Carb Icing Calculator
Check the carburettor icing risk from the temperature and dew point, at cruise or descent power.
We read the temperature and dew point (like 15/10 or M02/M05) and fill them in below.
Use the temperature and dew point where you’ll fly. In a METAR they sit together: 15/10 means 15 °C with a dew point of 10 °C.
The chart has two columns: cruise power and descent power. Climb uses the cruise column, the nearest one.
In cloud or rain the air is saturated, so we use 100% humidity.
A rough estimate: the air cools about 2 °C and the dew point about 0.5 °C per 1,000 ft. Leave blank or 0 to skip.
What to do
How your carb icing risk is worked out
The fuel evaporating in the venturi chills the air. Moist air plus that chill makes ice.
- 1
Spread and humidity
15 °C − 10 °C
= 5 °C, RH 72% - 2
Find the zone
15 °C, 10 °C
→ Serious, any power - 3
Your power setting
Cruise power
= Serious risk - 4
What to do
Carb heat check
about every 10 min
Read more: the chart, the formula, a worked example and what to do
A carburettor mixes fuel with the air going into the engine. The air speeds up through a narrow throat (the venturi), so its pressure and temperature drop, and the fuel sprayed in evaporates and takes more heat away. The air inside the carburettor can end up far colder than the air outside. If it holds enough water vapour, that water freezes on the venturi and the throttle valve, chokes the airflow and the engine loses power.
The formula
spread = temperature − dew point
relative humidity = 100 × e^(17.625 × dew point ÷ (243.04 + dew point)) ÷ e^(17.625 × temperature ÷ (243.04 + temperature))
risk = the zone where temperature and dew point meet on the icing chart, read at your power setting (cruise or descent)
The humidity uses the Magnus formula (Alduchov and Eskridge constants), the same as our cloud base calculator. The chart itself is not a formula: it comes from icing tests and is published by the UK CAA, CASA and others. We drew its zone edges as straight-sided shapes read off the chart by eye, so near an edge treat the answer as the worse of the two zones.
Worked example
The METAR says 15/10: a temperature of 15 °C and a dew point of 10 °C, with cruise power.
- Spread: 15 − 10 = 5 °C.
- Humidity: 17.625 × 10 ÷ 253.04 = 0.6965 and 17.625 × 15 ÷ 258.04 = 1.0246. e^(0.6965 − 1.0246) = e^−0.3280 = 0.720, so about 72%.
- On the chart, the serious-icing zone at 15 °C reaches down to a dew point of about 9 °C. A dew point of 10 °C is inside it, so the zone is serious icing at any power.
- At cruise power the risk is serious, and at descent power it is serious too. Check for ice regularly and put full carb heat on before you reduce power.
- In Advanced, 3,000 ft above the field: 15 − 2 × 3 = 9 °C, dew point 10 − 0.5 × 3 = 8.5 °C, humidity about 97%: still serious.
The four zones
| Zone on the chart | Cruise power | Descent power |
|---|---|---|
| Serious icing, any power | Serious | Serious |
| Moderate icing at cruise, serious at descent | Moderate | Serious |
| Serious icing at descent power | Light (not marked; we take the cautious reading) | Serious |
| Light icing, cruise or descent | Light | Light |
| Outside the zones | Low | Low |
Signs of carb ice and what to do
With a fixed-pitch propeller the first sign is a slow drop in rpm. With a constant-speed propeller the rpm stays put and the manifold pressure drops instead. Rough running follows. Apply full carb heat and leave it on: the engine may run rougher for a moment as the melted ice goes through it, then the power comes back. Put carb heat on before you reduce power for descent or landing, and follow your aircraft’s flight manual, which comes first.
Notes and sources
- Chart: UK CAA Safety Sense Leaflet 14, Piston Engine Icing (caa.co.uk); the same chart from CASA (casa.gov.au) and the ATSB (atsb.gov.au); EASA Safety Information Bulletin 2010-03 (easa.europa.eu); FAA SAIB CE-09-35.
- FAA Pilot’s Handbook of Aeronautical Knowledge (FAA-H-8083-25), chapter 7, carburetor icing (faa.gov): most likely below 21 °C (70 °F) with humidity above 80%; possible up to 38 °C (100 °F) and humidity down to 50%.
- The height estimate uses rules of thumb (2 °C and 0.5 °C per 1,000 ft) and ignores inversions. Use the forecast if you have it, or check the METAR & TAF decoder and the density altitude calculator.
Planning and training aid only. Always use your approved flight manual, operator procedures and official charts.
Common questions
What is carburettor icing?
As air speeds up through the carburettor venturi its pressure drops, and the fuel sprayed into it evaporates. Both cool the air, so water vapour can freeze on the venturi and the throttle valve. The ice chokes the airflow, so the engine loses power and can stop. It can happen on a warm day, in clear air, because what matters is the humidity and the cooling inside the carburettor.
At what temperature can carb icing happen?
The FAA Pilot’s Handbook of Aeronautical Knowledge says carburettor ice is most likely below about 21 °C (70 °F) with relative humidity above 80%, but can form at up to about 38 °C (100 °F) and humidity as low as 50%, because the air in the carburettor can cool by 60–70 °F. The icing chart shows a risk from about −10 °C up to the high 30s °C.
Why is carb icing worse at descent power?
With the throttle nearly closed, the air squeezes through a small gap around the throttle valve, which cools it more, and a little ice blocks that gap quickly. The engine is also making less heat, so the carb heat is weaker. That is why the chart shows serious icing at descent power over a much wider range than at cruise. Put carb heat on before you close the throttle.
How do you read the carburettor icing chart?
Find the air temperature along the bottom and the dew point up the side. Where the two meet tells you the zone: serious icing at any power, moderate at cruise and serious at descent, serious at descent power, or light icing. The closer the temperature and dew point are (the higher the humidity), the worse the risk. This calculator plots your point on an approximate copy of the chart.
Do fuel-injected engines get carb icing?
No. A fuel-injected engine has no carburettor venturi, so it can’t get carburettor ice. It can still suffer induction icing, where snow, sleet or supercooled water blocks the air filter or intake in cloud or precipitation near 0 °C. That is what the alternate air door is for. Follow your flight manual.
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