Only the moments when the product is being transformed in a way the customer values: machining, welding, assembling, testing. Setup time, waiting time, transport, inspection, storage, none of these are touch time. The strict definition is deliberate; it produces a small number that reveals the true value-adding fraction of the process.
In most traditional processes, touch time is a single-digit percentage of lead time, often 1-5%. That means 95-99% of the time a product spends in the factory is not adding value. Understanding this ratio is what motivates lean transformations: the largest opportunity is not to work faster, but to eliminate the waiting between the work.
Measure touch time and lead time on the same unit. Calculate the ratio. Compare it to industry benchmarks. Set targets to improve the ratio, not just to reduce touch time. And use the gap as the visible motivation for value stream improvements, because closing it delivers shorter lead time and lower cost simultaneously.
Touch time is the small honest number that shows how much of a process is actually working. Compared against lead time, it produces one of the clearest cases for lean transformation: most of the time and cost is in the waiting, not in the work.
