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Acceleration and G-Force Units: m/s², g, ft/s², and Gal

Acceleration gets measured in several different units depending on the field — here is how they relate.

m/s²: the SI base unit

Meters per second squared (m/s²) is the SI base unit of acceleration, describing how much an object's speed changes every second. It is the default unit in physics and engineering worldwide.

"g": a gravity-based reference, not an SI unit

"g" is not itself an SI unit — it is a reference value equal to standard gravity, 9.80665 m/s², used as a convenient yardstick. Saying something experiences "3g" means it feels three times the acceleration of standing still on Earth's surface.

ft/s² and Gal

Feet per second squared (ft/s²) is the imperial equivalent of m/s², mostly used in US engineering contexts. Gal (short for galileo, equal to 1 cm/s²) is a much smaller, specialized unit used almost exclusively in geophysics and seismology to measure gravitational and seismic acceleration.

Everyday g-force reference points

For scale: standing still is 1g, a hard car brake might reach 0.5-1g, a roller coaster can peak around 3-5g, and fighter pilots train to withstand sustained forces up to roughly 9g before blacking out without a pressure suit.

G-force is not the same as gravity

G-force describes the acceleration you feel, not gravity itself — an astronaut in orbit is in freefall and experiences near-zero g-force despite gravity still acting on them, while someone in an accelerating car on flat ground feels g-force with no change in actual gravitational pull.

Why g is a convenient reference

Using g as a reference makes accelerations intuitively comparable to something everyone already experiences — standing on the ground — rather than requiring people to reason in raw m/s² values that have no built-in reference point.

Why g became the go-to shorthand

Standard gravity is a constant every person has direct, lifelong experience with, which makes it an unusually intuitive reference unit compared to something abstract like m/s². Aerospace and automotive engineers adopted g specifically because it lets people compare unfamiliar accelerations — a rocket launch, a crash test, a fighter jet turn — to something as concrete as their own body weight.

Where Gal survives as a specialized unit

Named after Galileo, the Gal is a legacy CGS unit that never got replaced in geophysics the way it did elsewhere, largely because seismologists and gravimetry researchers were already using extensive historical datasets recorded in Gal and milliGal, and switching units would have complicated comparisons across decades of measurements.

Frequently Asked Questions

Why do astronauts talk about "g" during launch if they are weightless in orbit?

Those are two different phases: during launch, the rocket's acceleration pushes astronauts into their seats with real g-force, sometimes 3g or more. Once in stable orbit, the spacecraft is in continuous freefall around Earth, which produces the sensation of weightlessness even though gravity is still very much present.

Is g-force the same thing as gravity?

No — gravity is a constant pull toward the Earth, while g-force is the acceleration you actually feel, which changes with motion. You can feel high g-force with gravity unchanged (a fast car turn) or feel weightless despite gravity still acting on you (freefall or orbit).