According to the International System of Units, rpm is not a unit. This is because the wordrevolution is a semanticannotation rather than a unit. The annotation is instead done as a subscript of the formula sign if needed. Because of the measured physical quantity, the formula sign has to be for frequency and or for angular velocity. The corresponding basic SI derived unit is s−1 or Hz. When measuring angular speed, the unitradians per second is used. Formally, hertz and radian per second are two different names for the same SI unit, s−1. However, they are used for two different but proportional ISQ quantities: frequency and angular frequency. The conversion between a frequency and an angular velocity are: Thus a disc rotating at 60 rpm is said to be rotating at either 2 rad/s or 1 Hz, where the former measures the angular velocity and the latter reflects the number of revolutions per second. If the non-SI unit rpm is considered a unit of frequency, then. If it instead is considered a unit of angular velocity and the word "revolution" is considered to mean 2 radians, then.
Examples
On many kinds of disc recording media, the rotational speed of the medium under the read head is a standard given in rpm. Phonograph records, for example, typically rotate steadily at,, 45 or 78 rpm.
Modern air turbine dental drills can rotate at up to 800,000 rpm.
The second hand of a conventional analogue clock rotates at 1 rpm.
Audio CD players read their discs at a precise, constant rate and thus must vary the disc's rotational speed from 8 Hz when reading at the innermost edge, to 3.5 Hz at the outer edge.
DVD players also usually read discs at a constant linear rate. The disc's rotational speed varies from 25.5 Hz when reading at the innermost edge, to 10.5 Hz at the outer edge.
A washing machine's drum may rotate at 500 to 2,000 rpm during the spin cycles.
A power generation turbine rotates at 3000 rpm or 3600 rpm, depending on country – see AC power plugs and sockets.
Modern automobile engines are typically operated around 2,000–3,000 rpm when cruising, with a minimum speed around 750–900 rpm, and an upper limit anywhere from 4500 to 10,000 rpm for a road car or nearly 20,000 rpm for racing engines such as those in Formula 1 cars. The exhaust note of V8 F1 cars have a much higher pitch than an I4 engine, because each of the cylinders of a four-stroke engine fires once for every two revolutions of the crankshaft. Thus an eight-cylinder engine turning 300 times per second will have an exhaust note of 1,200 Hz.
A piston aircraft engine typically rotates at a rate between 2,000 and 3,000 rpm.
Computer hard drives typically rotate at 5,400 or 7,200 rpm, the most common speeds for the ATA or SATA-based drives in consumer models. High-performance drives rotate at 10,000 or 15,000 rpm, usually with higher-level SATA, SCSI or Fibre Channel interfaces and smaller platters to allow these higher speeds, the reduction in storage capacity and ultimate outer-edge speed paying off in much quicker access time and average transfer speed thanks to the high spin rate. Until recently, lower-end and power-efficient laptop drives could be found with 4,200 or even 3,600 rpm spindle speeds, but these have fallen out of favour due to their lower performance, improvements in energy efficiency in faster models and the takeup of solid-state drives for use in slimline and ultraportable laptops. Similar to CD and DVD media, the amount of data that can be stored or read for each turn of the disc is greater at the outer edge than near the spindle; however, hard drives keep a constant rotational speed so the effective data rate is faster at the edge.
Floppy disc drives typically ran at a constant 300 or occasionally 360 rpm with a constant per-revolution data density, which was simple and inexpensive to implement, though inefficient. Some designs such as those used with older Apple computers were more complex and used variable rotational speeds and per-track storage density to store more data per disc; for example, between 394 rpm and 590 rpm with the Mac's 800 KB double-density drive at a constant 39.4 KB/s – versus 300 rpm, 720 KB and 23 KB/s for double-density drives in other machines.
Gas turbine engines rotate at tens of thousands of rpm. JetCat model aircraft turbines are capable of over 100,000 rpm with the fastest reaching 165,000 rpm.
A Flywheel energy storage system works at 60,000–200,000 rpm range using a passively magnetic levitated flywheel in vacuum. The choice of the flywheel material is not the most dense, but the one that pulverises the most safely, at surface speeds about 7 times the speed of sound.
A typical 80 mm, 30 CFM computer fan will spin at 2,600–3,000 rpm on 12 V DC power.
A turbocharger can reach 290,000 rpm, while 80,000–200,000 rpm is common.
Molecular microbiology – molecular engines. The rotation rates of bacterial flagella have been measured to be 10,200 rpm for Salmonella typhimurium, 16,200 rpm for Escherichia coli, and up to 102,000 rpm for polar flagellum of Vibrio alginolyticus, allowing the latter organism to move in simulated natural conditions at a maximum speed of 540 mm per hour.
