Jetting Calculator: Air-Density Correction
This calculator compares air density between your known-good baseline (where the engine was last jetted right) and today's conditions, then reports which direction the mixture has drifted and roughly how far, as richer/leaner guidance in main-jet steps. Never a jet number, by design.
Getting this wrong in the lean direction can destroy an engine in minutes. This tool gives directional guidance only; confirm every change with plug reads or a qualified tuner. It will never print a jet number: a safe absolute jet size cannot be computed without your carburetor, your jet series, and a verified baseline setup.
Why air density changes your mixture
A carburetor meters fuel against the volume of air moving through the venturi, not its mass. The venturi pressure drop draws fuel through a fixed orifice, and flow through a jet scales with its area; jet sizes are effectively an area series. Nothing in that mechanism knows how much oxygen the air contains.
Combustion runs on mass. When air density drops 10%, the engine pulls the same volume per cycle, but that volume holds 10% less oxygen, and the fuel side is unchanged, so the mixture is now roughly 10% richer than calibrated. When density rises, the mixture goes lean instead, which is the direction that damages engines.
Elevation, temperature, and humidity all move density, and the engine cannot tell them apart; a hot day at low elevation can match a cool day at altitude. So this page asks for two complete sets of conditions: what matters is the ratio of today's density to the density your jetting was actually dialed in at.
The "one main-jet step per ~3% density change" heuristic
The step guidance uses a rule of thumb that appears throughout carburetor tuning guides and jetting-kit makers' density-correction charts: about one main-jet step per 3% change in air density. It is a heuristic, not a law; a "step" is one position in your carb's own jet series, and step granularity varies by manufacturer. Treat the count as a starting search region, not a destination.
Every rounding decision lands on the rich side. Thinner air calls for leaning out, so the step count rounds down: you lean less than the arithmetic suggests, then confirm on the engine. Denser air calls for richening, so the count rounds up. And when the change is worth less than one step, the output says leave the jetting alone: slightly rich is a nuisance, slightly lean is a gamble.
Worked example: jetted at 500 ft, riding at 3,500 ft
Baseline: jetted right at 500 ft elevation, 20 °C, dewpoint 10 °C. Today: 3,500 ft, 30 °C, dewpoint 15 °C. You rode somewhere higher and hotter. These are the calculator defaults, so every number below reproduces on the page.
- Baseline station pressure from elevation via the ISA barometric formula: 500 ft ÷ 3.28084 = 152.4 m; P = 1013.25 × (1 − 2.2557×10⁻⁵ × 152.4)^5.25588 ≈ 995.08 hPa.
- Baseline density via the moist-air formula: vapor pressure es(10 °C) = 6.1078 × 10^(7.5×10/(237.3+10)) ≈ 12.28 hPa, so Pv = 1,228 Pa and Pd = 99,508 − 1,228 = 98,280 Pa; T = 293.15 K; ρ_base = 98280/(287.058×293.15) + 1228/(461.495×293.15) ≈ 1.1679 + 0.0091 ≈ 1.177 kg/m³.
- Current conditions the same way: 3,500 ft ÷ 3.28084 = 1,066.8 m → P ≈ 891.49 hPa; es(15 °C) ≈ 17.05 hPa → Pv = 1,705 Pa, Pd = 89,149 − 1,705 = 87,444 Pa; T = 303.15 K; ρ_now = 87444/(287.058×303.15) + 1705/(461.495×303.15) ≈ 1.0049 + 0.0122 ≈ 1.017 kg/m³.
- Density change: 1.017 ÷ 1.177 = 0.8641, a −13.6% change. The air is 13.6% thinner than the air the engine was jetted in, so the mixture is running roughly 13.6% rich.
- Step guidance: 13.6 ÷ 3 = 4.5 steps by the heuristic, rounded down to 4 main-jet steps leaner, never up, because the leftover half-step stays on the rich side. Then plug reads decide whether step 5 ever happens.
Plug reads are the ground truth
Tuning guides describe the same picture with minor variations: after a sustained load run on a fresh plug, a safe mixture leaves the insulator tan to light brown. A lean mixture leaves it chalk white or glazed, blistered, or carrying specks of aluminum from the piston, in worse cases. The lean picture is a stop-riding signal, not a data point to iterate on.
Plug reading is coarse, and modern fuels mute the colors, but it fails safe: it reports the actual engine, fuel, and day, which no density calculation can. EGT or wideband AFR is better evidence if you have it. Either way, the calculator's output is the start of that verification, not a substitute for it.
Why the two directions are not symmetric
Too rich costs performance: soft response, four-stroking, a fouled plug, all fixed with a jet change and a new plug. Too lean costs hardware: hotter combustion, shrinking detonation margin, and on a two-stroke a seized piston within minutes of hard running, from feels-crisp to locked-rear-wheel in one long pull.
That asymmetry drives every conservative choice on this page. It is also why denser air deserves the most respect despite feeling like good news: more power, and a mixture that has silently gone lean against your baseline. Richen first, then ride. Thinner air merely makes the engine rich and lazy, annoying, and safe.
Density change chart
Every cell comes from the same tested Go functions the calculator runs, computed at build time. Baseline fixed at 500 ft / 20 °C / dewpoint 10 °C; dewpoint held at 10 °C; 10/20/30/35 °C are 50/68/86/95 °F. Use the calculator for your actual baseline; the percentages shift with it.
| Elev / Temp | 10 °C | 20 °C | 30 °C | 35 °C |
|---|---|---|---|---|
| 0 ft | +5.4 | +1.8 | -1.5 | -3.1 |
| 1000 ft | +1.7 | -1.8 | -5.0 | -6.6 |
| 2000 ft | -2.0 | -5.3 | -8.5 | -10.0 |
| 3000 ft | -5.6 | -8.8 | -11.8 | -13.2 |
| 4000 ft | -9.0 | -12.1 | -15.0 | -16.4 |
| 5000 ft | -12.4 | -15.3 | -18.1 | -19.5 |
| 6000 ft | -15.6 | -18.5 | -21.2 | -22.5 |
| 7000 ft | -18.8 | -21.5 | -24.1 | -25.3 |
| 8000 ft | -21.8 | -24.5 | -27.0 | -28.2 |
For the raw numbers behind this page (air density, density ratio, density altitude) use the Density Altitude Calculator; it shares the same tested formulas. Mixing fuel for the day is a separate question: the 2-Stroke Mix Calculator covers premix ratios.
FAQ
Why won't this tell me a jet size?
Because a jet size is only meaningful relative to a baseline this page cannot know: your carburetor and jet series, your engine's tune, your fuel, and the conditions the current jetting was dialed in under. Any absolute number printed here would be a guess dressed up as an answer, and a guess that lands lean can seize an engine. Density physics supports one honest claim (how far conditions moved from your baseline, and which direction), so that is all this page outputs.
Do I rejet for weather, or just for altitude?
Density is density; the engine does not care about the cause. In the chart, holding 3,000 ft while temperature goes from 10 °C to 35 °C moves the density change from −5.6% to −13.2%: a 7.6-point swing from weather alone, more than two main-jet steps by the heuristic. A cold snap deserves the most attention; it moves the mixture lean. Racers re-read conditions every session; a plug check after a big weather swing is the casual version.
What about fuel injection?
Modern EFI measures intake air (MAP or MAF, air temperature, often barometric pressure) and compensates automatically; density changes of this size are the ECU's job. This page is for carburetors, where the metering is fixed orifices and the correction is you. An EFI machine running poorly after an altitude change is a diagnostic question for the ECU's trim data, not a jetting question.
Does this apply to the pilot jet and needle too?
The direction applies to every circuit (thinner air richens pilot, needle, and main alike), but the step heuristic is calibrated to the main jet, which is why the guidance names it. Tuning guides correct the main first and treat pilot and needle as smaller, later adjustments many density changes never require. Change one thing, verify on the engine, then decide.
What is a baseline, and how do I get one?
The baseline is a jetting setup you have confirmed is correct, together with the conditions it was confirmed in: the elevation, temperature, and dewpoint on the day the engine last ran clean and a plug read backed it up. Every number this page produces is a correction relative to that baseline, so a baseline you have not verified on the engine turns the output into a correction built on a guess. If you do not have one, establish it first: jet until a sustained-load plug read comes back tan to light brown, then record that day's conditions as your baseline.
Is it better to run a little rich or a little lean?
A little rich, without hesitation. A slightly rich mixture costs power and fouls plugs, both annoying and both reversible. A slightly lean mixture raises combustion temperature and shrinks the detonation margin, and on a two-stroke it can seize a piston within minutes of hard running. The two mistakes are not equal, which is why every rounding decision on this page lands on the rich side and why the output says to leave the jetting alone when a change is worth less than one step. When in doubt, stay rich and let plug reads move you leaner, never the arithmetic.
Sources
Density physics: identical formulas and constants to this site's Density Altitude Calculator, using Magnus vapor pressure (WMO CIMO Guide coefficients as used by NOAA/NWS), moist-air density per ICAO Doc 7488/3 (Rd = 287.058, Rv = 461.495 J/(kg·K)), and the ISA barometric elevation-to-pressure conversion. Full citations on that page.
Tuning heuristics: the ~3%-per-step correction and main-jet-first practice are described across carburetor tuning guides and aftermarket jetting-kit makers' density-correction charts, cited descriptively as a class, because step granularity is carburetor- and jet-series-specific.
Plug reading: insulator-color guidance summarized conservatively from engine-builder and manufacturer tuning literature; the safe-vs-lean distinction above is the common core, stated in its most cautious form.