Nobody carries a ruler with them everywhere, but you can measure short distances accurately if you memorise the lengths of your feet and hand span.
Quora user Peter Baskerville suggests measuring your appendages, and committing them to memory.
Learning the actual measurement of your span and your foot will help you whenever your need to measure something but don't have access to a ruler or tape measure. You can then measure anything by number of spans or feet (toe to heel). Then multiply the spans or feet by the known measurement to estimate fairly accurately the actual length, depth or breadth of things.
For other measurements, you could use the same trick with the distance between the tip of your thumb and its first knuckle, or the distance between your nose and your thumb with your arm outstretched. If you have a poor memory, you could always store the lengths in something like Evernote too. This tip is pretty obvious in hindsight, but it's worth mentioning if you haven't done it.