How Fast Is a Fart?
Can a fart break the sound barrier? How far does the smell spread per minute? And how fast does the gas actually flow out of the body? The physics of farting is surprisingly complex — and the answers are both soberly scientific and delightfully absurd.
Outflow Speed: No Rocket Propulsion
Measured data on the actual gas outflow speed during farting are rare in scientific literature — the topic is not exactly a research focus of astrophysics. The few available measurements and physical estimates yield sobering results: outflow speed typically lies in the range of 3 to 10 km/h — roughly the speed of a brisk walk. That corresponds to approximately 1 to 3 metres per second. The speed of sound (343 m/s at 20°C room temperature) is therefore 100 to 300 times higher. A fart cannot physically break the sound barrier — at least not with the human digestive apparatus.
Where Does the Pressure Come From?
The gas in the large intestine is under pressure slightly above atmospheric pressure — typically a few millimetres of mercury (mmHg) above normal. For comparison: an inflated balloon has an internal pressure of around 10–20 mmHg above ambient pressure. A fart is therefore not a high-pressure event. The pressure is created by bacterial gas production and peristalsis transporting gas toward the rectum. The outflow speed is largely determined by how far the external anal sphincter is open — a tight sphincter creates higher local flow velocity and therefore louder sounds, while a relaxed sphincter lets gas escape more slowly and quietly.
Smell Propagation: Diffusion vs. Convection
How fast does the smell spread? Two physical processes work together: convection (air movement that carries the smell along) and diffusion (independent spreading of gas molecules). In a still room without air movement, hydrogen sulphide (H₂S) — the main odour component — spreads by diffusion: the diffusion constant of H₂S in air is approximately 1.6 × 10⁻⁵ m²/s. This means the molecules independently travel only a few centimetres in one minute. What actually carries the smell across several metres is air movement through convection — created by body heat, air conditioning, or simply standing up from a chair.
Loudness and Frequency: The Physics of Sound
The sound of a fart is produced by the flapping and vibrating of anal folds as gas exits — similar to a wind instrument. The frequency of the sound depends on the geometry of the sphincter, the gas flow rate, and tissue tension. Measured frequencies typically lie between 30 and 1000 Hz — within the audible range. Loudness varies considerably: quiet exhalations can be below 30 dB (barely perceptible), while especially loud events have been measured at up to 80 dB — comparable to a busy street. A fart can therefore be very clearly audible in a quiet room, but it doesn't blow the walls down.
Did you know?
- The outflow speed of a fart is 3–10 km/h — similar to a jogger.
- To break the sound barrier, a fart would need to reach 343 m/s — physically impossible with the human body.
- The smell spreads mainly through air movement (convection), not through independent diffusion of gas molecules.
- Farts can be up to 80 dB loud — as loud as a busy street.
Fun Fact
NASA actually conducted studies on gas propagation in space capsules — not because of the sound, but because of the flammability of hydrogen in the oxygen-rich atmosphere of early capsule models. Astronauts were instructed to avoid certain gas-producing foods before missions. One of the less glorious research threads in space exploration history.
Sources
- Moshfegh, A. et al. (2005). Usual nutrient intakes from food and water. USDA Agricultural Research Service.
- Levitt, M.D. & Bond, J.H. (1970). Volume, composition, and source of intestinal gas. Gastroenterology, 59(6), 921–929.
- Suarez, F.L. et al. (1997). Insights into human colonic physiology obtained from the study of flatus composition. American Journal of Physiology, 272(5), G1028–G1033.
- Matan, N. et al. (1999). Physical properties of hydrogen sulfide in air. Journal of Chemical Engineering Data.