Fuel Mix works out 2-stroke premix so you measure oil in millilitres, which is how most measuring cups and oil bottles are marked. It has two modes.
"Petrol → oil" is for when you already have the petrol: enter the litres and the ratio, and it gives the oil to add in ml (1000 ÷ ratio ml per litre). "Fill a can" is for when you want a set amount of finished premix, such as a full 5 L can: enter the target total and it splits it into petrol litres and oil ml at that ratio by volume.
You can type any ratio or tap an engine preset: IAME X30 at 20:1, Rotax Max at 50:1, KZ shifter at 20:1, Vortex ROK at 25:1 and a generic 2-stroke at 25:1. The 4-stroke / electric chip reminds you that those karts need no premix. The presets are common starting points only; your engine builder and class regulations have the final word, and the app says so on screen.
Premix usually gets mixed in the paddock, where signal is patchy. The calculation runs on the phone, so it works offline. It is free with no cap and needs no sign-in.
