Compression Ratio Calculator
Find an engine's static compression ratio from its bore, stroke, and combustion-chamber (clearance) volume. Enter bore and stroke in millimeters or inches and the chamber volume in cc.
Example: with Bore 86 · Stroke 86 · Bore/stroke unit millimeters (mm) · Clearance volume (cc) 50 → Compression ratio: 10.99:1.
- Swept volume499.6 cc per cylinder
- Fuel guidancePremium (91+ octane) recommended
Computed by the calculator below using its default values. Change any input to see your own numbers.
CR = (swept volume + clearance volume) / clearance volume. It sets how tightly the mixture is packed at the top of the stroke — and how much octane you need.
Why compression ratio matters
The compression ratio compares two volumes: how much space is above the piston at the bottom of the stroke versus at the very top. At the bottom you have the swept volume plus the clearance (combustion-chamber) volume; at the top only the clearance volume remains. The ratio of the two tells you how tightly the air-fuel mix gets squeezed.
Squeeze the mixture harder and each combustion event releases more of its energy, which lifts efficiency and power. Squeeze too hard for the fuel and it ignites early and knocks, which is destructive — so compression and octane have to be matched.
Getting the clearance volume right
The swept volume comes straight from bore and stroke, but the clearance volume is the fiddly part. It is not just the cylinder-head chamber: it also includes the head-gasket bore, any gap from the piston sitting below the deck, and the volume added or removed by a dished or domed piston crown.
Add those pieces into the single clearance figure you enter here. Skipping the gasket and deck volumes is the most common reason a calculated ratio reads too high compared with the real engine.
How it’s calculated
Static compression ratio CR = (Vs + Vc) / Vc, where the swept volume per cylinder Vs = (π/4) × bore² × stroke and Vc is the clearance volume at top dead center. Bore and stroke are converted to millimeters (1 in = 25.4 mm) and Vs is reported in cubic centimeters; the clearance volume is entered in cc.
This is the static (geometric) ratio. Dynamic compression, which depends on cam timing and when the intake valve closes, is lower and is what the fuel actually experiences.
Compression ratio and fuel
| Compression ratio | Typical fuel | Where you see it |
|---|---|---|
| Under 9:1 | Regular, low-compression | Older or heavily boosted engines |
| 9–10.5:1 | Regular to mid-grade | Most naturally aspirated engines |
| 10.5–12:1 | Premium, 91+ octane | Modern performance NA engines |
| 12–14:1 | Race gas or E85 | Track and high-performance builds |
| Over 14:1 | Methanol / race fuel | Purpose-built race engines |
General guidance; real octane needs also depend on ignition timing, boost, chamber design, and cooling.
Common mistakes
- Entering only the head-chamber volume as clearance — real clearance also includes the head gasket, deck gap, and piston dish or dome.
- Mixing bore/stroke units, such as a bore in inches with a chamber in cc, which inflates the ratio.
- Confusing static compression ratio with dynamic compression ratio; cam timing lowers the effective figure.
- Assuming higher is always better — too much compression on pump gas causes knock and can wreck the engine.
Frequently asked questions
What is the compression ratio formula?
CR = (swept volume + clearance volume) / clearance volume. The swept volume comes from bore and stroke; the clearance volume is the space left above the piston at top dead center.
What compression ratio needs premium gas?
As a rough guide, naturally aspirated engines above about 10.5:1 usually want 91+ octane. Timing, boost, and chamber design shift the line, so treat it as a starting point, not a rule.
What is the difference between static and dynamic compression?
Static compression is pure geometry from the volumes. Dynamic compression accounts for when the intake valve closes, so a long-duration cam lowers the effective ratio the fuel sees.
What counts as clearance volume?
Everything above the piston at top dead center: the combustion-chamber volume, the head-gasket bore, the deck clearance, and the effect of a dished or domed piston.
Does higher compression add power?
Up to a point, yes — it raises thermal efficiency. But past what the fuel's octane can handle, it causes knock, so gains are capped by the fuel you run.