This calculator computes static compression ratio the way an engine builder plans a build: from cylinder bore and stroke, plus the individual pieces that make up clearance volume — combustion chamber, head gasket, and piston dish or dome. It's the complement to cc'ing a finished head: use this one before you've measured anything, to figure out what combination of parts gets you to the ratio you want.

Why build from bore, stroke, and components instead of measured volumes

Some compression-ratio calculators assume you already know your swept and clearance volume — useful once heads are cc'd and pistons are on hand. But when you're still speccing a build, you usually start from catalog numbers instead: a bore and stroke from the block and crank you're using, a chamber volume from the head manufacturer's spec sheet, a gasket volume from the gasket's bore size and compressed thickness, and a dish or dome volume from the piston manufacturer. This calculator works directly from those numbers, so you can compare piston and gasket combinations before ordering parts.

Reading the dish/dome sign convention

Piston dish and dome are entered as a single signed field. A dish — a bowl-shaped depression in the piston crown — adds volume above the piston, so enter it as a positive number; it lowers compression ratio. A dome — a raised piston crown that pushes up into the chamber — removes volume, so enter it as a negative number; it raises compression ratio. Flat-top pistons use 0. Piston manufacturers publish this number directly, sometimes as "+dish" or "−dome" cc.

Using the breakdown and sensitivity views

The Volume Breakdown tab shows what share of total cylinder volume comes from swept volume versus each clearance component, which makes it obvious which lever has the most effect on your build. The Effect of Changes tab holds your bore, stroke, and chamber volume fixed and shows how the ratio moves with a few cc of gasket-thickness change or a piston swap toward more dish or more dome — useful for deciding, for example, whether a 2 cc thicker gasket alone is enough to hit a target ratio, or whether you also need a different piston.