Spark Plug Heat Range Calculator
You added power, raised compression or moved somewhere higher up, and now you are not sure the plug in the box is still the right one. Tell it what changed and it gives you the shift in steps and the number to actually order. Nothing uploaded.
How the shift adds up
The same tier in other brands ?
What the elevation is doing
Before you order
- A plug is not a tune. NGK's advice alongside a heat range change is to retard the timing a little and to increase fuel enrichment and octane, and it warns that failure to address all of those areas with forced induction will virtually guarantee engine damage.
- Heat range is only the thermal part of the choice. Reach, thread size, seat type, gap and electrode material all still have to match the head, and none of them are in this calculation.
- The engine has the final word. Pull the plugs after a few heat cycles and read them, because a plug that comes out clean and light brown is right whatever any calculator said.
Learn more: heat range, altitude and added power
Everything gets converted into horsepower first
There is one published rule with a number in it, and the whole calculator hangs off it. NGK says to select one heat range colder for every 75-100 hp you add, or when you significantly raise compression.
That gives a step a price in horsepower. So the altitude half of the problem is handled by converting thin air into horsepower too, rather than inventing a second rule for it.
The direction that half pushes is not in doubt. NGK's own basics page says air pressure and cylinder pressure decrease as altitude increases, and as a result spark plug tip temperature will also decrease. What no manufacturer publishes is how much altitude is worth in heat range steps, which is the gap this fills.
Air density comes from the International Standard Atmosphere, the same model aviation uses. Density is the ratio of local temperature to sea level temperature raised to the power 4.256, which works out at 85% of sea level density in Denver at 5,280 ft and 74% in Leadville at 10,150 ft.
A naturally aspirated engine's air mass tracks that density, so its power does too. The calculator multiplies your stock output by the density at each elevation, adds the power you gained, and turns the difference into steps.
Which end of NGK's range it divides by depends on the direction. Power up divides by the tighter 75 hp per step, power down by the looser 100 hp, so the recommendation leans cold either way. Both ends are shown in the breakdown so you can see the spread.
Three worked examples from the same starting plug
Start with an NGK 6, a 300 hp naturally aspirated engine and a factory plug specification, which means a sea level baseline.
Drive it to Denver and nothing changes. Density drops to 85%, so 300 hp behaves like 256 hp, a loss of 44 hp. At the loose end of NGK's rule that is 0.44 of a step, under the half step the calculator needs before it will move you, so it says to stay on the NGK 6.
Bolt a supercharger worth 150 hp onto the same engine and it is a different answer. Boost holds manifold pressure as the air thins, so no altitude credit applies and the full 150 hp counts: 150 divided by 75 is exactly 2 steps colder, taking the NGK 6 to an NGK 8. DENSO's chart puts that tier at a DENSO 24 or a Bosch 4.
Now take a 200 hp naturally aspirated engine up to Leadville at 10,150 ft. Density is 74%, so it behaves like 147 hp, down 53 hp, which is 0.53 of a step at the loose end. That is just over the half step threshold, so it suggests one step hotter, an NGK 5.
Why the numbers run backwards between brands
NGK states plainly that lower numbers indicate a hotter type, higher numbers indicate a colder type. DENSO's ladder runs the same way, from 9 at the hot end to 35 at the cold end.
Champion and Bosch go the other way, and so does Autolite, where a higher number indicates a hot plug and a low number a cold heat range. Ordering a bigger number in the hope of getting a colder plug is how people end up with the opposite of what they wanted.
The ladder is not evenly spaced either. DENSO's own comparison chart steps 20, 22, 24 and then jumps to 27, so a step is a move to the next number on the chart rather than an arithmetic plus one. The cross reference table above walks that chart instead of doing arithmetic on your number.
What one step is actually worth, and what it cannot fix
NGK puts the difference from one heat range to the next at approximately 70°C to 100°C of firing tip temperature, against an optimal window of roughly 500°C to 800°C. Autolite describes the same move as changing the tip temperature by 50 to 150 degrees depending on design, without naming which scale those degrees are on.
The two figures do not line up because a heat range is a property of a specific plug's insulator and electrode geometry rather than a standardised unit. Cross-brand equivalents are approximate for the same reason, and the cross reference chart carries that warning in capitals: the spark plugs of various manufacturers do not have exactly the same heat range.
The hotter direction is capped at a single step here on purpose. Autolite's guidance is to start with a colder plug when you are unsure, and not to move to a hotter heat range without consulting engine experts, because the failure mode is pre-ignition rather than a fouled tip.
Nothing in this calculation knows about your fuel, your ignition timing or how the engine is actually driven. A plug that is thermally correct for a track day can foul on a week of short cold trips, and the plugs themselves will tell you that long before a formula does.
FAQ
Does altitude really change which spark plug I need?
It changes the plug's temperature, which is not the same as changing the plug you need. NGK's own guidance is that air pressure and cylinder pressure fall as altitude rises, and that the spark plug tip temperature falls with them, so a plug that was correct at sea level runs cooler in the mountains. Whether that is enough to justify buying a different plug depends on how far up you went and how big the engine is. A 200 hp naturally aspirated engine at 10,150 ft loses about 53 hp worth of air, which is just past the half step this calculator needs before it will suggest anything, while the same engine at 5,280 ft is not close.
Which way do I move the number for a colder plug?
It depends on the brand, and this is the single most common way people order the wrong plug. NGK states that lower numbers indicate a hotter type and higher numbers a colder type, and DENSO runs the same way, so colder is a higher number for both. Champion, Bosch and Autolite run the opposite way, with the lower number being the colder plug. Set the brand in the calculator and it moves the number in the correct direction for you.
Can I go hotter to stop my plugs fouling?
Sometimes, but it is the riskier direction and this calculator will never suggest more than one step of it. A plug that runs too cold fouls and misfires, which is annoying but not destructive, while a plug that runs too hot can pre-ignite and damage the piston. Autolite's guidance is that if you are unsure of the correct heat range you should always start with a colder spark plug, and not move to a hotter heat range without consulting engine experts. Fouling from short trips or a rich condition is usually better fixed at the cause than with a hotter plug.