Density from first principles, not from a chart
Most jetting charts give you a correction factor against altitude alone, which quietly assumes a temperature and a humidity you never agreed to. Air density is a three-variable problem and the ideal gas law handles all three at once.
For dry air, density is pressure divided by the gas constant for air times absolute temperature:
ρ = p ÷ (287.058 × T)
with p in pascals and T in kelvin. At sea level standard conditions — 101,325 Pa and 288.15 K — that gives 101,325 ÷ (287.058 × 288.15) = 1.2250 kg/m³, which is the standard sea level figure, so the constant checks out.
Humid air is slightly lighter, which surprises people who expect wet air to be heavy. Water has a molar mass of 18 against about 29 for air, so every water molecule that displaces an air molecule at the same pressure and temperature reduces the mass in the box. The correction splits the total pressure into dry and vapour parts:
ρ = p_dry ÷ (287.058 T) + p_vapour ÷ (461.495 T)
The vapour pressure comes from the relative humidity times the saturation vapour pressure at that temperature, which this page takes from the Magnus approximation. On a hot humid day that is worth around one percent of density; on a cold one it is nearly nothing, because cold air cannot hold much water in the first place.
Pressure at altitude, when you do not have a barometer, comes from the standard atmosphere:
p = 101,325 × (1 − 2.25577×10⁻⁵ h)^5.25588
with h the elevation in metres. This is a model of an average day, not a measurement of yours. A real low-pressure system can move the number by two percent, which is the same order as a couple of thousand feet of altitude, so if you have a station pressure reading it is better than the model. Note that weather reports usually give sea-level-corrected pressure, which is not what this wants.
Density altitude as one number
Density altitude collapses altitude, temperature and humidity into a single figure: the elevation at which the standard atmosphere would have the density you actually have. A hot day at 5,000 feet can be an 8,000 foot day as far as the engine is concerned. Aviation uses it constantly for the same reason engines care about it — it is the only altitude the air itself knows about.
| Conditions | Density | Fraction of sea level standard |
|---|---|---|
| Sea level, 59 F, dry | 1.2250 kg/m³ | 100% |
| Sea level, 95 F, dry | 1.1690 kg/m³ | 95.4% |
| 5,000 ft, 59 F, dry | 1.0561 kg/m³ | 86.2% |
| 5,000 ft, 95 F, dry | 1.0078 kg/m³ | 82.3% |
| 10,000 ft, 59 F, dry | 0.9046 kg/m³ | 73.8% |
Those figures come from the same two formulas above at the stated conditions, and they show why temperature is not a footnote: 36 degrees of it is worth about four percent of density, which at 5,000 feet is comparable to another 1,500 feet of elevation.
From density to a factor, and no further
An engine at a given rpm and throttle opening is a fixed-volume pump, so the mass of air it swallows per minute is proportional to density. To hold the same air-fuel ratio, the mass of fuel has to move in the same proportion. That much is unarguable and it is the mass-flow figure shown in the results.
The jet does not deliver a fixed fuel mass, though. Flow through an orifice goes with the area times the square root of the pressure drop across it, and the pressure drop is the venturi depression, which scales with air density at a given air velocity. So a fixed jet in thinner air already delivers less fuel, by the square root of the density ratio, without anybody touching it. The shortfall the jet has to make up is what is left: the other square root. That is why the classical correction factor applied to jet area is the square root of the density ratio, and why diameter — which goes with the square root of area — moves by the fourth root.
Both figures appear in the results deliberately. The mass-flow ratio is what would be needed if fuel delivery did not respond to density at all; the square root is what is needed given that it does. Real carburettors sit near the second and are not exactly on it.
Why this stops at a number
A correction factor is arithmetic. A jet size is not, and this page will not produce one. Jet numbering schemes are not linear in diameter and are not consistent between manufacturers. Discharge coefficients change with size and with Reynolds number. The main jet only governs one part of the throttle range, with the pilot circuit, needle taper, needle position, emulsion tube and air jet governing the rest, and altitude affects each of them differently. Fuel itself varies — oxygenated pump fuel behaves differently from what a machine was set up on.
What the factor genuinely tells you is the size and direction of the change, and whether a change is worth making at all. A density ratio of 0.98 is inside the noise of a normal week of weather. A ratio of 0.80 is a different engine.
Lean running is the failure mode with consequences. A two-stroke jetted for thin air and run in dense air, or set on a hot afternoon and ridden on a cold morning, is running leaner than it was set for, and a lean two-stroke can pick up a piston without giving much warning first. Deciding jetting means reading a plug and knowing that engine, and it is work for someone who does it — not for a factor off a web page.
Questions people ask
What is a carburetor correction factor?
It is the ratio of air density between the conditions a machine was set up in and the conditions it is going to run in, converted into a scaling for fuel delivery. Because an engine at a given rpm swallows a fixed volume of air, the mass it takes in tracks density directly, and to hold the same mixture the fuel has to follow. The factor most tuners use is the square root of the density ratio applied to jet area, because a fixed jet already delivers less fuel in thin air on its own — the venturi depression that drives fuel through it falls with density too, and that accounts for the other square root. What a correction factor is not is a jet size. It tells you the size and direction of the change; converting that into a part number is a judgement about a specific engine on specific fuel, made with a plug reading in hand.
How much does altitude actually change air density?
Roughly three percent per thousand feet near sea level, falling off slowly as you climb because the atmosphere thins exponentially rather than linearly. In standard conditions, 5,000 feet has about 86 percent of sea level density and 10,000 feet about 74 percent. Temperature is a comparable effect and gets ignored far more often: air density varies inversely with absolute temperature, so going from 40 F to 100 F at the same elevation costs about eleven percent of density, which is worth roughly 4,000 feet of altitude. A cold morning at elevation and a hot afternoon at the coast can land at the same density. That is exactly what density altitude expresses, and it is why a correction based on elevation alone is only half a calculation.
Does humidity matter for jetting?
Less than people expect, and in the opposite direction to intuition. Humid air is less dense, not more, because a water molecule weighs about 18 against roughly 29 for the average air molecule, so vapour displacing air at the same pressure reduces the mass in a given volume. At 90 F and full saturation the effect is around one and a half percent of density; at 40 F it is a fraction of a percent, because cold air holds very little water. So on a hot muggy day it is a real if small contribution, and on a cold day it is noise. It is included above because it costs nothing to include and because it stops the number being silently wrong on exactly the days when it is largest.
Can I use the barometric pressure from a weather app?
Only if it is station pressure, and most reported figures are not. Weather services normally publish altimeter setting or sea-level-corrected pressure, which is the local reading mathematically adjusted to what it would be at sea level so that readings from different elevations can be compared on a map. Feeding that into a density calculation at your actual elevation double-counts the altitude and gives a density that is far too high. What this page wants is the pressure the air is actually at where you are standing. A phone barometer, an altimeter watch in absolute mode, or an aviation METAR that reports both will give it. If you do not have one, leaving the field empty and letting the standard atmosphere estimate from elevation is usually within a percent or two.
Why does my engine feel flat at altitude even after rejetting?
Because jetting restores the mixture, not the air. Power comes from burning fuel, and the amount of fuel you can burn is limited by the oxygen available, which is what the thin air is short of. Correct jetting at 8,000 feet means the engine is running at the right ratio on about three quarters of the air mass it had at sea level, so it makes something in the region of three quarters of the power. Rejetting stops it running rich and losing more than that, and it protects the plugs and the throttle response, but it cannot put back what the atmosphere did not supply. Forced induction is the only thing that does, which is why aircraft and high-altitude machinery use it. Gearing is the other lever riders reach for at elevation, and that arithmetic is on the sprocket gearing calculator on this site.