Engine Displacement Calculator (Bore and Stroke)

Displacement is the volume the pistons sweep between top dead centre and bottom dead centre. Each cylinder is a cylinder in the geometric sense, so its swept volume is the area of the bore times the stroke: π ÷ 4 × bore² × stroke. Multiply by the number of cylinders for the whole engine.

Units are the only trap. With the bore and stroke in millimetres, the calculator converts them to centimetres first, so the volume comes out in cubic centimetres, which are the same as millilitres. With inches, the volume comes out in cubic inches, and one cubic inch is exactly 16.387 cc. A litre is 1,000 cc.

KTM lists the 125 SX engine with a 54 mm bore and a 54.5 mm stroke. The calculator gives π ÷ 4 × 5.4² × 5.45 = 124.82 cc for its single cylinder, which is 7.62 cubic inches or 0.125 litres, against the 124.8 cc KTM publishes.

A twin works the same way. Tohatsu's M18E2 outboard has two cylinders of 60 mm bore and 52 mm stroke: 294.1 cc in total, 147 cc per cylinder, which Tohatsu rounds to 294 cc.

Manufacturers round, sometimes down. Honda's GCV170 mower engine, 60 × 59 mm, computes to 166.8 cc and is sold as 166. If your result lands within a cc or so of the published figure, your measurements are right.

American car engines are still sized in cubic inches. The classic small-block 350 V8 has a 4 inch bore and a 3.48 inch stroke across eight cylinders: 349.8 cubic inches, which is 5,733 cc or 5.7 litres, hence the "5.7 litre" badge.

For a conversion without bore and stroke, use the cubic inch ↔ cc converter in the calculator: multiply cubic inches by 16.387 to get cc, or divide cc by the same number. A 100 cubic inch engine is 1,638.7 cc, and a 50 cc chainsaw is 3.05 cubic inches.

The bore ÷ stroke ratio says a lot about an engine's character. Above 1 the engine is oversquare (short-stroke): the piston travels less per turn, so it can rev higher, which suits saws and racing bikes. Below 1 it is undersquare (long-stroke), which favours torque at low speed. The KTM 125 SX is almost exactly square at 0.99, the Honda GCV170 mower engine is 1.02, and Yamaha's E40X outboard is oversquare at 1.14.

Displacement on its own does not tell you the power; for that you need to know what kind of engine it is, which is the question the cc to hp converter asks first.

The formula is identical for both. What differs is what the volume does: a 2-stroke fills and fires its cylinder on every revolution, a 4-stroke on every second one. That is why a typical 2-stroke handheld engine makes about 1.9 times the horsepower per cc of a 4-stroke small engine, and why the cc to hp converter asks for the engine type before it estimates anything.

Displacement plays no part in the fuel mix either. A 2-stroke's gas-to-oil ratio is set by the maker for the engine's design and the oil it was developed with, not by its cc, so a 25 cc trimmer and a 70 cc saw from the same brand both run 50:1. The oil mix calculator handles the amounts, and the dirt bike page covers why the ratio shifts there.

• Bore: measure the cylinder's inside diameter with a caliper near the top, above the ring travel, where the wall is least worn. Take two readings at right angles and use the larger.
• Stroke: with the spark plug out, run a depth gauge or a thin rod through the plug hole and note the piston height at top dead centre and at bottom dead centre; the difference is the stroke. On a stripped engine, the stroke is twice the crank throw (the distance from the crank's centreline to the crankpin's).
• Overbore: a rebore enlarges the bore and with it the displacement. Taking the bore from 54 to 54.5 mm adds 2.3 cc to the KTM example, for 127.1 cc in total.