you can measure the distance along the floor using the valve as the marker for a single, full revolution and using a tape measure to get the circumference... it'll be accurate to within a few mm either way.
Or use a fabric tape measure (more flexible than a metal DIY one) and measure around the wheel (including tyre, pumped up) itself, which i feel is a little more accurate.
or measure the height of the wheel (including tyre, pumped up) and multiply by 3.14... I think? It's a while since school.
edited several times