Cardiopulmonary resuscitation (CPR) consists of alternating cycles of chest compressions and rescue breaths at a 30 compressions to 2 rescue breaths ratio (30:2). One ‘cycle’ of CPR is equivalent to one round of 30:2.
Last reviewed: September 2026 — checked against Resuscitation Council UK Guidelines 2025.
Why the chest compression rate matters
Rescuers are recommended to switch over after conducting 2 minutes of CPR in order to guarantee that their chest compressions stay successful (delivered at the correct depth and speed).
If you perform chest compressions at the proper tempo, you should be able to conduct roughly 5 cycles of CPR in 2 minutes (100 to 120 chest compressions per minute). The rate at which chest compressions are performed is critical in order to move blood around the body. Compressions that are too slow are unlikely to move enough blood, and compressions faster than 120 per minute are also less effective because the chest has too little time to recoil fully between compressions – so aim for the 100 to 120 per minute target range, not simply “as fast as possible”.
Keeping to that tempo is easier than it sounds. Count out loud as you compress, use a metronome or CPR app set to between 100 and 120 beats per minute, or compress in time with a familiar song of roughly that tempo. Training manikins with feedback devices, used on most accredited first aid courses, will prompt you whenever your rate or depth drifts outside the target range.
As a general rule, rescuers should aim for approximately 5 cycles of CPR before switching over. They should keep switching until expert medical help comes.
Before you start: call 999 first
Resuscitation Council UK changed the order of the opening steps in its 2025 guidelines. You no longer wait until you have confirmed that breathing is abnormal before calling for an ambulance.
Check that it is safe to approach, then check whether the person responds to you. If they are unresponsive, call 999 immediately – before you assess their breathing. Put your phone on speaker so your hands stay free, and assess breathing while you wait for the call to be answered. The ambulance service call handler is trained to help you decide whether breathing is normal and will talk you through CPR if it is not.
If the person is unresponsive and not breathing normally, assume cardiac arrest and start chest compressions straight away. Agonal gasping, slow or laboured breathing and, in someone who has collapsed during exercise, an almost normal panting pattern are all signs of cardiac arrest rather than signs of life. If in doubt, start CPR: the risk of harm from compressions given to someone not in cardiac arrest is low, and far smaller than the harm of doing nothing.
How deep should chest compressions be?
To make chest compressions as effective as possible, keep the following in mind. For an adult, press down to a depth of at least 5 cm and no more than 6 cm (about 5–6 cm, roughly one third of the depth of the chest), and allow the chest to recoil fully after each compression without leaning on it.
Depth and rate work together. Compressions that are deep enough but too slow, or fast enough but too shallow, will not generate the circulation the brain and heart muscle need. Under-compressing is by far the more common problem, so do not be afraid to push hard. CPR also works best on a firm surface, but do not waste time moving someone off a bed or sofa onto the floor – start compressions where they are and, if the surface is soft, compress a little deeper to compensate.
Hand position and technique
Kneel beside the person and place the heel of one hand on the lower half of the breastbone, in the centre of the chest. Place your other hand on top and interlock your fingers so the pressure goes through the breastbone rather than the ribs. Keep your arms straight and your shoulders directly above your hands, so the force comes from your body weight instead of your arms. Push down 5 to 6 cm, then release completely so the chest comes all the way back up before the next compression, holding the same 100 to 120 per minute rhythm throughout.
With or without rescue breaths
If you are trained and willing to give rescue breaths, use a ratio of 30 compressions to 2 breaths (30:2); if you are untrained, unable or unwilling to give breaths, hands-only (compression-only) CPR at the same 100 to 120 per minute rate is recommended.
When you do give rescue breaths, deliver just enough air to make the chest start to rise. There is no benefit in blowing harder or for longer, and over-inflating the lungs does harm. Keep each pair of breaths brief so compressions restart quickly, because long pauses are one of the biggest causes of poor-quality CPR. If two attempted breaths fail to make the chest rise, think about a blocked airway, reposition the head and carry on with compressions.
Keep interruptions to a minimum, and where more than one rescuer is available, swap over roughly every 2 minutes to avoid fatigue.
Using an AED alongside chest compressions
Continue CPR until emergency help or a defibrillator (AED) arrives and is ready to use. When the AED is ready, follow the proper placement of defib pads to deliver a shock safely.
Anyone can use an AED. No training is needed, the machine will not allow a shock to be given to someone who does not need one, and the earlier it is used the better the chances of survival – bystander CPR combined with early defibrillation increases the chance of surviving an out-of-hospital cardiac arrest by two to four times.
If you are on your own, do not leave the person to go looking for a defibrillator. Keep compressing and let the 999 call handler direct someone else to the nearest one; in the UK, call handlers can find registered public-access defibrillators through The Circuit. Where a second rescuer is available, they should fetch the AED while you carry on with compressions.
Once the AED arrives, switch it on and follow its spoken prompts. Attach the pads to bare skin in the positions shown on the pads themselves, and keep compressing while they are being applied. Then stand clear and make sure nobody is touching the person while the machine analyses the heart rhythm and while any shock is delivered. Some AEDs shock automatically; others ask you to press a flashing button. Either way, restart chest compressions immediately after the shock – or immediately if no shock is advised – and keep going at 100 to 120 per minute until the AED tells you to pause again or the ambulance crew take over.
Compression rate and depth for children and infants
The 100 to 120 compressions per minute rate is the same for adults, children and infants. What changes with age is the depth, the technique and the way the sequence starts.
Resuscitation Council UK asks rescuers to compress the chest by at least one third of its front-to-back depth in any child or infant, to use the adult 5 to 6 cm figure in adolescents, and never to exceed 6 cm at any age. Compressions go over the lower half of the breastbone whatever the age. For an infant under one year, use the two-thumb encircling technique – both thumbs on the lower half of the breastbone with your hands wrapped around the chest – rather than two fingers. For a child over one year, use one hand or two, whichever lets you reach the right depth.
Paediatric resuscitation also begins differently. Because cardiac arrest in children is far more often caused by a breathing problem than by a primary heart problem, give five initial rescue breaths before you start chest compressions. After that, use a ratio of 15 compressions to 2 breaths if you have been specifically trained in paediatric basic life support; if not, use the adult 30:2 ratio, or compression-only CPR if you are unable or unwilling to give breaths. Attach an AED as soon as one is available for a child of any age, but if you are the only rescuer, calling 999 and starting CPR come first.
Common mistakes that reduce compression quality
- Compressing too fast. Above 120 per minute the heart has too little time to refill between compressions.
- Compressing too shallow. Aim for the full 5 to 6 cm in an adult; hesitant compressions move very little blood.
- Leaning on the chest between compressions, which stops it recoiling and refilling.
- Long pauses for rescue breaths, pad placement or swapping over. Every pause loses the pressure you have built up.
- Waiting until you are certain before acting. Call 999 for any unresponsive person, and start CPR if their breathing is not normal.
- Working on past exhaustion. Compression quality falls away within about two minutes, so swap over whenever someone else can take a turn.
- Not using a nearby AED. It will not shock a heart that does not need it, and early defibrillation is the single biggest factor in survival.
Where this guidance comes from
This guidance follows the Resuscitation Council UK 2025 Adult Basic Life Support guidelines and NHS advice on CPR.
Resuscitation Council UK published its 2025 guidelines in October 2025, and UK first aid training materials have had to comply with them since January 2026. The rate of 100 to 120 compressions per minute and the adult depth of 5 to 6 cm are unchanged from the previous edition. What changed in 2025 is calling 999 for any unresponsive person before assessing breathing, and the expanded role of the ambulance service call handler in recognising cardiac arrest, guiding bystanders through CPR and directing them to the nearest defibrillator.
Learn more about CPR by attending one of our first aid training courses.
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