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Compression Ratio Calculator

The Compression Ratio Calculator determines an engine's compression ratio from swept volume (displacement) and clearance volume (combustion chamber). Optimize your engine build for power, efficiency, and fuel octane requirements.

Compression Ratio Calculator — Live Preview
Compression Ratio
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Proportion Solver
A : B = C : D — Enter any 3 values
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Results
Visual ratio breakdown
Solved Proportion
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Simplified
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Percentages
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Decimal
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Fraction
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Visual Ratio
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    Ratio Simplifier
    Reduce any ratio to its simplest form
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    Simplified Result
    Reduced to lowest terms
    Simplified Ratio
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    GCD Used
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    Decimal Ratio
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    Visual Ratio
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      Ratio Scaler
      Multiply a ratio by a scale factor
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      Scaled Result
      Ratio after scaling
      Scaled Ratio
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      Original
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        What is Compression Ratio?

        Compression ratio is the ratio of the total cylinder volume (when the piston is at bottom dead center) to the clearance volume (when the piston is at top dead center). A 10:1 compression ratio means the air-fuel mixture is compressed to 1/10th of its original volume before ignition.

        Higher compression ratios increase thermal efficiency and power output but require higher octane fuel to prevent detonation (knock). Stock gasoline engines typically run 9:1 to 12:1. Diesel engines use 14:1 to 25:1 because diesel fuel ignites through compression alone.

        Formulas & Equations Used

        This Compression Ratio Calculator uses the following core equations:

        1 Compression Ratio ▼
        CR = (Swept Volume + Clearance Volume) / Clearance Volume

        A cylinder with 500cc swept volume and 55cc clearance: CR = (500 + 55) / 55 = 10.09:1.

        2 Swept Volume (Single Cylinder) ▼
        Swept Volume = π/4 × Bore² × Stroke

        Bore = 86mm, Stroke = 86mm: V = 0.7854 × 86² × 86 = 499.6 cc.

        3 Clearance Volume from CR ▼
        Clearance Volume = Swept Volume / (CR - 1)

        To achieve 9.5:1 with 500cc displacement: CV = 500 / (9.5 - 1) = 58.82 cc.

        How to Use This Compression Ratio Calculator

        Follow these 3 simple steps:

        1

        Enter Your Values

        Type the known values into the input fields above. The Compression Ratio Calculator accepts any positive numbers.

        2

        Choose Calculation Mode

        Select Solve, Simplify, or Scale mode in the calculator. Each applies different equations to your inputs.

        3

        View Results

        Click Calculate to see your answer with a visual ratio bar, pie chart, and step-by-step solution breakdown.

        Example Problems & Step-by-Step Solutions

        Here are 3 worked examples using this Compression Ratio Calculator:

        Example 1 Engine with 600cc displacement and 60cc chamber
        1 Total Volume = 600 + 60 = 660cc
        2 CR = 660 / 60 = 11.0:1
        3 Requires 91+ octane fuel
        ✓ Compression ratio: 11.0:1
        Example 2 Calculate displacement for 88mm bore, 82mm stroke
        1 V = π/4 × 88² × 82
        2 V = 0.7854 × 7744 × 82
        3 V = 498.8 cc per cylinder
        ✓ 498.8 cc displacement per cylinder
        Example 3 Find clearance volume for 12:1 CR, 450cc displacement
        1 CV = Swept Volume / (CR - 1)
        2 CV = 450 / (12 - 1)
        3 CV = 450 / 11 = 40.9 cc
        ✓ Need 40.9 cc clearance volume

        Frequently Asked Questions

        What compression ratio is best for pump gas (87 octane)? ▼

        For 87 octane regular gas, stay at or below 9.5:1 static compression ratio. Modern engines with direct injection and variable valve timing can safely run 10:1 to 11:1 on regular fuel.

        How does compression ratio affect horsepower? ▼

        Each 1-point increase in compression ratio typically yields 3-4% more power, up to a limit. Going from 9:1 to 10:1 might add 10-15 HP on a 300 HP engine. Beyond about 13:1, gains diminish and knock risk increases.

        What is the compression ratio of a diesel engine? ▼

        Diesel engines run 14:1 to 25:1 compression ratios. The high compression heats air enough to ignite diesel fuel without spark plugs. Modern common-rail diesels typically use 16:1 to 18:1.

        How do I measure clearance volume? ▼

        Fill the combustion chamber with fluid (isopropyl alcohol or light oil) through a burette with the valves closed and piston at TDC. The volume of fluid needed equals the clearance volume. Typical values range from 40-70cc.

        Does milling the head increase compression ratio? ▼

        Yes. Milling (shaving) the cylinder head reduces clearance volume, increasing compression ratio. Removing 0.020 inches from the head surface typically raises CR by 0.3-0.5 points depending on chamber size.

        Learn About Ratios

        What is a ratio?

        A ratio is a comparison between two or more quantities showing the relative size of one to another. Written as A : B, it means 'for every A units of the first quantity, there are B units of the second.' For example, a ratio of 3 : 4 means for every 3 parts of A, there are 4 parts of B. Ratios are used in cooking, construction, finance, science, and everyday life.

        How do I solve a proportion?

        A proportion is an equation that says two ratios are equal: A : B = C : D. To solve for a missing value, use cross-multiplication. If D is unknown: D = (B × C) / A. This works because in equal ratios, the cross products are always equal: A × D = B × C. Our Proportion Solver does this automatically — just enter any 3 values and it finds the 4th.

        How do I simplify a ratio?

        To simplify a ratio, find the Greatest Common Divisor (GCD) of both numbers and divide each by it. For example, 24 : 36 — the GCD of 24 and 36 is 12. So 24 ÷ 12 = 2 and 36 ÷ 12 = 3, giving the simplified ratio 2 : 3. Our Simplifier automatically finds the GCD and reduces your ratio to its lowest terms.

        What is ratio scaling and when is it useful?

        Scaling a ratio means multiplying both parts by the same factor to create an equivalent, larger (or smaller) ratio. For instance, scaling 2 : 5 by a factor of 3 gives 6 : 15. This is extremely useful for recipes (tripling a recipe), construction (scaling blueprints), mixing solutions, or any scenario where you need to maintain the same proportion at a different magnitude.

        What's the difference between a ratio and a fraction?

        A ratio A : B compares two quantities to each other (part-to-part), while a fraction A/B typically represents a part-to-whole relationship. However, any ratio can be expressed as a fraction: 3 : 4 is equivalent to 3/4 = 0.75. The key difference is context — ratios compare quantities side-by-side, while fractions represent a portion of a total.