Compression Ratio Calculator
Static compression ratio from bore, stroke, chamber cc, head gasket, deck clearance and piston dish or dome, plus dynamic CR from rod length and intake closing.
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About this toolWhat is the Compression Ratio Calculator?
Compression ratio compares the space above the piston at bottom dead centre with the space left at top dead centre. Getting it right decides whether an engine runs happily on pump fuel or pings itself to pieces, and it is set by a handful of small volumes that are easy to get wrong: the combustion chamber, the head gasket, the gap between piston and deck, and the dish, dome or valve reliefs in the piston crown.
Enter those dimensions and this calculator adds up the clearance volume piece by piece, shows how much each part contributes, and returns the static ratio. Turn on dynamic compression and it also works out the ratio from the point where the intake valve actually closes, using the rod length and cam timing, which is a better guide to what the mixture really sees. A target field tells you what chamber volume would hit a chosen ratio, and a sensitivity list shows what a few cc or a thicker gasket would do.
- Static compression from bore, stroke, chamber, gasket, deck and piston volumes
- Dynamic compression from rod length and intake valve closing, using slider-crank geometry
- Breakdown bar of every part of the clearance volume
- Chamber volume needed for a target ratio, plus what ±2 or ±4 cc and a thicker gasket would do
- Inch or millimetre inputs, copy and share link
How to use the Compression Ratio Calculator
- Choose inches or millimetres and enter bore and stroke.
- Enter the combustion chamber volume in cc, measured or from the head maker.
- Enter the head gasket bore and its compressed thickness, then the deck clearance (positive if the piston sits below the deck at TDC).
- Enter the piston volume: dish and valve reliefs as a positive number, a dome as negative.
- Read the static ratio and the breakdown of clearance volume. Switch on dynamic compression and enter rod length and intake closing angle (degrees after BDC) to see the dynamic ratio.
- Optionally enter a target ratio to see the chamber volume that would give it.
Worked examples
Small-block street build
Input
4.000 x 3.480 in, 64 cc heads, 4.100 x 0.041 in gasket, 0.025 in below deck, 5 cc valve reliefs
Output
Swept 716.6 cc, clearance 83.02 cc, static 9.63:1
The gasket adds 8.87 cc and the deck 5.15 cc, so the chamber is only three quarters of the clearance volume. Ignoring the gasket would overstate compression by more than a full point.
Same engine, dynamic compression
Input
5.7 in rod, intake closes 60° ABDC
Output
Effective stroke 2.813 in, dynamic 7.98:1
A cam closing at 70° ABDC drops it to 7.39:1; one closing at 45° raises it to 8.70:1. This is why a bigger cam can tolerate more static compression.
Swapping to larger chambers
Input
Same short block with 76 cc heads
Output
Static 8.54:1
Twelve extra cc in the head cost more than a full point of compression. The target field shows that 9.63:1 needs about 64 cc.
Zero deck and a domed piston
Input
0 deck clearance, 6 cc dome (enter −6)
Output
Static 10.95:1
Reversing the sign of the piston volume swings the ratio by well over a point, the most common entry mistake.
Background and insight
Static compression ratio is (swept volume + clearance volume) / clearance volume. Swept volume is π/4 x bore² x stroke for one cylinder. Clearance volume is everything left above the piston at TDC: the chamber in the head, the cylinder of gasket (π/4 x gasket bore² x compressed thickness), the deck volume (π/4 x bore² x deck clearance), and the piston crown shape. In this calculator a dish or valve relief is entered as a positive volume because it adds space, and a dome as negative because it takes space away. A piston that rises above the deck at TDC has a negative deck clearance.
Dynamic compression accounts for the intake valve staying open after bottom dead centre. Until it closes, the rising piston pushes charge back into the port, so compression only really starts at intake valve closing (IVC). The calculator finds the piston position at a crank angle of 180° plus the IVC figure using slider-crank geometry, x = r + l − (r cos θ + √(l² − r² sin² θ)), where r is half the stroke and l the rod length, and uses the remaining travel as the effective stroke. Use the seat-to-seat (advertised) IVC from the cam card; a common rule of thumb is to add about 15° to the 0.050 in figure if that is all you have.
As rough guidance from engine builders, iron-headed street engines on premium pump fuel are usually kept near 8 to 8.5:1 dynamic, with aluminium heads tolerating a little more. These are rules of thumb, not limits: chamber shape, quench, ignition timing, fuel and boost all shift them.
Common use cases
Choosing heads, pistons and gaskets
Compare chamber sizes, gasket thicknesses and piston volumes to land on a target ratio before ordering parts.
Matching a camshaft
Check that the static ratio and the cam's intake closing give a dynamic ratio suited to the fuel you will run.
Rebuilding a stock engine
Work out what milling the heads or decking the block will do to compression after machining.
Boost planning
Pick a lower static ratio for a turbocharged or blown build and see how the parts choices get there.
Tips and limitations
- Measure chamber and piston volumes with a burette and a plate if you can; catalogue numbers can be off by several cc.
- Use the gasket's compressed thickness, which is usually printed on the package and is less than the free thickness.
- Deck clearance is measured at TDC with the piston rocked square; a piston that rises above the deck gets a negative number.
- Dynamic compression depends heavily on which IVC figure you use. Seat timing (often at 0.006 in lift) gives a more realistic result than the 0.050 in figure.
- Milling a head by a given amount removes roughly the chamber area times the cut, so measure again after machining rather than estimating.
Frequently asked questions
How do you calculate the static compression ratio?
CR = (swept volume + clearance volume) / clearance volume, all for one cylinder. Clearance volume is chamber + gasket + deck volume + piston dish (or minus a dome). For a 716.6 cc cylinder with 83.02 cc of clearance, CR = 799.6 / 83.02 = 9.63:1.
How do you work out head gasket volume?
Treat the gasket as a short cylinder: π/4 x gasket bore² x compressed thickness. A 4.100 in bore, 0.041 in gasket is 0.5413 cu in, or 8.87 cc.
Is a dish piston positive or negative in a compression ratio calculation?
A dish or valve relief adds clearance volume, so it is positive and lowers compression. A dome takes up volume, so enter it as negative; it raises compression.
What is dynamic compression ratio?
It is the compression ratio measured from the moment the intake valve closes rather than from bottom dead centre. Because the valve closes after BDC, the effective stroke is shorter and the dynamic ratio is always lower than the static one.
What dynamic compression ratio is safe on pump gas?
A common builder rule of thumb is around 8 to 8.5:1 for iron heads on premium fuel, with aluminium heads allowing a little more. Chamber design, quench, ignition timing and fuel quality all matter, so treat it as a starting point, not a guarantee.
How does deck clearance affect compression?
Every thousandth of an inch the piston sits below the deck adds a thin disc of volume. On a 4 in bore, 0.025 in of deck clearance adds 5.15 cc; taking it to zero raises the small-block example from 9.63 to 10.20:1.
Which cam timing figure should I use for dynamic compression?
Use the intake closing at seat-to-seat (advertised) duration from the cam card. If you only have the 0.050 in lift figure, adding about 15° is a common approximation.
Is there an in-depth guide to the Compression Ratio Calculator?
Yes. “Static vs Dynamic Compression Ratio: How to Calculate Both” covers the background, how it works, common pitfalls and worked examples. Read the full guide.
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