Clock rate or clock speed in
computing
Computing is any goal-oriented activity requiring, benefiting from, or creating computer, computing machinery. It includes the study and experimentation of algorithmic processes, and the development of both computer hardware, hardware and softw ...
typically refers to the
frequency
Frequency is the number of occurrences of a repeating event per unit of time. Frequency is an important parameter used in science and engineering to specify the rate of oscillatory and vibratory phenomena, such as mechanical vibrations, audio ...
at which the
clock generator of a
processor can generate
pulses used to
synchronize the operations of its components. It is used as an indicator of the processor's speed. Clock rate is measured in the
SI unit of frequency
hertz
The hertz (symbol: Hz) is the unit of frequency in the International System of Units (SI), often described as being equivalent to one event (or Cycle per second, cycle) per second. The hertz is an SI derived unit whose formal expression in ter ...
(Hz).
The clock rate of the first generation of computers was measured in hertz or kilohertz (kHz), the first
personal computer
A personal computer, commonly referred to as PC or computer, is a computer designed for individual use. It is typically used for tasks such as Word processor, word processing, web browser, internet browsing, email, multimedia playback, and PC ...
s from the 1970s through the 1980s had clock rates measured in megahertz (MHz). In the 21st century the speed of modern
CPUs is commonly advertised in gigahertz (GHz). This metric is most useful when comparing processors within the same family, holding constant other features that may affect
performance
A performance is an act or process of staging or presenting a play, concert, or other form of entertainment. It is also defined as the action or process of carrying out or accomplishing an action, task, or function.
Performance has evolved glo ...
.
Determining factors
Binning

Manufacturers of modern processors typically charge higher prices for processors that operate at higher clock rates, a practice called
binning. For a given CPU, the clock rates are determined at the end of the manufacturing process through testing of each processor. Chip manufacturers publish a "maximum clock rate" specification, and they test chips before selling them to make sure they meet that specification, even when executing the most complicated instructions with the data patterns that take the longest to settle (testing at the temperature and voltage that gives the lowest performance). Processors successfully tested for compliance with a given set of standards may be labeled with a higher clock rate, e.g., 3.50 GHz, while those that fail the standards of the higher clock rate yet pass the standards of a lower clock rate may be labeled with the lower clock rate, e.g., 3.3 GHz, and sold at a lower price.
Engineering
The clock rate of a CPU is normally determined by the
frequency
Frequency is the number of occurrences of a repeating event per unit of time. Frequency is an important parameter used in science and engineering to specify the rate of oscillatory and vibratory phenomena, such as mechanical vibrations, audio ...
of an
oscillator crystal. Typically a crystal oscillator produces a fixed
sine wave
A sine wave, sinusoidal wave, or sinusoid (symbol: ∿) is a periodic function, periodic wave whose waveform (shape) is the trigonometric function, trigonometric sine, sine function. In mechanics, as a linear motion over time, this is ''simple ...
—the frequency reference signal. Electronic circuitry translates that into a
square wave at the same frequency for digital electronics applications (or, when using a
CPU multiplier, some fixed multiple of the crystal reference frequency). The
clock distribution network inside the CPU carries that
clock signal to all the parts that need it. An
A/D Converter has a "clock" pin driven by a similar system to set the
sampling rate. With any particular CPU, replacing the crystal with another crystal that oscillates at half the frequency ("
underclocking") will generally make the CPU run at half the performance and reduce
waste heat produced by the CPU. Conversely, some people try to increase performance of a CPU by replacing the oscillator crystal with a higher frequency crystal ("
overclocking"). However, the amount of overclocking is limited by the time for the CPU to settle after each pulse, and by the extra heat created.
After each clock pulse, the signal lines inside the CPU need time to settle to their new state. That is, every signal line must finish transitioning from 0 to 1, or from 1 to 0. If the next clock pulse comes before that, the results will be incorrect. In the process of transitioning, some energy is wasted as heat (mostly inside the driving transistors). When executing complicated instructions that cause many transitions, the higher the clock rate the more heat produced. Transistors may be damaged by excessive heat.
There is also a lower limit of the clock rate, unless a fully
static core is used.
Historical milestones and current records
The first fully mechanical digital computer, the
Z1, operated at 1 Hz (cycle per second) clock frequency and the first electromechanical general purpose computer, the
Z3, operated at a frequency of about 5–10 Hz. The first electronic general purpose computer, the
ENIAC
ENIAC (; Electronic Numerical Integrator and Computer) was the first Computer programming, programmable, Electronics, electronic, general-purpose digital computer, completed in 1945. Other computers had some of these features, but ENIAC was ...
, used a 100 kHz clock in its cycling unit. As each instruction took 20 cycles, it had an instruction rate of 5 kHz.
The first commercial PC, the
Altair 8800 (by MITS), used an Intel 8080 CPU with a clock rate of 2 MHz (2 million cycles per second). The original
IBM PC
The IBM Personal Computer (model 5150, commonly known as the IBM PC) is the first microcomputer released in the List of IBM Personal Computer models, IBM PC model line and the basis for the IBM PC compatible ''de facto'' standard. Released on ...
(c. 1981) had a clock rate of 4.77 MHz (4,772,727 cycles per second). In 1992, both Hewlett-Packard and Digital Equipment Corporation (DEC) exceeded 100 MHz with
RISC
In electronics and computer science, a reduced instruction set computer (RISC) is a computer architecture designed to simplify the individual instructions given to the computer to accomplish tasks. Compared to the instructions given to a comp ...
techniques in the PA-7100 and AXP 21064
DEC Alpha respectively. In 1995,
Intel's P5 Pentium chip ran at 100 MHz (100 million cycles per second). On March 6, 2000,
AMD demonstrated passing the 1 GHz milestone a few days ahead of Intel shipping 1 GHz in systems. In 2002, an Intel
Pentium 4 model was introduced as the first CPU with a clock rate of 3 GHz (three billion cycles per second corresponding to ~ 0.33
nanoseconds per cycle). Since then, the clock rate of production processors has increased more slowly, with performance improvements coming from other design changes.
Set in 2011, the
Guinness World Record for the highest CPU clock rate is 8.42938 GHz with an
overclocked AMD FX-8150
Bulldozer
A bulldozer or dozer (also called a crawler) is a large tractor equipped with a metal #Blade, blade at the front for pushing material (soil, sand, snow, rubble, or rock) during construction work. It travels most commonly on continuous tracks, ...
-based chip in an
LHe/
LN2 cryobath, 5 GHz
on air
On air or On Air may refer to:
Music
*On Air (The Yardbirds album), ''On Air'' (The Yardbirds album), 1991
*On Air (Alan Parsons album), ''On Air'' (Alan Parsons album), 1996
*On Air (John Fahey album), ''On Air'' (John Fahey album), 2004
*On Air ...
. This is surpassed by the
CPU-Z overclocking record for the highest CPU clock rate at 8.79433 GHz with an AMD FX-8350
Piledriver-based chip bathed in
LN2, achieved in November 2012. It is also surpassed by the slightly slower AMD FX-8370 overclocked to 8.72 GHz which tops off the
HWBOT frequency rankings.
[ ] These records were broken in 2025 when an Intel Core i9-13900KF was overclocked to 9.12 GHz.
The highest
boost clock rate on a production processor is the
i9-14900KS, clocked at 6.2 GHz, which was released in Q1 2024.
Research
Engineers continue to find new ways to design CPUs that settle a little more quickly or use slightly less energy per transition, pushing back those limits, producing new CPUs that can run at slightly higher clock rates. The ultimate limits to energy per transition are explored in
reversible computing
Reversible computing is any model of computation where every step of the process is time-reversible. This means that, given the output of a computation, it's possible to perfectly reconstruct the input. In systems that progress deterministica ...
.
The first fully reversible CPU, the Pendulum, was implemented using standard CMOS transistors in the late 1990s at the Massachusetts Institute of Technology.
[
Matthew Arthur Morrison]
"Theory, Synthesis, and Application of Adiabatic and Reversible Logic Circuits For Security Applications"
2014.
Engineers also continue to find new ways to design CPUs so that they complete more instructions per clock cycle, thus achieving a lower
CPI (cycles or clock cycles per instruction) count, although they may run at the same or a lower clock rate as older CPUs. This is achieved through architectural techniques such as
instruction pipelining and
out-of-order execution
In computer engineering, out-of-order execution (or more formally dynamic execution) is an instruction scheduling paradigm used in high-performance central processing units to make use of instruction cycles that would otherwise be wasted. In t ...
which attempts to exploit
instruction level parallelism in the code.
Comparing
The clock rate of a CPU is most useful for providing comparisons between CPUs in the same family. The clock rate is only one of several factors that can influence performance when comparing processors in different families. For example, an IBM PC with an
Intel 80486
The Intel 486, officially named i486 and also known as 80486, is a microprocessor introduced in 1989. It is a higher-performance follow-up to the i386, Intel 386. It represents the fourth generation of binary compatible CPUs following the Inte ...
CPU running at 50 MHz will be about twice as fast (internally only) as one with the same CPU and memory running at 25 MHz, while the same will not be true for MIPS R4000 running at the same clock rate as the two are different processors that implement different architectures and microarchitectures. Further, a "cumulative clock rate" measure is sometimes assumed by taking the total cores and multiplying by the total clock rate (e.g. a dual-core 2.8 GHz processor running at a cumulative 5.6 GHz). There are many other factors to consider when comparing the performance of CPUs, like the width of the CPU's
data bus, the latency of the memory, and the
cache architecture.
The clock rate alone is generally considered to be an inaccurate measure of performance when comparing different CPUs families. Software
benchmarks are more useful. Clock rates can sometimes be misleading since the amount of work different CPUs can do in one cycle varies. For example,
superscalar processors can execute more than one
instruction per cycle (on average), yet it is not uncommon for them to do "less" in a clock cycle. In addition, subscalar CPUs or use of parallelism can also affect the performance of the computer regardless of clock rate.
See also
*
Crystal oscillator frequencies
*
Double data rate
*
Quad data rate
*
Pulse wave
*
Instructions per second
Instructions per second (IPS) is a measure of a computer's Central processing unit, processor speed. For complex instruction set computers (CISCs), different Machine code, instructions take different amounts of time, so the value measured depen ...
*
Moore's law
References
{{CPU technologies
Clock signal
Temporal rates
it:Clock#Velocità di clock