Engine Displacement & Compression Ratio Calculator
Displacement, static compression ratio and bore/stroke ratio from the engine’s geometry.
Displacement
Enter the bore and the stroke.
Static compression ratio
Uses the bore and stroke above. Volumes in cc; lengths in millimetres.
Results are estimates from the formulas shown, not a professional design or certification. Have a qualified engineer verify anything safety-critical (structures, electrical installations, gas or pressure systems).
About the Engine Displacement & Compression Ratio Calculator
Work out an engine’s displacement (swept volume) from its bore, stroke and number of cylinders, in cc, litres and cubic inches, or find the bore or stroke that gives a displacement you have in mind. The second part works out the static compression ratio from the combustion chamber volume, the head gasket, the deck clearance and the piston’s dish or dome, and the chamber volume that would give a target ratio. You also get the bore/stroke ratio, which says whether an engine is oversquare or undersquare.
It is pure cylinder geometry: the formulas are below with a worked example, and everything is calculated in your browser.
How to use it
- Choose millimetres or inches, then what to work out: the displacement, the bore or the stroke.
- Enter the bore, the stroke and the number of cylinders — or the displacement and whichever of bore and stroke you know.
- Read the displacement in cc, litres and cubic inches, the volume of one cylinder and the bore/stroke ratio.
- For the compression ratio, enter the combustion chamber volume in cc and, if you know them, the head gasket’s bore and compressed thickness, the deck clearance and the piston’s dish or valve reliefs (positive) or dome (negative) in cc.
- Optional: enter a target compression ratio to see the chamber volume that would give it with the same parts.
Examples
Bore 86 mm · stroke 86 mm · 4 cylinders
1998 cc (2.00 L, 121.9 cu in); 499.6 cc per cylinder; square (1.00)
Bore 4.000 in · stroke 3.480 in · 8 cylinders
349.8 cu in · 5733 cc · 5.7 L
Bore 72 mm · stroke 85.8 mm · 1 cylinder
349.3 cc; undersquare (0.84)
The 350 V8 with a 64 cc chamber, a 4.100 in × 0.041 in gasket, 0.025 in deck clearance and 5 cc valve reliefs
Clearance volume 83.0 cc → 9.63 : 1
Same parts, target 10 : 1
A chamber of about 60.6 cc
Common uses
- Checking the displacement of an engine from its bore and stroke, or working out what a bigger bore or a stroker crank would give.
- Planning an engine build: the compression ratio from the heads, gasket, deck height and pistons you have chosen.
- Choosing a head gasket thickness or chamber size to reach a target compression ratio.
- Converting an engine size between cc, litres and cubic inches for spec sheets, insurance or registration forms.
The formulas
- Swept volume of one cylinder = π ÷ 4 × bore² × stroke. With bore and stroke in millimetres, divide by 1000 for cc (cm³).
- Displacement = swept volume × number of cylinders. 1 litre is 1000 cc, and 1 cubic inch is exactly 16.387064 cc because an inch is exactly 25.4 mm.
- Bore from a displacement V = √(4 × V ÷ (π × stroke × cylinders)); stroke = 4 × V ÷ (π × bore² × cylinders).
- Static compression ratio = (swept volume + clearance volume) ÷ clearance volume. The clearance volume is everything above the piston at top dead centre: the combustion chamber + the gasket (π ÷ 4 × gasket bore² × compressed thickness) + the deck clearance (π ÷ 4 × bore² × the gap) + the piston’s dish or valve reliefs (a dome counts as negative).
- Chamber for a target ratio = swept volume ÷ (target − 1) − gasket − deck − piston volume.
Worked example
A small-block V8 with a 4.000 in bore and a 3.480 in stroke: one cylinder sweeps π ÷ 4 × 4.000² × 3.480 = 43.73 cu in, or 716.6 cc, so eight make 349.8 cu in (5733 cc).
With a 64 cc chamber, a 4.100 in gasket compressed to 0.041 in (π ÷ 4 × 4.100² × 0.041 = 0.541 cu in = 8.87 cc), the piston 0.025 in below the deck (0.314 cu in = 5.15 cc) and 5 cc of valve reliefs, the clearance volume is 64 + 8.87 + 5.15 + 5 = 83.0 cc, and the compression ratio (716.6 + 83.0) ÷ 83.0 = 9.63 : 1.
Oversquare, square and undersquare
The bore/stroke ratio compares the width of the cylinder with the piston’s travel. Above 1 is oversquare (a short stroke), which leaves room for larger valves and is common in engines that rev high. Below 1 is undersquare (a long stroke), common where pulling power at low engine speeds matters more. About 1 is called square.
Limitations
- Geometry only: this is the static (mechanical) compression ratio. The dynamic ratio also depends on when the intake valve closes, and no power, torque or fuel figures are worked out.
- Use measured volumes where you can: listed chamber and piston volumes are nominal, and measuring them with a burette gives the real figures.
- Makers round their quoted displacement and sometimes use rounded bore and stroke figures, so a result can differ from the badge by a few cc.
- Rotary (Wankel) engines and the trapped compression ratio of two-stroke engines are not covered.
Privacy
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Frequently asked questions
How do I calculate engine cc from bore and stroke?
Multiply π ÷ 4 by the bore squared and by the stroke, all in centimetres, then by the number of cylinders. For 86 mm × 86 mm: 0.7854 × 8.6² × 8.6 = 499.6 cc per cylinder, or 1998 cc for four cylinders.
How many cc is a cubic inch?
Exactly 16.387064 cc, because an inch is exactly 2.54 cm and 2.54³ = 16.387064. So 350 cubic inches is about 5735 cc, and 2.0 litres is about 122 cubic inches.
Should a dished piston be positive or negative?
Positive: a dish and valve reliefs add space above the piston and lower the ratio. A dome takes space away, so enter it as a negative number of cc.
What does the deck clearance do?
If the piston stops below the top of the block at top dead centre, the gap adds a thin disc of volume (π ÷ 4 × bore² × gap) to the clearance volume and lowers the ratio. A piston that comes up above the deck takes volume away: enter it as a negative number.
What does oversquare mean?
The bore is larger than the stroke, so the bore/stroke ratio is above 1. Undersquare means the stroke is longer than the bore, and square means they are about equal.