Compression Ratio Calculator

What is a compression ratio calculator?

A compression ratio calculator works out an engine’s static compression ratio from two volumes you measure per cylinder: the swept volume and the clearance volume, both in cubic centimetres (cc). The compression ratio compares how much space the cylinder holds when the piston is at the bottom of its stroke against how much is left when the piston reaches the top. Engine builders, tuners, students and anyone speccing pistons or a head gasket use it to predict squeeze, fuel-octane needs and power potential before turning a single bolt.

Enter your two volumes in the tool above and it returns the ratio instantly as X.XX : 1, with no sign-up and no data leaving your browser.

How is compression ratio calculated?

The formula is compression ratio = (swept volume + clearance volume) ÷ clearance volume, applied to a single cylinder with both volumes in cc:

• Swept volume — the cylinder displacement, the volume the piston sweeps through as it travels from top dead centre (TDC) to bottom dead centre (BDC). It is set by the bore and stroke and must be a positive number.
• Clearance volume — the space left above the piston when it sits at top dead centre. It is the sum of the combustion chamber volume, the head gasket bore volume, the deck clearance volume and any piston dish (a dome subtracts), again in cc and positive.

The numerator, swept + clearance, is the total cylinder volume at BDC; the denominator, clearance alone, is the trapped volume at TDC. Dividing one by the other gives a unitless number because the cc units cancel, which is why the answer is expressed as a ratio to 1. A larger swept volume or a smaller clearance volume both push the ratio up.

The result is rounded and shown to two decimal places, for example 11.00 : 1.

Examples

Each example uses only the formula above, so you can reproduce every answer by typing the same two volumes into the calculator.

Example 1 — the baseline 11:1 build

Swept volume 500 cc, clearance volume 50 cc.

CR = (swept + clearance) ÷ clearance = (500 + 50) ÷ 50 = 550 ÷ 50 = 11.00 : 1

The cylinder holds 550 cc at BDC and traps 50 cc at TDC, giving a static 11.00 : 1.

Example 2 — smaller displacement, same clearance

Swept volume 450 cc, clearance volume 50 cc.

CR = (450 + 50) ÷ 50 = 500 ÷ 50 = 10.00 : 1

Reducing only the swept volume (a shorter stroke or smaller bore) drops the ratio to 10.00 : 1, because there is less air to compress into the same space.

Example 3 — shaving the clearance volume

Swept volume 500 cc, clearance volume 45 cc.

CR = (500 + 45) ÷ 45 = 545 ÷ 45 = 12.11 : 1

Keeping the swept volume at 500 cc but cutting the clearance from 50 cc to 45 cc — by milling the head, using a thinner gasket or a domed piston — raises the ratio to 12.11 : 1. A smaller clearance volume always increases compression.

Compression ratio reference table

This table holds the swept volume fixed at 500 cc and varies the clearance volume, computed straight from (500 + clearance) ÷ clearance. It shows how sharply the ratio climbs as the clearance volume shrinks.

Swept volume (cc)	Clearance volume (cc)	Compression ratio
500	100	6.00 : 1
500	70	8.14 : 1
500	60	9.33 : 1
500	50	11.00 : 1
500	45	12.11 : 1
500	40	13.50 : 1
500	35	15.29 : 1

The relationship is non-linear: each cc you remove from a small clearance volume changes the ratio far more than the same cc removed from a large one.

Common uses

The static compression ratio is a core number whenever an engine is built, rebuilt or modified:

• Engine building and rebuilds — choosing pistons, head gaskets and how much to mill a head to hit a target ratio.
• Performance tuning — matching the ratio to a fuel octane and to forced induction, since boost effectively raises cylinder pressure.
• Restoration and swaps — verifying the compression of an unfamiliar or modified engine before it runs.
• Education and exams — automotive and engineering coursework that asks students to derive CR from cylinder volumes.
• Buying and selling — quoting a build’s compression spec so a buyer knows its fuel and tuning requirements.

Tips and common mistakes

• Work per cylinder, not for the whole engine. Both volumes are for one cylinder; the ratio is the same for every identical cylinder, so you do not multiply by cylinder count.
• Add every part of the clearance volume. Clearance is not just the combustion chamber — include the head gasket bore, the deck clearance and the piston dish. Leaving one out gives a clearance that is too small and a ratio that reads too high.
• A dish adds, a dome subtracts. A dished piston increases clearance volume (lower ratio); a domed piston reduces it (higher ratio). Get the sign right.
• Keep both numbers in cc. Mixing cubic inches or millilitres of one with cc of the other corrupts the result. Convert everything to cc first.
• Both values must be positive. A zero or negative clearance volume is unphysical and the formula would break; the swept volume must be positive too.
• Remember this is static CR. It assumes the intake valve closes exactly at BDC. Real valve timing makes the dynamic (running) ratio lower than this static figure.

Limitations and notes

This calculator gives the static compression ratio — a pure geometric comparison of two volumes you supply. It does not measure cylinder pressure, account for valve timing (which sets the lower dynamic compression ratio), or correct for altitude, boost, temperature or gasket crush at running conditions. It also trusts your inputs: if a clearance-volume component is missing or mismeasured, the ratio will be off by the same proportion. Treat the result as an accurate planning and education figure rather than a guaranteed in-cylinder pressure. Everything runs privately in your browser — your volumes are never uploaded or stored, so you can iterate on a build spec as much as you like.

For related engine math, try the engine displacement calculator to find swept volume from bore and stroke, the horsepower calculator to estimate output, or the speed converter for road-speed figures — and browse more in the automotive category.