Calculators · Forced induction
Boost pressure and pressure ratio calculator
A boost gauge shows pressure above the surrounding atmosphere, but a turbocharger compressor works on absolute pressure, and what it has to deliver is a pressure ratio. At altitude the same boost reading means a higher ratio and a harder-working turbo. These calculators convert between gauge and absolute pressure at any altitude, include intake and intercooler losses, and estimate the boost needed for a target power increase.
Gauge boost to absolute pressure and pressure ratio
Converts a boost gauge reading into absolute manifold pressure and the compressor pressure ratio, accounting for lower air pressure at altitude. The pressure ratio is what you read against a compressor map.
P_atm = 1.01325 × (1 − 2.25577×10⁻⁵ × h)^5.25588 P_abs = P_atm + boost PR = P_abs / P_atm
Compressor pressure ratio with system losses
The compressor must make more pressure than the manifold sees, because the air filter lowers pressure before the turbo and the intercooler and pipes lose pressure after it. This gives the true pressure ratio to check on a compressor map.
P_in = P_atm − inlet loss P_out = P_atm + boost + outlet loss PR = P_out / P_in
Boost needed for a power increase
Estimates the boost needed to raise power by a chosen factor, assuming power rises with air density in the cylinder. Warmer charge air is less dense, so the charge temperature after the intercooler matters as much as the boost itself.
PR = multiplier × T_charge / T_ambient (kelvin) boost = P_atm × (PR − 1)
Worked example: 1 bar of boost at sea level and in the mountains
- At sea level: 1.013 + 1.0 = 2.013 bar absolute, a pressure ratio of 1.99.
- At 1,500 m the air is at 0.846 bar, so the same 1.0 bar of boost needs a ratio of 2.18.
- Adding 0.05 bar of filter loss and 0.1 bar of intercooler loss at sea level raises the real compressor ratio to 2.19.
- For 50% more power with the charge at 45 °C on a 25 °C day, you need a ratio of 1.60 — about 0.61 bar of boost.
Standard atmospheric pressure by altitude
| Altitude | Pressure | Pressure ratio at 1 bar boost |
|---|---|---|
| 0 m | 1.013 bar (14.7 psi) | 1.99 |
| 500 m | 0.955 bar (13.8 psi) | 2.05 |
| 1,000 m | 0.899 bar (13.0 psi) | 2.11 |
| 1,500 m | 0.846 bar (12.3 psi) | 2.18 |
| 2,000 m | 0.795 bar (11.5 psi) | 2.26 |
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About these results
Results are theoretical estimates from standard engineering formulas. Real-world figures depend on conditions these formulas do not capture. Always follow the manufacturer's specifications for maintenance, loading and safety decisions.
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