Work out the static compression ratio from bore, stroke, combustion-chamber cc, gasket bore and compressed thickness, signed deck clearance and a signed piston dome or dish — with the full clearance-volume breakdown — then switch to dynamic compression to fold in the connecting-rod length and the intake-valve-closing angle. Enter your own numbers; nothing is looked up and nothing is sent anywhere.
Cylinder
Clearance volume parts (per cylinder)
Signs: a dome is negative (it raises CR), a dish or relief is positive. Use the compressed gasket thickness and the gasket's own bore. A piston that sits below the deck is a positive deck clearance (lowers CR); poking above is negative.
Dynamic compression extras
The valve-closing angle at 0.050″ lift is later than the advertised figure, so the two give different dynamic numbers — use whichever basis your cam card quotes.
Combustion chamber volume from a measured fill
Fill the assembled, sealed chamber with a graduated burette and read the cc it takes. That reading is the chamber volume — there is no reliable way to compute it from outside dimensions because casting shapes vary. Carry this figure into the chamber box of the Static or Dynamic tab.
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The matching engine tool
Need the size rather than the ratio? The engine displacement calculator shares the same bore and stroke but reports cc, litres and cubic inches, reverse-solves a bore or stroke and estimates carb CFM. For a tank, drum or any general container that uses the same πr²h volume idea, see the cylinder & tank volume calculator.
How to figure compression ratio
The compression ratio formula is a ratio of two volumes: the full cylinder volume with the piston at the bottom, over the volume left when it reaches the top. Written out, CR = (swept volume + clearance volume) ÷ clearance volume, where the swept volume is the displacement of one cylinder, (π ÷ 4) × bore² × stroke. To figure compression ratio properly you have to account for everything still above the piston at top dead centre, and that clearance volume is the part people get wrong. This is the same calculation whether you have seen it called calculating engine compression, figuring the comp ratio, or simply the compression ratio formula.
The four parts of the clearance volume — and their signs
The clearance volume is a sum, and three of its four parts can trip you up:
Combustion chamber: the measured cc of the head's chamber. Measure it after any milling rather than trusting a published figure.
Head-gasket volume: uses the gasket's own bore, not the cylinder bore or the chamber, and the compressed thickness, because that thin torqued-down gap is what is left.
Deck clearance: a piston sitting below the deck at top dead centre leaves extra space, so it is a positive clearance that lowers the ratio; a piston poking above the deck is negative and raises it.
Piston dome or dish: a dome fills the chamber, so it subtracts — enter it negative; a dish or valve relief adds space, so it is positive. Reversing this sign is the classic compression-calculation mistake.
The static formula then becomes CR = (Vd + Vc) ÷ Vc, with Vd the swept volume per cylinder. The calculator internally works in cubic centimetres, accepting bore, stroke, gasket and deck in inches or millimetres and the chamber, dome and dish in cc.
Static vs dynamic compression
The static ratio assumes compression starts the instant the piston leaves bottom dead centre, but in a real engine the intake valve is still open well past that point, so some charge is pushed back out before compression truly begins. The dynamic compression ratio fixes this by starting the effective stroke at the moment the intake valve closes. It is always lower than the static figure, and a bigger cam — one that holds the intake valve open longer — lowers it further. The Dynamic tab adds the connecting-rod length and the intake-valve-closing angle and shows static and dynamic compression ratio side by side, so this works as both a dynamic compression calculator and a static compression ratio calculator in one place.
It uses the long-established crank-slider relationship to find how far the piston has travelled down the bore when the valve closes:
where IVC is the intake-valve-closing angle in degrees after bottom dead centre. A note on cam cards: the angle quoted at 0.050″ of lift is later than the advertised duration figure, and the two produce different dynamic numbers, so feed in whichever basis your cam card uses and read the result as a guide rather than a guarantee.
Combustion chamber volume
The chamber volume is measured, not computed: you seal the assembled chamber, fill it from a graduated burette through a clear plate, and read the cubic centimetres. The chamber-cc helper here simply takes that burette reading as the chamber volume, because the casting shape is too irregular to derive from outside dimensions. If you only have a published chamber figure you can type it straight into the chamber box, but a measured cc on your actual heads — especially after milling — is what makes the compression number trustworthy.
2-stroke compression
One wrinkle worth flagging: a 2-stroke engine compression ratio is often quoted two ways. The full or geometric ratio uses the entire stroke just like the static calculation here, but many two-stroke builders quote a trapped (or corrected) ratio measured only from the point the exhaust port closes, which is lower. If you are comparing a trapped figure, measure the stroke from port closure rather than from bottom dead centre; the arithmetic in this tool is the geometric ratio unless you feed it that shortened effective stroke.
A worked example
A V8 with a 4.00″ bore and 3.48″ stroke, a 64 cc chamber, a 4.10″ gasket bore at 0.040″ compressed, 0.010″ deck (piston below) and a flat-top piston (0 cc):
Add a 5.70″ rod and a 60°-ABDC valve closing and the dynamic ratio drops to roughly 8.8:1.
Compression ratio chart — CR vs chamber volume
Below is a small computed reference showing how the static ratio moves with the combustion-chamber cc at a fixed 4.00″ bore, 3.48″ stroke, 4.10″ gasket bore at 0.040″, 0.010″ deck and a flat-top piston. Every value is calculated live from the formula above, not copied from any proprietary table — change a chamber number in the calculator and you will reproduce the same figures.
Static CR vs chamber cc — computed at a fixed bore, stroke, gasket and deck
Chamber (cc)
Clearance (cc)
Static CR
Terms and spellings
For readers outside the US: chamber volume is given in cc, the same as cm³; “degrees” and the ° symbol mean the same crank angle; and a compression ratio cal, a comp ratio calculator and an online compression calculator are all this one job. The arithmetic is identical whether you enter inches or millimetres for the lengths; only the labels change.
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Frequently asked questions
How do I figure compression ratio?
Compression ratio is the cylinder volume with the piston at the bottom divided by the volume left at the top: CR = (swept volume + clearance volume) ÷ clearance volume. The swept volume is (π ÷ 4) × bore² × stroke. The clearance volume is the sum of everything still above the piston at top dead centre — the combustion chamber, the head-gasket gap, the deck space and the piston dish or dome. The single number that drives the result is that clearance volume, which is why getting each part and its sign right matters more than the bore and stroke. This tool adds them up for you and shows the breakdown.
Do I enter a piston dome as negative and a dish as positive?
Yes. A dish or a valve relief adds space above the piston, so it adds clearance volume and is entered as a positive cc figure. A dome sticks up into the chamber and takes space away, so it reduces clearance volume and is entered as a negative cc figure. Getting that sign backwards is the most common error in a compression calculation: a domed piston raises compression, a dished piston lowers it. The calculator labels the box dome negative, dish positive so you can match the sign to the part in front of you.
Which gasket thickness and bore do I use, and how does deck clearance work?
Use the compressed (installed) gasket thickness, not the thicker as-supplied value, because once the head is torqued down that thin gap is what is left. The head-gasket volume uses the gasket's own bore, not the cylinder bore or the chamber, since the gasket opening is what defines that disc of space: HGV = (π ÷ 4) × gasket_bore² × compressed_thickness. Deck clearance is the gap between the piston crown and the block deck at top dead centre; in this tool a piston that sits below the deck is a positive deck clearance, which adds clearance volume and lowers the ratio, while a piston poking above the deck is negative and raises it.
What is the difference between static and dynamic compression ratio?
Static compression ratio assumes compression begins the instant the piston leaves the bottom, but in a running engine the intake valve stays open well past bottom dead centre, so part of the charge is pushed back out before the cylinder actually starts to compress. Dynamic compression ratio accounts for that by starting the effective stroke at the point the intake valve closes. It is always lower than the static figure, and a longer-duration camshaft lowers it further. Because the intake-valve-closing angle quoted at 0.050 inch of lift is later than the advertised figure, the two give different dynamic numbers, so use the same basis your cam card uses and treat the result as a guide.
How does the dynamic compression calculation work?
It uses the crank-slider geometry to find how far the piston has come down the bore at the moment the intake valve closes, then treats only the volume above that point as the effective swept volume. With crank radius r = stroke ÷ 2 and rod length l, the piston's distance below top dead centre at crank angle θ is x = (r + l) − [ r·cosθ + √(l² − r²·sin²θ) ]. With the intake valve closing a given number of degrees after bottom dead centre, θ is measured from top dead centre as 180° minus that angle, and the dynamic ratio is (clearance + the swept volume above that piston position) ÷ clearance. A shorter rod, a wider bore or a later valve closing all change the figure.
How do I find the combustion chamber volume?
The combustion chamber volume is normally measured by filling the assembled chamber with a measured liquid using a graduated burette and a clear plate, and reading off the cubic centimetres it takes. The helper in this tool simply accepts that measured fill in cc as the chamber volume, since that burette reading is the chamber volume; there is no reliable way to compute it from outside dimensions because casting shapes vary. If you only have a published chamber figure you can enter it directly, but a measured cc on your actual heads, especially after any milling, gives the trustworthy compression number.
Why don't you list compression ratios for specific engines?
Because a published compression figure belongs to one specific factory combination and stops being true the moment the heads are milled, the pistons are swapped, the deck is decked or a different gasket goes on — all the everyday changes that this calculation exists to track. So the tool ships no engine database and no proprietary tables; it computes the ratio from the chamber cc, gasket, deck and piston volumes you measure on your own build, using public clearance-volume geometry. Measure your own chamber and clearances and the answer reflects your engine rather than a generic chart.