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To check if your bikes chain is in need of replacement pinch some chain between your thumb and forefinger with the chain on the bikes largest sprocket (chainring) as shown. If the chain comes away from the sprocket so that you can see a gap between the chain and the cog then its time to replace the chain. The chain on the left is well overdue for replacement. Or as we say in Bristol its girt knackered.
Because the Machinehead Chain Length Calculator calculates is cog sizes from chain pitch, the assumption is that you are calculating for a new chain and sprockets that have been precisely manufactured. Worn out chains will always have more slack than new ones and will therefore require longer chainstay lengths than quoted.
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