The boost horsepower calculator above scales a naturally aspirated power figure by a compressor pressure ratio and a derate you control, then converts the answer into the airflow and fuel flow that power would demand. The first half is a rough estimate. The second half is the genuinely useful part, because airflow in pounds per minute is what you take to a compressor map, and fuel flow in pounds per hour is what sizes injectors and a pump.
Arb Digital builds free calculators that cite the relation they implement and are explicit about where estimating stops. On this page that boundary is sharp: arithmetic can size a component, and it cannot tell you whether an engine will tolerate the result. Fuelling, ignition timing and knock margin are decisions made on a dynamometer by a qualified tuner, with data, on the specific engine.
What This Boost Horsepower Calculator Does
You give it a naturally aspirated power figure, a boost pressure, the ambient pressure it is running in, and a derate factor. It computes the compressor pressure ratio, applies it to the base power, applies your derate, and reports the estimate along with the gain over the naturally aspirated figure.
It then takes that power and works out mass airflow using the relation published in Garrett's own turbo selection guide, along with the fuel flow implied by the same brake specific fuel consumption figure. Those two numbers are what actually get used: airflow against a compressor map to pick a turbo, fuel flow against injector and pump ratings to check the fuel system is not the limit.
Everything that could be a hidden constant is an input instead: ambient pressure, inlet depression, the derate, the air/fuel ratio and the BSFC. None of them has a universally right value, and a tool that picked one for you would be asserting something it cannot know.
The Formula: How It's Calculated
Power from a piston engine is very close to proportional to the mass of air it ingests, and for a fixed charge temperature the mass of air is proportional to absolute pressure. That proportionality is the ideal gas relation, which NASA states as p = ρRT on its equation of state page: at constant temperature, density and pressure move together.
So the estimate is:
pressure ratio = (ambient + boost) ÷ (ambient − inlet depression)
boosted power = naturally aspirated power × pressure ratio × derate
airflow, lb/min = power × air/fuel ratio × BSFC ÷ 60
fuel flow, lb/hr = power × BSFC
The pressure ratio and airflow relations are the ones published in Garrett Motion's turbo selection calculations guide, where pressure ratio is compressor outlet absolute pressure over compressor inlet absolute pressure, and airflow is horsepower multiplied by air/fuel ratio and BSFC, divided by sixty.
Work the defaults through. At sea level with no inlet restriction, 8 psi of boost gives a pressure ratio of 22.7 ÷ 14.7 = 1.544. A 200 hp engine scaled by that ratio would be 308.8 hp, and a 0.90 derate brings it to 278.0 hp, a gain of 78. At an air/fuel ratio of 12 and a BSFC of 0.55, that power needs 278.0 × 12 × 0.55 ÷ 60 = 30.6 lb/min of air and 278.0 × 0.55 = 152.9 lb/hr of fuel.
Why the Pressure Ratio Alone Overstates the Gain
The naive version of this calculation — multiply power by the pressure ratio and stop — is why "boost equals free power" is such a persistent piece of folklore, and why the derate field exists.
Compressing air heats it, and hot air is less dense at the same pressure. That is the same ideal gas relation working against you: raising pressure raises density, but the temperature rise that comes with compression takes some of it back. A compressor operating away from its efficient region heats the charge far more for the same pressure ratio, which is exactly why compressor maps have efficiency islands on them and why a mismatched turbo can deliver the boost number and not the power.
Charge cooling recovers much of that, and an effective intercooler is worth more than most people expect. But an intercooler has pressure drop of its own, and a poor one at high flow can cost more in restriction than it gains in density.
The third term hidden in the derate is timing. An engine under boost is at far higher cylinder pressure and temperature, which brings it much closer to knock. The usual response is to pull ignition timing, and retarded timing costs power directly. So an engine that could theoretically make the full pressure-ratio number on race fuel and optimal timing will not make it on pump fuel with a safe knock margin. The derate absorbs all of that in one figure, which is honest about the fact that it cannot be predicted from a spreadsheet.
Altitude, Depression, and Why Gauge Boost Deceives
A boost gauge reads pressure above ambient, and ambient is not a constant. That single fact accounts for a lot of confusion.
At sea level, ambient is about 14.7 psia, so 8 psi gauge is a pressure ratio of 1.54. At a mile of altitude ambient is closer to 12.2 psia, so the same 8 psi gauge reading is a ratio of 20.2 ÷ 12.2 = 1.66. The turbo is working harder, further up its map, and the air behind it is hotter — yet the engine is still making less absolute power than at sea level, because 20.2 psia is less air than 22.7 psia. The gauge reads the same. Everything else has changed.
Inlet depression is the other end of the same problem. A restrictive filter or a small intake pipe drops the compressor's inlet pressure below ambient, which raises the pressure ratio needed for a given manifold pressure. It is the cheapest thing on the list to fix and the most commonly ignored. Our pressure converter handles psi, bar and kPa if your figures are in mixed units, and the air density calculator works the density side directly from pressure, temperature and humidity.
Airflow Is the Number That Actually Buys a Turbo
Horsepower does not appear on a compressor map. Mass flow and pressure ratio do, which is why the airflow figure here matters more than the headline.
The procedure is simple once you have both numbers: find your pressure ratio on the vertical axis of a candidate compressor's map, your corrected airflow on the horizontal, and see where the point lands. Near the left-hand surge line the compressor will be unstable. Past the right-hand choke line it has run out of flow. Outside the efficiency islands it is heating the charge badly, which sends you back to the derate field.
The fuel flow figure does the equivalent job for the fuel system. Injectors are rated in mass or volume flow at a reference pressure, and a pump is rated in flow at a pressure. Both need headroom over the calculated requirement rather than matching it exactly, because an injector run at very high duty cycle loses linearity and a pump at the edge of its curve does not maintain rail pressure. Our companion fuel pump size calculator works that side of the problem, and the BSFC calculator lets you derive a real BSFC from measured fuel flow rather than assuming one.
Where This Estimate Breaks Down
Several things sit outside the arithmetic entirely, and they are not small.
Compression ratio is the first. A high static compression ratio and meaningful boost together produce cylinder pressures that pump fuel cannot support, which is why boosted engines commonly run lower static compression. The engine compression ratio calculator handles that geometry, but the interaction between static compression, boost, fuel octane and timing is a dyno problem, not a formula.
Second, the naturally aspirated figure you started from was measured with a particular camshaft, exhaust and intake. Boost changes what those components want, and an engine optimised for atmospheric induction is rarely optimal under pressure.
Third, nothing here says anything about durability. Rods, pistons, head gasket, clamping load, cooling capacity, oil control and the transmission behind the engine all have limits that this page cannot see. A number that looks achievable on airflow may be far past what the rest of the assembly tolerates.
For a cross-check on the power figure itself, our horsepower calculator estimates output from trap speed, elapsed time or torque and rpm, and the pressure converter covers the surrounding unit arithmetic. The air fuel ratio calculator works the mixture side properly.
Arb Digital designs and builds free interactive calculators that cite the relation they implement, state where estimating stops, and earn links because they are genuinely useful. Browse what we have already published, or tell us what your audience keeps searching for.
Browse the Free Tools Hub Talk to Arb DigitalCommon Mistakes to Avoid
- Using gauge boost as if it were absolute pressure. The ratio is taken on absolute pressures. Eight psi of boost is a ratio of 1.54 at sea level, not 8.
- Setting the derate to 1.0. That assumes perfect charge cooling and no timing retard. It is the best case, not the expected case.
- Ignoring altitude. The same gauge reading is a higher pressure ratio and less absolute air at altitude. The gauge does not tell you this.
- Sizing injectors to the calculated fuel flow exactly. Injectors lose linearity at very high duty cycle, and pumps lose rail pressure at the edge of their curve. Both need headroom.
- Treating an airflow number as approval for a build. It sizes a compressor. It says nothing about rods, pistons, head gasket clamping, cooling or the gearbox.
Related Free Tools From Arb Digital
Derive a real BSFC from measured fuel flow with the BSFC calculator, size the fuel system with the fuel pump size calculator, and check the mixture with the air fuel ratio calculator. The engine compression ratio calculator covers engine geometry, the horsepower calculator estimates output from track or dyno figures, and the pressure converter and air density calculator handle the units and the air side. Everything else is on the free online tools hub.
Frequently Asked Questions
Approximately in proportion to the compressor pressure ratio, which is absolute manifold pressure divided by compressor inlet absolute pressure. At sea level, 8 psi of boost is a ratio of about 1.54, so a 200 hp engine scales to roughly 309 hp before any derate for charge heating and timing, and about 278 hp at a 0.90 derate.
Because the relation works on absolute pressures, not gauge ones. Eight psi of boost is eight psi above atmospheric, so what matters is 22.7 psia against 14.7 psia. That is why altitude changes the answer even when the boost gauge reads the same number.
There is no correct value, which is why it is an input. It absorbs charge heating from compression, intercooler effectiveness and pressure drop, and the ignition timing you can actually run without knock on your fuel. Setting it to 1.0 assumes a best case that pump fuel and a real intercooler rarely deliver.
Garrett's published turbo selection guide gives 0.50 to 0.60 and higher as the general range for turbocharged gasoline engines, and notes that reaching lower figures requires race fuels and aggressive tuning. If you have measured fuel flow and power on your own engine, calculate your own figure instead of assuming one.
Because horsepower does not appear on a compressor map and mass flow does. Airflow in pounds per minute and pressure ratio together give you a point to plot on a candidate compressor's map, which is how a turbo is actually chosen.
No. Past a compressor's efficient region the charge gets hotter for each additional psi, so density gains shrink while knock risk grows. Beyond that the engine's timing has to be retarded to survive, which costs power directly, and the airflow may be past what the turbo can supply at all.
You can use it to size a compressor and a fuel system, which is what the airflow and fuel flow figures are for. It cannot tell you whether an engine will tolerate the result. Fuelling, ignition timing and knock margin are dynamometer decisions made by a qualified tuner on the specific engine.
This page performs arithmetic on figures you supply and sizes components; it does not approve a build. Fuelling, ignition timing and knock margin must be set on a dynamometer by a qualified tuner on the specific engine, and engine modification affects reliability, emissions compliance, insurance and road legality — all matters for the relevant authorities and professionals, not for a calculator.
