Push down at least five centimetres — roughly one third of the depth of an adult’s chest. That is the depth set in ANZCOR Guideline 6, the Australian and New Zealand resuscitation guideline that accredited CPR training in Australia is built on, and it is a minimum rather than a target to creep up on.
What surprises most people who read the guideline itself is that ANZCOR sets no upper limit on depth at all. It notes evidence of harm from compressions deeper than six centimetres, then explains why it still declines to cap them: shallow compressions are definitely linked to poor outcomes, and rescuers cannot reliably tell five centimetres from six under their hands. If you are uncertain whether you are deep enough, the safer error is to press harder.
What ANZCOR Guideline 6 actually specifies
The guideline’s depth section is short and specific. The lower half of the sternum should be depressed approximately one third of the depth of the chest with each compression, which equates to more than 5 cm in adults, approximately 5 cm in children and 4 cm in infants.
Three details in that sentence matter more than the numbers:
- It is a proportion first and a measurement second. One third of the chest is the instruction; five centimetres is what that usually works out to on an average adult. A small, frail adult and a large adult both get the proportion, which is why a fixed number alone is a poor guide at the bedside.
- Adults get “more than” 5 cm. The word more matters. A compression that stops at exactly five centimetres is at the bottom edge of the recommendation, not in the middle of it.
- There is no ceiling in the Australian guideline. ANZCOR acknowledges the evidence of detriment beyond 6 cm, but states plainly that inadequate depth is definitely associated with poor outcomes and that the risk of compressions that are too shallow outweighs the risk of compressions that are too deep.
How to reach that depth without losing your position
Depth is a whole-body technique problem, not an arm-strength problem. The guideline’s placement rules exist because a compression delivered a few centimetres off the mark loses effectiveness or adds injury:
- Hands on the lower half of the sternum. Heel of one hand in the centre of the chest, the other hand on top. Compression placed too high does little; placed too low it risks regurgitation and damage to internal organs.
- Straight arms, shoulders over the hands. Body weight does the work, which is what makes sustained depth possible.
- A firm, flat surface. ANZCOR is explicit that the person should be on their back on a firm surface before compressions begin — a soft mattress absorbs the depth you are trying to deliver.
- Straight down, no rocking, no jabs. The guideline warns against rocking backwards and forwards or using thumps and quick jabs; compressions should be rhythmic, with equal time given to compression and relaxation.
- Full recoil between compressions. The chest must be allowed to come all the way back up so the heart can refill. Leaning on the chest between pushes keeps the pressure inside the thorax and undoes part of the work.
Depth also has a partner. Compressions are delivered at 100 to 120 per minute, so a run of thirty takes roughly fifteen to eighteen seconds, and the ratio for every age is 30:2 — the tempo and the pattern are covered in our guides to how many chest compressions per minute is correct and the compression to breath ratio for adult CPR.
Why shallow compressions are the failure you should worry about
ANZCOR’s section on CPR quality is a warning about self-assessment. It notes that compression rate and depth vary between rescuers and that quality can be worse in the first five minutes of an arrest. One manikin study it cites found compressions became shallow within one minute, yet the providers only became aware of their fatigue after five minutes.
That gap — a minute of deteriorating depth against five minutes of felt effort — is the reason instructors insist on changing rescuers at least every two minutes where there are enough people, specifically to protect depth. It is also the reason counting out loud and watching the chest, rather than trusting how your arms feel, is taught as the way to judge your own compressions.
Will pushing that hard break ribs?
Rib and sternal fractures do occur in real CPR, particularly in older people with fragile bone, and they are a known and accepted risk of doing the job properly. A fracture you can hear is not a reason to ease off: the certain harm of a heart and brain without circulation outweighs the possible harm of a compression a centimetre too deep, which is the same trade-off ANZCOR makes when it declines to set an upper limit.
Keep compressions going until the person responds or breathes normally, until a health care professional takes over or directs you to stop, or until you are physically unable to continue — the four conditions are set out in when should you stop CPR in Australia.
Depth for children and infants
The proportion stays the same across ages; the numbers shrink. ANZCOR puts the depth at approximately 5 cm for children and 4 cm for infants, again as roughly one third of the chest. Children can be compressed with one hand or two, depending on their size and yours.
The ratio stays at 30:2 for all ages, and the sequence before compressions is unchanged — danger, response, send for help, airway, normal breathing. Our guide to performing CPR on a child or infant walks through the differences in breathing and hand placement.
Where compression depth gets trained
Depth is one of the few parts of first aid that cannot be learned by reading. You need a manikin, a trainer watching your hands, and repetition until the movement is automatic, because in a real arrest you will not be measuring anything.
ALIT delivers HLTAID009 Provide CPR as a 3.5 hour face-to-face course at the West Melbourne campus, with participants practising compressions on a manikin and learning to use an automated external defibrillator. The course fee is $75, all learning materials are included, and the qualification is nationally recognised; a replacement certificate costs $25. Entry requires no previous CPR experience, but students must be at least 14 years old and physically capable of performing CPR compressions — a genuine consideration once you know what sustained depth demands. Annual refresher training is recommended to keep the skill current. Enrolments are open, so confirm the next start date with ALIT.
If you want the broader qualification, HLTAID011 Provide First Aid includes CPR along with the rest of the workplace first aid scope.
Frequently asked questions
How deep should chest compressions be for an adult?
More than five centimetres, which is approximately one third of the depth of an adult’s chest. ANZCOR Guideline 6 sets that as the depth for adults, with approximately 5 cm for children and 4 cm for infants.
Can you push too hard during CPR?
Depth beyond about six centimetres has been associated with harm, but ANZCOR deliberately sets no upper limit, because shallow compressions are clearly linked to poor outcomes and rescuers cannot reliably judge five against six centimetres. Pressing firmly is safer than holding back.
How do I know I am deep enough without a manikin?
Use the proportion: aim for one third of the chest, delivered with straight arms and body weight, with the chest rising fully between compressions. In training, a manikin and a trainer give you the feedback that real life does not.
Should the chest come all the way back up?
Yes. ANZCOR instructs rescuers to allow complete recoil after each compression so the heart can refill, with equal time given to compression and relaxation.
Does depth change for a very small or very large adult?
The proportion is the guide, so one third of the chest is the aim in both cases. More than five centimetres remains the working figure for an average adult, and a compression that visibly moves the chest is doing useful work.
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