Turbo Pressure Ratio Calculator

What this page is for

This page helps you calculate turbocharger pressure ratio, which is one of the main numbers used when reading a compressor map and sizing a turbo. If you want to know whether a turbo is being asked to work too hard or whether it is operating in a healthy part of the map, pressure ratio is one of the first things you need.

In simple terms, boost pressure in psi tells you what you see on the gauge, while pressure ratio tells you what the turbo itself is actually being asked to do. That is why pressure ratio matters more than boost alone when comparing turbo setups.

Why pressure ratio matters

Turbo compressor maps are plotted using pressure ratio on the vertical axis and airflow on the horizontal axis. If you cannot calculate pressure ratio, you cannot accurately place your engine’s operating point on the map.

This matters because two engines can both run “12 psi of boost” and still have different pressure ratios if they are at different elevations or if one has more inlet restriction. In other words, the same boost gauge reading does not always mean the turbo is doing the same amount of work.

The simple formula

The standard sea-level shortcut is:

 This version assumes:

• Sea-level atmospheric pressure is 14.7 psi absolute.

• There is no meaningful inlet restriction before the compressor.

The better real-world formula

A more accurate formula is:

where:

• P_2c= compressor outlet absolute pressure.

• P_1c = compressor inlet absolute pressure.

In shop terms, that becomes:

This version is more accurate because it takes into account things like a restrictive air filter or intake piping that slightly lowers pressure at the turbo inlet.

What the inputs mean

• Boost pressure: the gauge pressure you want to run, in psi above atmosphere.

• Atmospheric pressure: your local barometric pressure in psi absolute. At sea level, this is commonly 14.7 psi.

• Inlet depression: the pressure drop before the compressor inlet, often caused by filters, piping, or other restrictions.

How to calculate it

1. Take your target boost pressure in psi.

2. Add atmospheric pressure to convert it from gauge pressure to absolute pressure.

3. Divide that by compressor inlet absolute pressure.

That gives you pressure ratio, which is a unitless number.

Worked example 1

Let’s use a simple sea-level setup at 12 psi boost.

That gives a pressure ratio of about 1.82. This exact example is commonly used in turbo sizing references.

Worked example 2

Now use the more accurate method with 12 psi boost, 14.7 psi atmospheric pressure, and 1 psi inlet depression.

That gives a pressure ratio of about 1.95, which is noticeably higher than the simple 1.82 result. That difference shows why inlet restriction matters more than many people realize.

Worked example 3

Here is a quick example at a lower pressure ratio. If you want a pressure ratio of 1.50 and you are working from standard atmospheric pressure, the outlet absolute pressure would be 1.50 times inlet pressure. One turbo tuning example explains that this works out to about 7.25 psi of boost when atmospheric pressure is about 14.5 psi.

That is a useful reminder that pressure ratio is just another way of expressing how much the compressor is raising pressure.

Reverse formula for target boost

If you already know the pressure ratio you want and need to estimate the boost pressure that goes with it, you can rearrange the equation:

That makes it easier to work backward from a compressor map when you are trying to see what boost level lines up with a particular operating zone.

Example using reverse calculation

If you want a pressure ratio of 2.0 at sea level:

That means a pressure ratio of 2.0 is basically the same thing as about 14.7 psi of boost at sea level, before correcting for inlet losses.

How to think about the result

A higher pressure ratio means the turbo is working harder to raise pressure. In general, as pressure ratio goes up, compressor outlet temperature rises and the turbo moves farther into the harder-working area of the map.

That does not automatically mean a setup is bad, but it does mean you need to pay closer attention to efficiency, intercooling, and whether the chosen turbo is actually comfortable there.

Why this matters for turbo sizing

Compressor maps show pressure ratio and airflow together, not just boost pressure by itself. That is why pressure ratio is the bridge between your target boost number and the compressor map you use to choose a turbo.

One major turbo manufacturer also notes a useful gasoline-engine rule of thumb: about 1 lb/min of airflow can support roughly 10 horsepower, which means pressure ratio becomes part of the bigger airflow-and-power picture when choosing the right turbo.

What this formula does not know

Pressure ratio is only one part of turbo sizing. By itself, it does not tell you whether the turbo has enough flow for the engine, whether it is in the efficiency island you want, or whether turbine sizing is appropriate.

It also does not account for charge cooling, cam timing, engine efficiency, fuel type, or compressor map choke and surge limits. That is why pressure ratio should be used together with airflow, not by itself.

Plain-English takeaway

If you want the short version: pressure ratio tells you how hard the turbo is working, while boost psi only tells you what the gauge says. Calculate pressure ratio first, then use it with airflow to see whether a turbo actually fits your engine and your power goals.