Turbo Size Calculator

Turbocharger sizing is one of the most technically demanding parts of a forced-induction build. Get it wrong and you end up with a turbo that either spools lazily and tops out at 80 % of your power target, or one that operates outside its efficient range and generates excessive heat and lag. This calculator applies the same first-principles thermodynamic and empirical formulas used by professional engine builders to give you a data-driven starting point before you commit to a purchase.

How turbo sizing works

At the core of every sizing exercise is one question: how much air does the engine need, and at what pressure? The answer drives every other decision — compressor wheel diameter, pressure ratio, turbine A/R, wastegate sizing and intercooler capacity.

Mass airflow rate

For a given engine the theoretical mass airflow (MAF) at peak power is:

MAF = (Vd × N × VE × ρ_charge) / (2 × 60)

where Vd is displacement in m³, N is crankshaft RPM, VE is volumetric efficiency (typically 0.80–0.95 for a naturally aspirated engine, sometimes above 1.0 with aggressive cam timing), and ρ_charge is the density of the intake charge. The factor of 2 accounts for the four-stroke cycle (one induction stroke every two crank revolutions). The density of the charge air is computed from the ideal-gas law at the pressure and temperature downstream of the intercooler.

Alternatively, if you have a wheel-horsepower target, you can work backwards using the Corky Bell method:

MAF (lb/min) = HP × BSFC × AFR / 60

BSFC (brake specific fuel consumption) is typically 0.45–0.55 lb/(hp·h) for a petrol turbo engine.

Pressure ratio

Pressure ratio is simply the ratio of absolute outlet pressure to absolute inlet pressure:

PR = (P_atm + P_boost_gauge) / P_atm

At sea level (101.3 kPa) and 15 psi of boost, PR ≈ 2.02. This is the most important single number when selecting a turbocharger — every compressor map is plotted against PR on the vertical axis and corrected mass flow on the horizontal axis.

Corrected mass flow

Because compressor maps are referenced to standard atmospheric conditions (288.15 K, 101.325 kPa), the actual airflow must be corrected before plotting:

W_c = MAF × sqrt(T1_K / 288.15) / (P1_kPa / 101.325)

On a hot day at altitude you may need a significantly larger turbo than standard conditions would suggest.

Compressor power and charge temperature

The work done by the compressor is:

P_comp = ṁ × Cp × T1 × (PR^((k−1)/k) − 1) / η_c

where ṁ is mass flow in kg/s, Cp = 1005 J/(kg·K), k = 1.4 for air and η_c is the isentropic efficiency. The temperature rise out of the compressor is:

T2 = T1 × PR^((k−1)/k)

An intercooler with effectiveness ε reduces this rise: the charge temperature entering the manifold is approximately T1 + (1 − ε) × (T2 − T1). A 70 % effective intercooler on a 2.0× PR build typically brings the charge from around 93 °C down to about 52 °C — a meaningful density recovery.

Compressor inducer estimate

Once you have the corrected flow, an empirical fit across a wide range of off-the-shelf turbos gives a first estimate of the required compressor inducer (inlet) diameter:

inducer_mm ≈ 10.16 × W_c^0.5

This aligns with commonly published sizing tables and is accurate to within roughly ±5 mm for most street and track applications. The exducer (outlet) of the compressor wheel is typically 1.35–1.50× the inducer diameter.

Worked example

A 2.0-litre four-cylinder engine running at 6,500 RPM with 85 % VE, 15 psi of boost, a 70 % effective intercooler and standard sea-level conditions (25 °C, 101.3 kPa) produces:

A 53 mm inducer falls in the mid-range performance bracket — turbos like the Garrett GT3076R, Precision 6266 or Borg Warner EFR 6758 are all candidates in this range.

Formula reference

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