The same idea at a different scale
Both appliances cook by circulating hot air with a fan. That's the whole trick. The practical differences come from size, airflow speed, and preheat time — not from any magical physics the air fryer added.
A full-size convection oven holds 5–6 cubic feet of air and has a fan tuned for that volume. A basket air fryer holds under a cubic foot and has a fan that is, proportionally, a small jet engine. The air in an air fryer moves past the food much faster, which is why you get more aggressive browning per minute.
Where air fryers are better
- Small batches. Reheating two slices of pizza is trivial in an air fryer and a waste of energy in a full oven.
- Crispy surfaces on pre-cooked or frozen foods — fries, nuggets, wings — where speed and airflow matter more than even heat.
- Quick snacks. They preheat in 60–90 seconds instead of 8–10 minutes.
- Keeping your kitchen cool in summer. They dump a lot less ambient heat than a full oven.
Where a convection oven still wins
- Anything over about 1.5 pounds. A whole chicken, a sheet pan of vegetables for four people, a lasagna — the air fryer either won't fit it or cooks it unevenly.
- Baking. Big ovens stage their heat more gently. Cakes, loaves, and anything with a delicate crumb benefit from the larger, slower thermal environment.
- Multiple trays at once. Convection ovens have the height to stack; air fryers usually don't.
- Anything that needs a long, steady 300–325°F. Air fryers at low temperatures often run uneven.
Yes, the 20/20 rule still applies — with caveats
If your convection oven has a dedicated "convection" mode (not just a fan-assisted bake), most recipes already assume you'll knock off 25°F or so from standard oven times. An air fryer compounds that: you're already cooking with a fast fan, so the further reduction to keep things from browning too fast is real.
Converting between the two is rarely a clean multiplier. If you've been running a recipe in a convection oven at 375°F for 30 minutes, a good air fryer starting point is 350°F for 22–24 minutes, with the understanding that you'll check early on your first try and adjust.
The practical answer
Most kitchens that already own a decent oven don't strictly need an air fryer, but they probably want one anyway. The appeal is speed and small-batch convenience, not some textural capability the big oven lacks. If you're choosing between them and can only have one — and you regularly cook for more than two people — a convection oven is the better tool. If you cook mostly for one or two, and a lot of what you cook is frozen, pre-breaded, or reheated, an air fryer is genuinely a better fit than keeping a full oven hot for 40 minutes at a time.