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An 86 mm bore, an 86 mm stroke and 50 cc of clearance give a compression ratio of 10.99:1 — near enough to 11.0:1. Each cylinder sweeps 499.56 cc, four cylinders displace 1,998.2 cc (2.0 L), and the ratio calls for premium 91 fuel. Enter your own bore, stroke, cylinder count and clearance volume below.
Compression ratio is the volume of a cylinder at bottom dead centre divided by its volume at top dead centre. Enter the bore and stroke in millimetres, the number of cylinders, and the clearance volume in cubic centimetres, and the calculator works out the swept volume, the total displacement, the ratio, and the octane the engine needs.
A piston engine squeezes a fuel-air charge before it burns it, and the compression ratio says how hard the squeeze is. At bottom dead centre the cylinder holds its full volume — the swept volume the piston travels through, plus the small clearance volume left above the piston at the top. At top dead centre only the clearance volume remains. The ratio of those two volumes is the compression ratio: 10.99:1 means the charge is squeezed into roughly a tenth of its full volume before the spark fires. A higher ratio extracts more work from the same fuel, which is why modern engines run 10:1 to 12:1, but it also raises the temperature of the charge, so a high ratio needs high-octane fuel to avoid knock — the uncontrolled detonation that damages pistons.
At the defaults — 86 mm bore, 86 mm stroke, 4 cylinders, 50 cc of clearance per cylinder — one cylinder sweeps 499.56 cc, four cylinders displace 1,998.2 cc (2.0 L), and the ratio is (499.56 + 50) ÷ 50 = 10.99:1. That is a typical modern four-cylinder car engine: a 2.0-litre with an 86 mm square bore and stroke, running on premium 91.
| Specification | Value | Notes |
|---|---|---|
| Bore | 86 mm | Cylinder diameter |
| Stroke | 86 mm | Piston travel, top to bottom |
| Cylinders | 4 | Square engine: bore = stroke |
| Clearance volume | 50 cc per cylinder | Combustion chamber + gasket + deck space |
| Swept volume | 499.56 cc per cylinder | π/4 × 86² × 86 ÷ 1000 |
| Total displacement | 1,998.2 cc (2.0 L) | 499.56 × 4 cylinders |
| Compression ratio | 10.99:1 | (499.56 + 50) ÷ 50 |
| Octane guidance | Premium 91 recommended | CR 9.5–11.0 band |
Setup: bore 86 mm; stroke 86 mm; cylinders 4; clearance volume 50 cc per cylinder.
Line 1 — the bore area. π/4 × 86² = 0.7854 × 7,396 = 5,808.8 mm². This is the cross-section of one cylinder.
Line 2 — the swept volume. 5,808.8 mm² × 86 mm = 499,557 mm³. Divide by 1,000 to convert cubic millimetres to cubic centimetres: 499.56 cc per cylinder.
Line 3 — the total displacement. 499.56 cc × 4 cylinders = 1,998.2 cc, or 2.0 L.
Line 4 — the total cylinder volume. Swept volume plus clearance: 499.56 + 50 = 549.56 cc at bottom dead centre.
Line 5 — the compression ratio. 549.56 ÷ 50 = 10.99, written 10.99:1 — near enough to 11.0:1.
Line 6 — the octane. A ratio of 10.99 sits in the 9.5–11.0 band, so the guidance is premium 91 recommended.
Clearance 60 cc: the ratio falls to (499.56 + 60) ÷ 60 = 9.33:1, and the octane guidance drops to regular 87. Ten extra cubic centimetres of clearance buys a full point of ratio — and a cheaper tank of fuel.
Bore 92 mm and stroke 92 mm: the swept volume grows to π/4 × 92² × 92 ÷ 1000 = 611.6 cc per cylinder, and the ratio climbs to (611.6 + 50) ÷ 50 = 13.23:1 — race fuel / high-octane territory. A bigger bore and stroke raise the ratio when the clearance stays put, because the swept volume grows while the denominator does not.
Clearance 40 cc: the ratio climbs to (499.56 + 40) ÷ 40 = 13.49:1 — race fuel / high-octane only. Skimming 10 cc off the clearance is the classic way to raise a ratio, and it shows why head work is not a free lunch: the fuel bill follows.
6 cylinders: displacement grows to 499.56 × 6 = 2,997.3 cc (3.0 L), but the ratio stays 10.99:1. Adding cylinders adds displacement, never ratio — the ratio is a per-cylinder number.
2 cylinders: displacement shrinks to 499.56 × 2 = 999.1 cc (1.0 L), and the ratio still stays 10.99:1. Same cylinders, same ratio, half the engine.
Reverse — the clearance for a target ratio: clearance = swept ÷ (target ratio − 1). For a 10.0:1 ratio, 499.56 ÷ 9 = 55.5 cc of clearance per cylinder. For 12.0:1, 499.56 ÷ 11 = 45.4 cc. If you are choosing a head gasket or a piston dish to hit a target ratio, this is the number to aim at.
The defaults describe a square 2.0-litre four-cylinder — 86 mm bore and 86 mm stroke — with 50 cc of clearance volume in each cylinder, measured at top dead centre and including the combustion chamber, the head gasket volume, and the piston-to-deck space. The ratio is the static compression ratio: the geometric ratio of the two volumes, with no allowance for the intake valve closing late, for boost, or for the fuel. Rounding convention: swept volume and displacement are computed from unrounded values and rounded only for display, to two decimals for swept volume and one decimal for displacement. The octane bands are guidance for pump fuel, not a manufacturer specification — always follow the fuel rating in your owner’s manual.
| Output | Formula | What it means |
|---|---|---|
| Swept volume per cylinder | π/4 × 86² × 86 ÷ 1000 | 499.56 cc — the volume one piston sweeps. |
| Total displacement | 499.56 × 4 | 1,998.2 cc (2.0 L) — the engine’s size. |
| Compression ratio | (499.56 + 50) ÷ 50 | 10.99:1 — the squeeze, near 11.0:1. |
| Octane guidance | CR 9.5–11.0 band | Premium 91 recommended. |
| Clearance 60 cc | (499.56 + 60) ÷ 60 | 9.33:1 — Regular 87 octane. |
| Bore 92 mm, stroke 92 mm | π/4 × 92² × 92 ÷ 1000 | Swept 611.6 cc, CR 13.23:1 — Race fuel / high-octane. |
| Clearance 40 cc | (499.56 + 40) ÷ 40 | 13.49:1 — Race fuel / high-octane. |
| 6 cylinders | 499.56 × 6 | 2,997.3 cc (3.0 L), CR 10.99:1. |
| 2 cylinders | 499.56 × 2 | 999.1 cc (1.0 L), CR 10.99:1. |
| Reverse: clearance for 10.0:1 | 499.56 ÷ (10 − 1) | 55.5 cc of clearance per cylinder. |
The math is exact for the numbers you enter: swept volume is π/4 × bore² × stroke, displacement is swept volume times the cylinder count, and the ratio is (swept + clearance) ÷ clearance. What the model does not do is measure your engine: the clearance volume is the hardest number to get right, because it includes the combustion chamber, the gasket, the piston dish or dome, and the deck clearance, and a 5 cc error in a 50 cc clearance moves the ratio by a full point. The ratio shown is the static compression ratio, not the dynamic or effective ratio: a real engine’s intake valve closes after bottom dead centre, so the charge actually trapped is smaller, and a turbocharged engine’s effective ratio is higher than its static number. The octane guidance is approximate — direct injection, variable valve timing, and knock sensors let some engines run higher ratios on lower octane than the bands suggest. Read the engine oil viscosity chart for the oil your engine needs, and the gas vs electric car calculator if the fuel bill is what you are really comparing.
Bright Side Kit. "Engine Compression Ratio Calculator — Bore & Stroke." https://brightoolbox.com/engine-compression-ratio-calculator/ Accessed October 24, 2026.The volume of a cylinder at bottom dead centre divided by its volume at top dead centre. At the defaults — 86 mm bore, 86 mm stroke, 50 cc clearance — the cylinder holds 549.56 cc at the bottom and 50 cc at the top, so the ratio is 549.56 ÷ 50 = 10.99:1. It says how hard the engine squeezes the fuel-air charge before it burns it.
Most modern petrol engines run 10:1 to 12:1. A ratio around 10.99:1, like the default here, is typical for a modern four-cylinder car and calls for premium 91. Older engines ran 8:1 to 9.5:1 on regular fuel, and high-performance engines run 12:1 and beyond — but only on high-octane or race fuel.
With a burette and fluid, with the head off or through the spark plug hole. The standard method is to set the piston at top dead centre, seal the ring gap with grease, and fill the space above the piston with a measured volume of fluid from a burette until it reaches the deck — the volume used is the clearance volume. Add the head gasket volume and the combustion chamber volume for the total. A 5 cc error in a 50 cc clearance moves the ratio by a full point, so measure carefully.
Generally yes — a higher ratio extracts more work from the same fuel. Raising the ratio from 9.33:1 to 10.99:1 makes the engine more thermally efficient, so it makes more power on the same amount of fuel. The limit is knock: squeeze the charge too hard on fuel that cannot take it, and it detonates before the spark, which damages pistons. That is why a higher ratio demands higher-octane fuel.
Below 9.5:1, regular 87 is enough; 9.5 to 11.0, premium 91 is recommended; 11.0 to 12.5, premium is required; above 12.5, race fuel or high-octane. The default 10.99:1 sits in the premium-91-recommended band. These are guidance bands, not a manufacturer specification — always follow the fuel rating in your owner’s manual, because direct injection and knock sensors let some engines run higher ratios on lower octane.
Static ratio is geometry; dynamic ratio is what the engine actually traps. The static ratio — what this calculator shows — is the volume at bottom dead centre divided by the volume at top dead centre. The dynamic ratio accounts for the intake valve closing after bottom dead centre, so some of the charge is pushed back out before compression starts, and the trapped ratio is lower. A turbocharged engine’s effective ratio is higher than its static number, because boost packs more charge in.
No — displacement and compression ratio are independent. Displacement is swept volume times the cylinder count: 499.56 cc × 4 = 1,998.2 cc, and 499.56 cc × 6 = 2,997.3 cc. The ratio stays 10.99:1 in both cases, because the ratio is a per-cylinder number. Changing the bore or stroke changes both displacement and ratio; changing the cylinder count changes displacement only.
Yes — by shrinking the clearance volume. A thinner head gasket, a skimmed cylinder head, or domed pistons all reduce the volume above the piston at top dead centre. Cutting clearance from 50 cc to 40 cc raises the ratio from 10.99:1 to 13.49:1 — but the fuel bill follows, because 13.49:1 needs race fuel. Raise the ratio only as far as the fuel you are willing to buy can support.