Almost every packet in a British supermarket still gives oven instructions, and a fair number now add an air fryer line underneath. The trouble is that the air fryer line is often written for a machine nothing like yours — tested on one model, one basket size, one portion. Your 5.5-litre dual drawer with a full load of chips behaves differently, and the gap shows up as a burnt outside and a cold middle.
The conversion itself is simple enough. Knowing when to ignore it takes a bit of experience.
The Short Answer
Drop the oven temperature by 20°C and cut the cooking time by roughly 20 per cent. A tray of sausages at 200°C for 30 minutes becomes 180°C for about 24 minutes in an air fryer. That rule works for the majority of everyday food — chips, chicken pieces, roast vegetables, breaded items, most things you'd otherwise spread on a baking tray.
One important caveat that trips people up constantly: if the packet gives a fan oven temperature, you're already most of the way there. An air fryer is a small fan oven, so reducing a fan temperature by another 20°C can leave food pale and undercooked. Take the 20°C off conventional oven figures, and only about 10°C off fan figures. UK packaging usually lists both, with the fan figure in brackets — that bracketed number is your real starting point.
What Actually Matters
An air fryer cooks faster than an oven for two reasons, and neither is magic. The chamber is tiny, so it reaches temperature in a couple of minutes rather than fifteen. And the fan is proportionally far more powerful relative to that chamber, so air moves across the food much faster, stripping away the insulating layer of cooler, moister air that sits against anything cooking in a big oven.
That second point is what matters for conversions. The speed advantage comes from air movement, not higher heat, so the more surface area exposed to that moving air, the better the rule holds. A single layer of chips converts beautifully. A dense casserole dish in the basket barely converts at all, because the air only reaches the outside of the dish — at which point you've simply built a slower, smaller oven.
It's also why load size matters more than most charts admit. Doubling the food doesn't double the time, but it adds 25 to 40 per cent for a full basket versus a half one. An oven barely notices. Your air fryer very much does.
Detailed Analysis
The basic conversion, written out properly
Take the oven instruction. If it's a conventional (non-fan) temperature, subtract 20°C. If it's a fan temperature, subtract 10°C, or leave it alone entirely for anything you want well browned. Then reduce the time by around 20 per cent and check the food at that point rather than trusting it.
A few worked examples using typical UK packet instructions:
| Food | Oven instruction | Air fryer equivalent |
|---|---|---|
| Oven chips | 220°C (200°C fan), 20 mins | 190°C, 15–16 mins, shake twice |
| Chicken breasts (fresh) | 200°C (180°C fan), 25 mins | 180°C, 18–20 mins |
| Sausages | 200°C (180°C fan), 30 mins | 180°C, 22–25 mins, turn once |
| Breaded fish fillets | 200°C (180°C fan), 22 mins | 180°C, 14–16 mins |
| Roast potatoes (par-boiled) | 200°C fan, 45 mins | 190°C, 28–32 mins, shake twice |
| Frozen pizza | 220°C (200°C fan), 12 mins | 180°C, 8–10 mins |
| Cupcakes | 180°C (160°C fan), 20 mins | 150°C, 12–14 mins |
| Bacon | Grill, 8 mins | 180°C, 7–9 mins |
Note how the drop is larger for baked goods and pizza than for chips. That's not inconsistency — it's the fan doing damage to anything with a delicate surface, which brings us to where the rule breaks down.
Where the rule fails
Anything with a wet or loose batter. Yorkshire puddings, toad in the hole, tempura — anything where liquid batter needs to set before the air hits it. A fan at full tilt blows the batter sideways before it can rise. Air fryers with a bake or low-fan setting manage it; most don't, and you end up with a flat, misshapen thing that tastes fine and looks terrible.
Light items that can lift. Filo, tortilla wraps, sliced bread, baking paper without food weighing it down. The fan picks them up and pins them against the element, which is a fire risk rather than a nuisance. Use perforated liners sized for your basket, and load the food before the machine starts.
Large, dense joints. A whole chicken doesn't obey the 20 per cent reduction, because heat has to travel to the centre and conduction happens at its own pace regardless of airflow. Expect a 10 per cent time saving at best, and drop the temperature by 30°C or the outside will be dark before the middle is safe. Use a probe rather than a clock — poultry needs 75°C in the thickest part.
Frozen food is a different case
Frozen items are where air fryers genuinely excel, and where the conversion is more forgiving than people expect. Surface moisture flashes off quickly and the coating crisps rather than steaming, which is exactly what a domestic oven struggles with when you load a cold tray into it.
Use the fan temperature as printed and cut the time by 25 to 30 per cent, shaking at least once. Frozen chips are worth checking early the first time — the difference between good and overdone is about ninety seconds.
Batch size, and the mistake nearly everyone makes
Overcrowding is the single biggest cause of disappointing air fryer results, and it's rarely the machine's fault. If food is piled up, the air can't circulate, and what you get is steaming rather than crisping — soggy, pale, and taking longer than the oven would have done.
The honest guidance is a single layer with visible gaps. For a 4-litre basket that's realistically two portions of chips, not four. Cooking two batches back-to-back is quicker than you'd think, because the machine is already hot for the second run. If you regularly cook for four or more, a dual-drawer or an air fryer oven with shelves is a better fit than trying to make a small basket do the work.
Preheating: does it matter for conversions?
For most food, no. Air fryers reach temperature in two to three minutes, and the conversion figures above assume a cold start. Where it does matter is anything you want seared or crisped on contact — steak, halloumi, reheated pizza slices, chips you want properly crunchy. Two minutes of preheating for those, and then treat the timing as normal.
Real-World Considerations
Air fryers vary far more than ovens do. Two 5-litre baskets from different brands, both set to 180°C, can differ by 15°C or more in practice, and the fan speeds are not comparable at all. Cheap models tend to run hot and fast; larger air fryer ovens with a bigger chamber behave much more like a conventional fan oven and need less aggressive conversion.
So treat the first attempt at any new dish as calibration. Set a timer for around 70 per cent of what you think it needs, look at the food, and add time in small increments. After a fortnight the published charts become a starting point rather than gospel.
On running costs, the savings are real but modest and depend on portion size. A 1,500W air fryer running for 20 minutes uses about 0.5kWh — roughly 12p at typical UK rates. A 2,500W fan oven running 40 minutes including preheat is closer to 40p. For a couple's dinner that adds up over a week. For a full Sunday roast, the oven does more work in one go and the gap narrows considerably.
Common Mistakes Buyers Make
Using the packet's air fryer line without question. It's tested on one machine. Treat it as a suggestion, not an instruction.
Double-reducing from a fan temperature. Taking 20°C off an already-fan figure is the most common cause of pale, undercooked results, and it makes people think their air fryer is faulty.
Trusting the timer over the food. Air fryers go from perfect to overdone quickly because the surface is exposed to fast-moving hot air. Check early, check often, particularly for anything with sugar in the coating.
Never shaking or turning. A basket air fryer heats from above. Whatever's on the bottom of the pile does not cook at the same rate, no matter what the conversion says.
Buying too small. A 3.5-litre basket is fine for one or two people. For a family it forces exactly the overcrowding that ruins results.
Alternatives Worth Considering
If most of your cooking is single-tray family meals rather than small portions, an air fryer oven with wire shelves handles conversion far more predictably — the larger chamber behaves like a scaled-down fan oven, so oven timings often transfer with only a 10°C reduction and little time change.
For households that already own a good fan oven and cook for four or more, the honest position is that an air fryer is a convenience appliance rather than a replacement. It's excellent for chips, frozen food, reheating and small portions on a weeknight. It won't do a roast dinner for six, and pretending otherwise leads to disappointment.
Frequently Asked Questions
Do you always reduce the temperature in an air fryer?
Reduce by 20°C from a conventional oven figure, and by about 10°C from a fan oven figure. For very short cooks, or anything you want heavily browned, you can leave a fan temperature unchanged and simply shorten the time.
Do frozen foods convert the same way?
They convert better. Use the fan temperature as printed and cut the time by 25 to 30 per cent, shaking at least once. Frozen chips, breaded items and frozen vegetables are where air fryers most clearly beat an oven.
Can I use my oven recipe timings for a whole chicken?
No. Large joints need a bigger temperature drop — around 30°C — and only a modest time reduction, because the heat still has to reach the centre by conduction. Use a meat thermometer and cook to 75°C rather than to a clock.
Final Thoughts
The 20°C and 20 per cent rule is a good starting point and it holds up for the majority of everyday cooking. Where it falls down is entirely predictable: wet batters, light items that blow about, dense joints, and anything crammed into a basket that's too small for the job.
Learn your own machine's tendencies within the first couple of weeks, keep the food in a single layer, and check by eye rather than by timer. Do that and you'll rarely need a conversion chart again — which is really the point of one.
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