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Jetting Change Calculator
Size a jet change for changing air density
A jetting change recalculates carburetor jet size for new air density, keeping the air/fuel ratio constant as the weather changes.
Formula: New jet diameter = Jet × (air density ratio)^(1/4), density ratio from the standard atmosphere at each DA
What this number means
This estimates a carburetor jet change from a change in air density or density altitude. It gives a mathematically grounded first step, in jet sizes, from a baseline jetting you know is right.
What to change next
Make only the suggested change, then judge by trap MPH and plug reading before stepping again. Log the baseline and the air it was set in so you can return to it exactly.
Worked example
Jetted correctly at 78 primary jets at 1,000 ft DA, moving to 3,500 ft DA suggests ≈ 76 primary jets as a first step.