CPR Depth, Rate and Recoil: Why the Numbers Matter

Every CPR course gives you the same three numbers for an adult: push at least 5 cm deep, at a rate of 100 to 120 compressions a minute, and let the chest come all the way back up between each one. Most people remember the first two. Almost everyone forgets the third, and it matters just as much.

These are not arbitrary. Each one exists because of what is happening inside the chest while you push. When you understand what a compression is doing, it becomes much easier to do it well, and much easier to notice when you have started doing it badly, which everyone eventually does.

This post explains the reasoning behind each number. It is not a substitute for putting your hands on a manikin, and it does not certify you in anything. The numbers only turn into skill with practice. a CPR course covering compressions for adults, children and babies.

What a compression is actually doing

When the heart stops, blood stops moving. Chest compressions are a way of moving it from the outside. Pushing down on the centre of the chest squeezes the heart between the breastbone and the spine and raises the pressure inside the whole chest. That pressure pushes blood out of the heart and into the arteries, including the ones feeding the brain and the heart muscle itself.

Even done perfectly, compressions produce only a fraction of the blood flow a beating heart manages. That is why the details matter so much. You are working with a small margin, and small losses in depth, rate or release take a large share of what little flow there is.

Depth: at least 5 cm

For an adult, compressions need to go at least 5 cm, about 2 inches, deep. Shallower than that and the heart is not squeezed enough to move a useful amount of blood. It feels much deeper than people expect. On a manikin with a depth indicator, most first-time learners are surprised how hard they have to push.

The way to get there is body position, not arm strength. Kneel close to the person's side, put the heel of one hand in the centre of the chest with the other hand on top, lock your elbows, and bring your shoulders directly over your hands. Then push by bending at the hips, letting your body weight do the work. Arms that bend at the elbow tire within a minute and lose depth without the rescuer noticing.

The surface matters too. A person lying on a soft mattress sinks away from your hands, and a lot of your effort compresses the bed instead of the chest. Current guidance is to do compressions on a firm surface. On the floor is almost always better.

Rate: 100 to 120 a minute

The target rate is 100 to 120 compressions a minute. That works out to about two a second, or 30 compressions in roughly 15 to 18 seconds.

Too slow and the total amount of blood moved each minute drops. Too fast is also a problem, and it is the more common mistake under stress. Above about 120 a minute there is not enough time between compressions for the heart to fill again, so each push moves less blood. Very fast compressions also tend to get shallower, because there is no time to reach full depth.

Many people use a song to hold the rhythm. Anything around 100 to 120 beats a minute works. What helps more in practice is counting out loud: it keeps your pace steady and it tells everyone around you what you are doing.

Recoil: the part everyone forgets

Between compressions, the chest has to come all the way back up. This is recoil, and it is when the heart refills. As the chest springs back, the pressure inside it drops, and blood flows back into the heart from the veins. The next compression can only push out what came in during the last recoil.

The common fault is leaning: keeping some weight on the chest between pushes, usually because the rescuer is tired or resting their weight on their hands. The chest never fully comes back up, the heart never fully refills, and the next compression moves less blood even though it looks just as hard. Leaning is very difficult to see from the outside and very easy to do.

The fix is to lift your weight completely off the chest at the top of every compression while keeping your hands in contact. Push hard, release fully. Down and up should take about the same time.

Why rescuers drift, and what to do about it

Good compressions are hard physical work, and quality falls off before the person doing them feels tired. Depth drops first, then leaning creeps in. This is the reason professional teams swap the person on the chest regularly, commonly about every two minutes, and do it quickly so the pause is as short as possible. the BLS course, where two-rescuer CPR and the swap are drilled as a team.

If there is more than one person able to do compressions, plan the swap before the first person is exhausted. The new rescuer gets into position beside the first, and they change over on a count with the smallest gap you can manage. Every pause lets the pressure in the arteries fall away, and it takes several compressions to build it back up. why every pause in compressions costs more blood flow than it seems.

Training helps here more than anywhere. The 2025 resuscitation guidelines recommend feedback devices during CPR training for the public, because it is so hard to judge your own depth, rate and recoil by feel. Practising on a manikin that tells you when you are too shallow, too fast, or leaning is how most people discover their habits before it matters.