How do you calculate turbo pressure ratio?

Pressure ratio = absolute outlet pressure ÷ absolute inlet pressure = (boost + atmospheric) ÷ atmospheric. At sea level (14.7 psia), 10 psi of boost is (10 + 14.7) ÷ 14.7 = 1.68. For a compressor map, add the intercooler pressure drop to the outlet and subtract the air-filter loss from the inlet.

How much horsepower does 1 psi of boost add?

As a rough rule, each 14.7 psi (one atmosphere) could double power if the charge stayed cool, about 7% per psi at sea level. Heat from compression reduces that: with a 70% compressor and 70% intercooler, 10 psi gives about 57% more power (density ratio 1.57) rather than 68%.

How much boost do I need for a target horsepower?

Use the target section. Example: 300 hp from a 2.0 L engine at 6,500 rpm with 90% VE, 11.5:1 AFR, 0.50 BSFC and 130 °F manifold air needs about 28.8 lb/min of air and roughly 30.4 psia manifold pressure, about 15.7 psi of boost at sea level (compressor pressure ratio about 2.36 with a 2 psi intercooler and 1 psi inlet loss).

What does lb/min mean on a compressor map?

It is the mass of air the compressor flows per minute, corrected to the map's reference inlet conditions (Garrett uses 545 °R and 13.95 psia). A common rule of thumb is roughly 10 hp per lb/min of air for gasoline engines.

Does altitude change boost?

Gauge boost is measured relative to the local air pressure. At 5,000 ft the air is about 12.2 psia, so 10 psi gauge is 22.2 psia absolute and a 1.82 pressure ratio, compared with 24.7 psia and 1.68 at sea level. The turbo works harder at altitude, and a boost controller that targets gauge pressure makes less power.

Why is the estimated horsepower only an estimate?

The model scales power by charge-air density. It ignores changes in volumetric efficiency with rpm, intercooler and pipe pressure losses, ignition timing, fuel and knock limits, exhaust back pressure and drivetrain losses. Wheel horsepower is typically 10–20% below crank horsepower.

What is typical compressor and intercooler efficiency?

Centrifugal turbo compressors run about 65–78% isentropic efficiency in the main map islands. Street air-to-air intercoolers are typically 60–85% effective.

Turbo Boost Calculator

Enter boost (psi or bar), altitude or air pressure, temperatures and stock horsepower to get the pressure ratio, charge temperature and estimated turbo horsepower, or enter a target horsepower to get the boost and airflow (lb/min) it needs.

Turbo pressure ratio = (boost + atmospheric pressure) ÷ atmospheric pressure, so 14.7 psi of boost at sea level is a 2.0 pressure ratio and 10 psi is 1.68.

Engine & Turbo Parameters

Target Boost Pressure (Gauge)

Gauge pressure above ambient inlet (1 bar = 14.5038 PSI).

Ambient Air Temperature

Altitude (optional)

Air pressure (psia)

Sea level standard = 14.696 psia (101.325 kPa). Typing an altitude fills the standard-atmosphere pressure.

Compressor Isentropic Efficiency (η_c): 70%

50% (low)70% (typical)90% (max)

Intercooler Effectiveness (η_ic): 70%

0% (None)70% (Street)100% (Ideal)

Baseline NA Engine Power (HP)

Naturally aspirated crank horsepower at ambient pressure.

Pressure Ratio (PR)

—

Absolute discharge / inlet pressure

Density Ratio (DR)

—

Post-IC air density vs. ambient

Compressor Outlet Temp (T2)

—

Discharge before intercooler

Intake Charge Temp

—

Air entering intake manifold

Estimated Horsepower with Boost

Density-ratio model, crank HP

Baseline NA

—

Est. Power Gain

—

Est. Turbo HP

—

0 HP—

—

Airflow for the compressor map

Air-fuel ratio

BSFC (lb/hp·h)

Gasoline turbo: AFR 11–12.5, BSFC 0.50–0.60. E85 and methanol need much richer AFR and higher BSFC.

Airflow at est. HP: —
Corrected airflow: —
Map point: —

Boost needed for a target horsepower

Garrett method. Uses the AFR, BSFC, air pressure and ambient temperature above.

Target HP (crank)

Displacement (L)

At rpm

Vol. efficiency (%)

Manifold air (°F)

IC loss (psi)

Inlet loss (psi)

Airflow: —
Manifold pressure: —
Boost needed: —
Compressor PR: —

—

Boost to pressure ratio chart

Sea level 14.7 psia and 5,000 ft (12.2 psia, standard atmosphere). Outlet temperature and density ratio assume 25 °C (77 °F) air, a 70% compressor and a 70% intercooler.

Boost	Absolute (sea level)	PR sea level	PR 5,000 ft	Outlet temp	Density ratio
5 psi (0.34 bar)	19.7 psia	1.34	1.41	62 °C	1.29
7 psi (0.48 bar)	21.7 psia	1.48	1.57	75 °C	1.41
10 psi (0.69 bar)	24.7 psia	1.68	1.82	93 °C	1.57
12 psi (0.83 bar)	26.7 psia	1.82	1.98	104 °C	1.68
14.7 psi (1.01 bar)	29.4 psia	2.00	2.20	118 °C	1.83
18 psi (1.24 bar)	32.7 psia	2.22	2.47	134 °C	2.00
20 psi (1.38 bar)	34.7 psia	2.36	2.64	143 °C	2.11
25 psi (1.72 bar)	39.7 psia	2.70	3.04	165 °C	2.37
30 psi (2.07 bar)	44.7 psia	3.04	3.45	184 °C	2.62

Formulas & Model Limitations

Pressure: 1 bar = 14.5037738 psi; standard sea-level atmosphere = 14.696 psia (101.325 kPa). PR = (P_atm + P_boost) / P_atm.

Temperature and density: T₂ = T₁ + T₁(PR^0.2857 − 1)/η_c (γ = 1.4); T_charge = T_amb + (1 − η_ic)(T₂ − T_amb); DR = PR × T_amb/T_charge (kelvin).

Airflow (Garrett): W_a = HP × AFR × BSFC/60 lb/min; corrected W = W_a√((T₁+460)/545)/(P₁/13.95); MAP_req = W_a × 639.6 × (460 + T_m)/(VE × N/2 × V_d) psia with V_d in cubic inches. Source: Garrett Motion — how to select a turbo, part 2.

Limits: horsepower is a density-scaled estimate. Real output depends on volumetric efficiency across rpm, intercooler and pipe pressure drop, ignition timing, fuel and knock limits, exhaust back pressure and drivetrain losses. Check the operating point against the actual compressor map for surge and choke.

Quick answer

Turbo pressure ratio = (boost + atmospheric pressure) ÷ atmospheric pressure, so 14.7 psi of boost at sea level is a 2.0 pressure ratio and 10 psi is 1.68. Power rises with air density, not pressure alone: with a 70% efficient compressor and a 70% effective intercooler, 10 psi on a 150 hp engine gives a density ratio of about 1.57, or roughly 236 hp before losses. At 5,000 ft the air pressure is about 12.2 psia, so the same 10 psi gauge boost is a 1.82 pressure ratio.

Examples

Boost (sea level, 25 °C, 70% compressor and intercooler)	Pressure ratio	Charge temperature	Estimated power
7 psi on 300 hp	1.48	40 °C	≈ 422 hp
10 psi on 150 hp	1.68	45 °C	≈ 236 hp
14.7 psi (1 bar) on 200 hp	2.00	53 °C	≈ 366 hp
10 psi at 5,000 ft (12.2 psia)	1.82	—	turbo works harder for the same gauge boost

Enter your boost, altitude and stock horsepower, or use the target-horsepower section to find the boost you need.

Summary

This turbo boost calculator answers three questions: what pressure ratio a boost level means, roughly how much horsepower it adds, and how much boost and airflow a horsepower target needs. Pressure ratio uses absolute pressures, so altitude matters; type an altitude to set the atmospheric pressure. Compressor outlet temperature comes from the isentropic relation and your compressor efficiency, the intercooler brings it back toward ambient, and the resulting air density ratio scales the naturally aspirated power. The airflow section converts horsepower into mass airflow (lb/min) with the air-fuel ratio and BSFC and corrects it to the reference conditions used on Garrett compressor maps. The target section uses the Garrett method to find the manifold pressure, boost and compressor pressure ratio for a horsepower goal from displacement, rpm and volumetric efficiency. Results are estimates; only a dyno measures real power.

How it works

Enter gauge boost (psi or bar), ambient temperature, and either an altitude or the atmospheric pressure (14.7 psia at sea level).
Pressure ratio PR = (P_atm + boost) ÷ P_atm. Altitude sets P_atm from the standard atmosphere: P = 14.696 × (1 − 2.25577×10⁻⁵ × h)^5.25588 with h in metres.
Compressor outlet temperature T₂ = T₁ + T₁ × (PR^0.2857 − 1) ÷ η_c. After the intercooler T_charge = T_amb + (1 − η_ic) × (T₂ − T_amb).
Density ratio DR = PR × T_amb ÷ T_charge (kelvin) scales the naturally aspirated horsepower into an ideal turbo horsepower.
Airflow Wa (lb/min) = HP × AFR × BSFC ÷ 60. Corrected airflow for the compressor map = Wa × √((T₁ + 460) ÷ 545) ÷ (P₁ ÷ 13.95), with T₁ in °F and P₁ in psia.
Target horsepower: MAP_req = Wa × 639.6 × (460 + T_manifold) ÷ (VE × rpm ÷ 2 × displacement in cubic inches); boost = MAP_req − P_atm and compressor PR = (MAP_req + intercooler loss) ÷ (P_atm − inlet loss).

Use cases

Converting a boost gauge reading in psi or bar into a pressure ratio for a compressor map.
Estimating how much horsepower 7, 10, 15 or 20 psi adds to a naturally aspirated engine.
Working out how much boost and airflow a 300, 400 or 500 hp goal needs from your displacement and rpm.
Seeing why the same boost works the turbo harder at high altitude.
Checking compressor outlet and intake charge temperatures and the value of a better intercooler.
Plotting an operating point (corrected lb/min, pressure ratio) on a Garrett or BorgWarner compressor map.

Frequently Asked Questions

Last updated: 2026-10-10 · Reviewed by Nham Vu