CPR follows a specific set of numbers: 30 compressions followed by 2 rescue breaths makes one cycle, and five cycles make up a roughly two-minute round before you pause to check for a pulse. These ratios, along with compression rate, depth, and timing, are the core calculations that determine whether CPR is effective. Here’s how each one works and why the numbers matter.
The 30:2 Ratio
The foundational calculation in CPR is the compression-to-ventilation ratio of 30:2. For every 30 chest compressions, you deliver 2 rescue breaths. This applies to adults, children, and infants when a single rescuer is performing CPR. Starting with 30 compressions delays the first breath by only about 18 seconds, which is a trade-off that keeps blood flowing to the brain and heart during the most critical window.
When two trained rescuers are working on a child or infant, the ratio shifts to 15:2. The higher proportion of breaths reflects the fact that pediatric cardiac arrests are more often caused by breathing problems rather than heart rhythm issues, so ventilation plays a larger role. For adults with two rescuers, the ratio stays at 30:2 until an advanced airway is placed.
Compressions Per Minute
The target compression rate is 100 to 120 per minute. That’s roughly two compressions per second. Pushing faster than 120 tends to reduce compression depth, and pushing slower than 100 doesn’t generate enough blood flow. A common way to keep pace is to compress to the beat of “Stayin’ Alive” by the Bee Gees, which hits about 104 beats per minute.
For adults, each compression should push the chest down at least 2 inches (5 cm) but no more than 2.4 inches (6 cm). For children, aim for about 2 inches. For infants, the target is 1.5 inches (4 cm). Depth matters as much as speed: shallow compressions don’t squeeze the heart hard enough to push blood forward.
How One Cycle Is Timed
One cycle of CPR equals 30 compressions plus 2 breaths. At a rate of 100 to 120 compressions per minute, the 30 compressions take roughly 15 to 18 seconds. Each rescue breath should last about 1 second. Including the brief pause to open the airway and deliver those breaths, one full cycle takes approximately 24 seconds.
Five complete cycles add up to about two minutes. At the two-minute mark, you pause for no more than 10 seconds to check for a pulse or rhythm. If there’s no pulse, you restart immediately. This two-minute block is the basic unit of CPR timing, and it repeats until help arrives or the person recovers.
Chest Compression Fraction
Chest compression fraction (CCF) is the single most important quality metric in CPR. It’s calculated by dividing the total time you spend actively compressing the chest by the total duration of the resuscitation, then expressing that as a percentage.
For example, if a resuscitation lasts 10 minutes and you spent 8 minutes doing compressions, your CCF is 80%. The target is at least 80%, though anything above 60% is considered the minimum threshold for maintaining adequate blood flow to the heart and brain. Every pause for breath delivery, rhythm checks, or rescuer switches chips away at this number, which is why minimizing interruptions is so heavily emphasized.
When to Switch Rescuers
Current guidelines recommend switching the person doing compressions every 2 minutes, which lines up with the five-cycle rhythm check. The switch should happen in under 10 seconds to keep the compression fraction high. If you feel fatigued before the two-minute mark, switch sooner. Compression quality drops significantly when a rescuer is tired, even if they don’t realize it.
Research comparing one-minute and two-minute rotation intervals found that switching every minute produced a higher number of adequate-depth compressions and less rescuer fatigue. In practice, if you have enough people available, rotating more frequently is better than waiting the full two minutes.
How Professionals Measure CPR Quality in Real Time
In hospital and advanced EMS settings, rescuers use a CO2 sensor attached to the airway tube to monitor how well compressions are working. The device measures the amount of carbon dioxide in exhaled air, which directly reflects how much blood the heart is pumping. During CPR, readings of at least 20 mmHg indicate that compressions are generating meaningful blood flow. Readings in the 28 to 36 mmHg range suggest particularly effective chest compressions.
This same measurement can signal recovery. A sudden jump of more than 10 mmHg during CPR, without any change in breathing support, strongly suggests the heart has restarted on its own. An initial reading of at least 15 mmHg correctly predicted eventual return of a pulse with 98% specificity in one study. If you ever see a medical team pause and look at a monitor during resuscitation, this is often what they’re watching.
Infant Compression Technique
For infants, the compression calculation changes not just in depth but in hand position. The preferred method when two rescuers are available is the two-thumb encircling technique: you wrap both hands around the infant’s torso and compress the breastbone with your thumbs. A single rescuer typically uses two fingers placed just below the nipple line on the breastbone.
A newer variation positions the rescuer behind the infant’s head, using two thumbs from above. This approach frees up space for a second rescuer to manage the airway and can be easier to sustain without fatigue. Regardless of technique, the compression point is the same: the lower third of the breastbone, pressing down about 1.5 inches with each compression.
Quick Reference for CPR Numbers
- Compression-to-breath ratio (single rescuer, all ages): 30:2
- Compression-to-breath ratio (two rescuers, child/infant): 15:2
- Compression rate: 100 to 120 per minute
- Adult compression depth: 2 to 2.4 inches (5 to 6 cm)
- Child compression depth: about 2 inches (5 cm)
- Infant compression depth: about 1.5 inches (4 cm)
- One cycle: 30 compressions + 2 breaths (roughly 24 seconds)
- Rhythm/pulse check interval: every 2 minutes (5 cycles)
- Pulse check duration: no more than 10 seconds
- Target chest compression fraction: 80% or higher
- Rescuer switch interval: every 2 minutes or sooner if fatigued