (−4 vs +4): ±1.0 CR error (huge!)
Pre-Build Checklist — Have You Measured Everything?
- ✅ Bore mic'd at 3 depths × 2 axes — recorded largest
- ✅ Stroke verified from crank throw (not brochure)
- ✅ Chamber cc burette'd 3× per head — averaged
- ✅ Piston dome/dish cc confirmed (manufacturer or CMM)
- ✅ Gasket thickness measured compressed (not advertised)
- ✅ Deck clearance measured with piston at true TDC
- ✅ Used consistent units (all inches → cu in or all mm → cc)
- ✅ Run the numbers on the calculator before ordering pistons
Related Guides & Resources
CR to PSI Calculator
Convert compression ratio to cranking PSI and back. Polytropic vs isentropic, altitude correction, visual CR-to-PSI table 7:1–15:1.
ECR with Boost
SCR or DCR plus boost PSI → effective compression ratio. Turbo vs supercharger ECR difference, target octane → max safe SCR, detonation risk matrix.
Metric CR Calculator
Pure metric-unit compression ratio calculator. Bore/stroke in mm, altitude in meters, results in bar and kPa. European and Japanese engine presets.
What Is Compression Ratio in an Engine? Complete Guide with Formulas & Examples
Complete guide to engine compression ratio — physical meaning, formula, every variable explained, BDC vs TDC, swept vs clearance volume, 8:1 vs 12:1 comparison, thermal efficiency.
Static vs Dynamic Compression Ratio — Why the Difference Is Everything for Fuel Choice
Formula-level deep dive: SCR pure geometry vs DCR real physics, IVC angle, slider-crank derivation, rod ratio effect, octane decision matrix, real LS3 stock vs cammed example.