The 25-degree rule, explained — and when it fails

Search for how to convert an oven recipe to an air fryer and you will get the same sentence everywhere: drop the temperature by 25°F, cut the time by about 20%, check early. It is repeated so uniformly that it reads like something handed down from a standards body.

We went looking for that standards body. It does not exist.

Nobody official publishes a conversion formula

Here is what we actually found, checked in August 2026.

Cosori publishes a detailed cook chart — sixty-odd rows covering vegetables, poultry, beef, seafood, frozen food and baking, each with a temperature and a time range. It contains no conversion formula at all. Neither does Ninja’s guide, which gives oven times and air fryer times side by side for ten popular foods and lets you draw your own conclusions. Philips publishes recipes.

The USDA’s Food Safety and Inspection Service has a page specifically about air fryers. It describes them as “essentially countertop convection ovens”, notes that food “generally require[s] 5 to 25 minutes of cooking at a temperature range of 350 degrees F to 400 degrees F”, and warns about overcrowding. It gives no arithmetic for adapting a recipe.

The closest thing to an authoritative statement comes from a land-grant university. University of Illinois Extension writes that to transition most conventional oven recipes to an air fryer you should “reduce temperature by 25 to 30°F and reduce cooking time by 20%.”

Read that again: 25 to 30. The rule that everyone quotes as a precise number was, in its most credible published form, already a range. Every subsequent retelling quietly rounded it into something more confident than the original.

Sources that do commit to a figure land between 25 and 50°F on temperature and between 20% and 25% on time. That spread is not noise — it is the honest answer.

Why the rule works at all

An air fryer is a small convection oven with a fast fan very close to the food. Two things follow.

First, moving air transfers heat to a surface far more efficiently than still air does. The air temperature might be identical, but the rate at which that heat arrives at the food is much higher. Left at the oven temperature, the surface browns and sets before the interior has caught up. Dropping the dial compensates.

Second, the chamber is tiny. An oven heats a large volume of air and a lot of metal; an air fryer heats a few litres. Less thermal mass means faster recovery when you open it, and it means the food is never far from the element.

That is the whole mechanism, and it explains why the rule is a rule of thumb rather than a formula: how much it matters depends entirely on the shape of the food and what you want to happen to its surface.

Where the rule fails

Bacon, which goes hotter, not cooler

Ninja’s own chart runs bacon at 220°C in the air fryer against 200°C in the oven. That is hotter than the oven, in the opposite direction to the rule, and it cuts the time from 10–15 minutes to 6–8.

The reasoning holds up. Bacon is a race between rendering the fat and burning the lean. Moving air carries rendered fat away from the surface faster than still air does, so a higher temperature reaches the crisp point before the meat dries out.

Interestingly, Cosori disagrees, putting bacon at 385°F — around 40 degrees cooler than Ninja. Both produce good bacon; they produce different bacon. Hot and fast gives you brittle, shattering rashers. Moderate gives you chewier ones and more margin for error. Our bacon page covers this in more detail.

Searing, where you want all the heat there is

Cosori runs New York strip, ribeye, flank steak, pork chops, shrimp, scallops and calamari all at 450°F — the top of most dials. These are not 25 degrees below an oven equivalent. They are simply as hot as the machine goes, because searing is a surface reaction that wants maximum heat and minimum time.

Apply the 25-degree rule to a steak recipe and you will get a grey band around the outside and no crust. The steak page shows how far two manufacturers can differ on the same cut.

Baking, where 25 degrees is nowhere near enough

Cosori puts cookies, cupcakes and cake at 300°F. A cookie recipe typically calls for 350–375°F. That is a reduction of 50 to 75 degrees — two or three times what the rule suggests.

Batter is the hardest case for a fan. A cookie needs to spread before it sets; a cupcake needs to rise before its dome hardens. Moving air sets and colours the top surface early, locking the structure before the inside has done its work. The only fix is a much lower temperature and more time. See cookies for the specifics, including the parchment fire risk that is genuinely worth knowing about.

Frozen food, where the reduction is bigger than you think

Ninja suggests dropping frozen chips from an oven’s 230°C to 180°C — a 50-degree cut, double the rule. A basket concentrates heat on a much smaller load than an oven tray, and frozen food benefits from time for the core to thaw while the outside crisps. Frozen fries go into this properly.

What to do instead

If the food has its own sourced time, use that. A conversion is an estimate built on a general rule. A manufacturer’s chart entry for that specific food is a measurement. The cook time index exists for this reason.

If it does not, convert — but treat the result as a window. Reduce by 25°F, reduce the time by 20%, and then look at the food at 60% of the original time. That first look is the actual technique. Everything before it is arithmetic.

Adjust for what you want the surface to do. If you want browning and the food is thin, keep the heat up and cut the time harder. If the food is thick, or is a batter, take the heat down further than the rule says.

Do not adjust safety temperatures. Ever. This is the one thing the USDA is completely unambiguous about: “Each air fryer will have varying cooking times due to the size and power of the appliance, but the safe internal temperature is always the same.” Chicken is 165°F in an oven, in an air fryer, and on a barbecue. See safe internal temperatures.

The honest summary

The 25-degree rule is a reasonable starting point that somebody sensible wrote down, that got repeated until it sounded official, and that is wrong often enough to be worth understanding rather than memorising.

Use the converter for the arithmetic. It gives you a range rather than a single number, for exactly the reasons above.