Chain Length tells you how many links a chain needs for a given gearing. You enter the front and rear sprocket teeth and the centre distance in millimetres, measured sprocket centre to sprocket centre with the engine mid-slot. Then pick the chain pitch: #219 (the karting standard, 7.774 mm), #35 (9.525 mm) or #428 (12.70 mm). The app notes that KZ and some 4-strokes run larger pitches.
The tool uses the standard chain-length formula: twice the centre distance over the pitch, plus half the sum of the teeth, plus a correction for the difference between the sprockets. It shows the exact, unrounded link count first.
Chains join in even link counts, so the main answer, "Chain to buy / make", rounds up to the next even number. You take up the remaining slack with engine position. It also gives the length of that chain in metres.
This is useful when you change sprockets and want to know what chain the new gearing needs. It runs entirely on the phone, so you can check it in the paddock with no signal. It is free with no cap and needs no sign-in.
