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Mean Piston Speed Calculator

Enter the stroke length in mm and the engine RPM to get the mean piston speed in m/s and ft/min.

How it is calculated

The piston travels one stroke up and one down for every crankshaft turn, so in one minute it covers 2 × stroke × RPM of distance. The tool converts the stroke from millimetres to metres and divides by 60 to get seconds: MPS (m/s) = 2 × (stroke ÷ 1000) × RPM ÷ 60. The second figure is the same speed in feet per minute, ft/min = m/s × 196.85, rounded to a whole number. Stroke is the piston travel from top to bottom dead centre, not the bore, and RPM is crankshaft speed. Both inputs must be above zero. The result is an average over a cycle; instantaneous piston speed is higher at mid-stroke and zero at both ends of travel.

Worked example

An engine with an 86 mm stroke turning 7,000 RPM. Stroke in metres: 86 ÷ 1000 = 0.086 m. Distance per minute: 2 × 0.086 × 7,000 = 1,204 m. Per second: 1,204 ÷ 60 = 20.07 m/s (20.0667 before rounding). In imperial units: 20.0667 × 196.85 = 3,950 ft/min.

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Common mistakes

  • Entering the cylinder bore instead of the stroke: bore and stroke are different dimensions and only the stroke (top to bottom dead centre) belongs here.
  • Typing the stroke in centimetres or inches: the field is millimetres, so 8.6 for 86 mm would give a speed ten times too low.
  • Using the redline for a daily-use figure, or the idle speed for a peak figure, without noting which RPM you entered.
  • Halving the result for a four-stroke engine: the factor of 2 is already the up and down travel per crank revolution, whatever the engine cycle.
  • Reading the mean speed as the peak speed of the piston: the highest instantaneous speed is noticeably above the mean.

About this calculator

Mean piston speed measures the average linear velocity of a piston as it travels up and down inside the cylinder, calculated as 2 multiplied by the stroke length multiplied by the engine's RPM, divided by 60 to convert to meters per second — the factor of 2 accounts for the piston completing two strokes (up and down) per revolution.

Engine designers and tuners use mean piston speed as a practical proxy for the mechanical stress the reciprocating assembly endures, because it's a more meaningful comparison across different engines than RPM alone — a long-stroke engine reaches high piston speeds at relatively low RPM, while a short-stroke, “oversquare” engine can spin to much higher RPM before hitting the same piston speed, which is why high-revving sports car and motorcycle engines are almost always designed with short strokes. Most production engines are designed to keep mean piston speed under roughly 20-25 m/s at their redline, since speeds beyond that dramatically accelerate wear on rings, wrist pins and rod bearings.

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